Termperature sensors are used today in many industrial and everyday environments for control and monitoring purposes. To make the most of your existing temperature sensors or to learn more about this sensing technology, Sensor Tips has provided the latest news and information for your reading pleasure.

Sensor Measures Dissolved Nitrate In Water

Featuring a precision nitrate ISE electrode sensor with an integral self-cleaning sprayer, the new HYDRA Nitrate Analyzer System from Electro-Chemical Devices, Inc. (ECD), offers superior measurement, monitoring and control with virtually no maintenance.

The highly intelligent HYDRA Nitrate Analyzer System measures the concentration of dissolved nitrate as nitrogen (NO3–N) in water. The sensor uses two electrodes to determine the NO3–N concentration: a nitrate ion electrode and a chloride ion electrode. An optional electrode is also available for pH measurement.

The system’s HYDRA Analyzer is configured to periodically actuate a cleaning cycle using the integral spray cleaner in the nitrate sensor, minimizing the formation of biofilms or other coatings on the electrodes and keeping maintenance to a minimum. The cleaning cycles feature a user configurable period and duration. During the cleaning cycle the 4-20 mA output is held at either a preset value or the last value.

While useful in all types of water treatment applications, the HYDRA Nitrate Analyzer System is especially well suited for municipal wastewater treatment plants. Nitrogen primarily enters a municipal wastewater system as ammonia/ammonium compounds. Nitrification oxidizes the toxic ammonium ion into a much less toxic nitrate ion using an aerobic activated sludge process.

De-nitrification reduces the nitrate ion (NO3-) to nitrogen gas (N2) through an anoxic reaction in the same treatment basin or in a separate anaerobic digester. The NO3–N measurement helps optimize the methanol being fed to the digester, which minimizes cost and provides trend measurement of the total nitrogen (TN) in the effluent.

The nitrate ion electrode provides the primary measurement. A second electrode measures the Chloride ions in the sample. The HYDRA Analyzer subtracts the appropriate amount of signal from the nitrate measurement for accurate monitoring. The sensor also detects temperature, and the analyzer provides a temperature-compensation calculation for superior measurement accuracy.

The rugged nitrate sensor offers 1.25-inch NPT rear facing threads for attaching an extension/immersion tube for easy installation from catwalks or handrails. Internal signal conditioning allows the sensor to be mounted up to 200 meters from the analyzer. The sensor is extremely low-maintenance, featuring a movable electrode guard to facilitate easy electrode replacement when necessary.

ECD’s HYDRA Nitrate Analyzer System measures nitrate and chloride in concentrations from 0.1 to 1000 ppm, pH from 0 to 14 and temperature from 0 to 50°C (32 to 122°F). Accuracy is ±3 percent of reading with a response time of T90 1 minute.

Featuring a backlit LCD display that provides up to 4 lines of text and graphics, the HYDRA Analyzer includes two 4-20 mA outputs with two SPDT alarm relays. Input power is 110/220 Vac. It is housed in a rugged NEMA 4X enclosure.

www.ecdi.com

New Sensors & Instruments Solutions Guide from Minco

Minneapolis, MN — Minco has a new, free guide that walks you through defining your temperature sensing application requirements and choosing the best sensor and instrument solution. This guide will also help you determine what sensing technology is best, show you how to compare sensor alternatives and help you obtain parts for testing and prototyping sensors. The Sensors & Instruments Solutions Guide can be ordered in print from Minco’s website or downloaded free.

If you have any responsibility regarding temperature sensing, transmitting or recording, you must have this Catalog in your library. This new publication has a number of improvements including:

• It highlights the 255 parts that are in stock and ready for fast delivery. It also shows thousands of other standard sensor designs and explains how to work with Minco engineers to build a custom sensing solution.

• It includes several brand new products such as a compact plug sensor, integrated sensor/transmitter assembly, conductivity level sensor, programmable “smart” transmitters and more.

• It offers a full line of sensor and transmitter assemblies with calibration accuracy options to meet virtually any temperature sensing application requirement.

• Streamlined, simplified ordering and quick access to part drawings.

• Choose from many Explosion-proof and Intrinsically-safe sensor designs for your applications in hazardous areas.

The 172-page guide explains various temp sensing solutions for process control, building automation, defense, aerospace, machinery and industrial and commercial equipment. The guide has complete technical data on sensor assemblies, probes, miniature sensors, sanitary sensors, stator RTDs, HVAC temperature and humidity sensors, flexible sensors, elements, instruments, transmitters and accessories. In addition to the Guide, a Non-Invasive Sensors Design Kit is also available.

Minco
www.minco.com

Low Power Requirements – High Temperature Ratings

The Low power requirements on the Microchip Technology Inc. sensor doesn’t keep the MCP9804 Temperature Sensor from providing a high temperature accuracy of +0.25° C (typical) and +/- 1° C from -40 to +125°C, as well as static current consumption of just 200 µA.

Many temperature-sensing designs require the use of several external components, making them large, complex and expensive. Silicon-based temperature sensors are becoming more popular because they do not require external components and can be used with little to no design experience. In addition to low power and high accuracy, the MCP9804 sensor features programmable shutdown to extend battery life; an alert feature for over- and under-temperature window monitoring; and a critical temperature-alert feature that provides over-temperature protection, helping to further lengthen system life.

“The MCP9804 temperature sensor represents a significant expansion of Microchip’s temperature-sensor family,” said Bryan J. Liddiard, vice president of marketing with Microchip’s Analog and Interface Products Division. “The sensor gives designers a tremendous amount of flexibility to design smaller, higher-performing temperature sensing systems at lower costs.”

Example applications for the MCP9804 temperature sensor include industrial freezers that require high accuracy at lower temperatures such as -20° C to +45° C; consumer electronic devices that require high accuracy at +85° C, such as personal computers; and automotive applications that demand high accuracy at temperatures up to +125° C, such as engine temperature monitoring.

Microchip Technology Inc

Understanding Infrared Thermometry

John R. Gyorki
Editorial Director

Infrared (IR) thermometry appears to be rather straight forward: point, press the button, and read the temperature. However, measurement results will be quite disappointing without a thorough understanding of the instruments’ principle of operation and specifications.

Temperature measurement instruments can be divided into contact and noncontact types. Sensors used in contact-type instruments include thermocouples, resistance temperature detectors (RTDs), thermistors, and semiconductor temperature sensors. Since contact sensors measure their own temperature they require physical contact with the measured object to bring the sensor body to the object’s temperature.

In some applications this contact creates problems: The measured object or media may be located at a distance or in a hazardous environment with no easy access. Measurements of moving objects are also difficult. A small object’s temperature may be altered when a relatively large sensor touches it and acts as a heat sink.

Noncontact infrared (IR) thermometers, if used properly, offer convenient solutions for these and many other measurement applications. However, you should select the measuring instrument and measurement techniques to be compatible with the application.

How IR Thermometry works
Heat is transferred from one body to another through conduction, convection, or radiation. Radiation is a process where heat energy in a form of electromagnetic waves is emitted by a hot object and absorbed by a colder object. Most of this radiation is in the infrared (IR) region of the electromagnetic spectrum, but some also spreads into the visible light band. The IR wavelength band stretches from 0.7 to 1000 microns, however practical IR measurement systems use only certain wavelength bands between 0.7 and 14 microns because the radiation is the strongest in this range.

If an object is exposed to IR energy radiated by a heat source, such as an electric heater, light bulb, sun, or other source, the energy reaching the object is called incident energy.  Part of this energy is reflected off the object surface. Theoretically, the object’s coefficient of reflectivity can vary from 0 (no reflection) to 1.0 (100% reflection). Rough, matt surfaces have low reflectivity. Polished and glossy surfaces, especially metals, have high reflectivity.

Depending on the object material, thickness, and the radiation wavelength, part of the radiation can go through the object or be transmitted. The coefficient of transmission can vary from 0 (no energy transmitted through object) to 1.0 (100% energy transmitted through object). High transmittance examples include glass, quartz, plastic film, and various gasses. Materials opaque in the IR spectrum have close to zero transmission coefficients.

The remaining energy is absorbed by the object and raises its temperature. A hypothetical body that has no reflection or transmission and absorbs all incident energy across the entire spectrum has a coefficient of absorption equal to 1.0 and is called a blackbody. Real-life objects, referred to as gray bodies, have coefficients of absorption that fall between 0 and 1.0.

Incident energy, W_I, is defined as:

W_I = W_R + W_T + W_A

Where:
W_I =  incident energy received by the object, W
W_R = energy reflected off the object’s surface, W
W_T = energy transmitted by the object, W
W_A = energy absorbed by the object, W

As the object absorbs energy and heats, it also emits energy.  When an object is in a state of thermal equilibrium, the amount of energy it absorbs (W_A) equals the amount of energy it emits (W_E): W_A = W_E. When an object absorbs more energy and its temperature increases, the amount of radiation it emits also increases.

IR thermometry is based on the fact that any body (solid, liquid, or gaseous) that has a temperature above absolute zero (0^oK or -273^oC) emits radiant energy. This energy is proportional to the forth power of the body temperature, and the body’s ability to absorb and emit IR energy is called emissivity. Energy radiated by a body can be expressed as follows:

W = E σ T4 A

Where:

W = energy, W
E = emissivity
σ = Stefan-Boltzmann Constant = 5.6703 10^-8, W/m²K⁴
T = absolute temperature, ^oK
A = emitting area, m²

Emissivity can range from 0 to 1 for various bodies. A hypothetical blackbody emits and absorbs all energy and thus has an emissivity equal to 1. Real-life objects have an emissivity between 0 and 1.

When an IR thermometer measures an object’s temperature, consider the energy that actually enters the lens. That is, in addition to emitting energy related to its own temperature, the object may reflect energy coming from another source, or transmit energy passing through it from a source behind it. For accurate measurements, survey the surrounding area for possible sources of extraneous IR radiation and choose the thermometer position and aiming angle to minimize the effects of those sources.

IR Thermometers
Infrared temperature measurement instrument design varies from simple hand-held thermometers that can be purchased for less than a hundred dollars to complex special-purpose instruments that cost hundreds and even thousands of dollars. However, some building blocks are common for most designs.

A typical infrared thermometer consists of optical components, IR detector, electronics, and a display or interface output stage. Optical parts focus radiation energy onto the IR detector and filter out radiation outside the desired wavelength band.  These components include collecting optics, lenses, fiber optics, and spectral optical filters.

IR Detectors
The majority of IR detectors are either single-wavelength (also called single-color), or dual-wavelength (also called two-color) type. The single-wavelength detectors measure IR energy within a certain wavelength band, and the instrument calculates object temperature based on the detector output and the preset emissivity. Some thermometers have adjustable emissivity, and most simple units have fixed emissivity.

Dual-wavelength detectors measure energy at two different wavelength bands, and the instrument calculates temperature based on the ratio of the two readings. If emissivity or the energy changes by the same amount at both bands, the measurement accuracy is not affected. Emissivity or the amount of radiated energy may change due to object change or movement, lens contamination or misalignment, or view obstruction.  The dual-wavelength detector’s drawback is higher cost and lower accuracy under certain conditions.

Emissivity of many materials and surfaces remains relatively constant over the IR wavelength range, and measuring energy in any narrower band will be acceptable. Other materials have wavelength bands with higher and lower emissivity due to high reflectivity or transmission, and require narrow band detectors tuned to high emissivity wavelengths.

Another factor is the atmosphere. Its transmission coefficient vs. wavelength curve has many peaks and valleys, which swing from almost 1.0 to near zero and block the IR energy transmission. Most general-purpose IR thermometers use the largest high-transmission band between 7 and 14 microns to minimize atmospheric attenuation.

To measure temperature of objects with emissivity that varies greatly over the IR wavelength spectrum and objects obscured by glass, smoke, steam, or other barriers, engineers need to use narrow band IR detectors. For example, short-wavelength detectors handle variable emissivity objects, lens contamination, and measurements through glass windows. Long-wavelength detectors are more prone to errors due to emissivity changes, but have a wide temperature range.

Special applications, such as measuring the temperature of glass, crystal, flame, gas, and thin film require detectors with specific narrow bands.  For example, detectors with a narrow band centered on 5 microns give the best results when measuring glass temperature.  Metals and metallic foils usually require 1 micron detectors where they have the highest level of radiation.

Based on the principle of operation, IR detectors fall into one of two categories: thermal detectors and photo detectors (photodiodes). Thermal IR detectors absorb the incident energy, raise the sensing element temperature, and change the detector’s electrical properties: thermopiles generate thermoelectric voltage, bolometers change resistance, and pyroelectric devices change their polarization. In general, they are slower than photo detectors.

A thermopile is made by connecting several thermocouples in series and placing their hot junctions in contact with a black body that absorbs the incident IR energy and heats the hot junctions. The cold junctions are placed in the area of the detector with adequate heat sinking. These detectors have fast response, broad band, large dynamic range, and are frequently used in general-purpose, automotive, air conditioning, and human-body thermometers.

Bolometers use a slab of material that changes its resistance in response to a change of temperature. The circuit converts resistance change to a voltage change, which is further processed by the instrument. Bolometers are frequently used for measuring low-level IR energy, often as an attachment to a telescope.

Pyroelectric devices become electrically charged when their body temperature changes. To produce a usable signal, the incident IR energy has to “pulse”. The output peak-to-peak AC signal is proportional to the pulse energy. Since energy emitted by measured objects is usually steady, thermometers that use pyroelectric detectors have a mechanical or optical chopper in front of the sensor. These sensors are used in many home security systems.

Photo detectors are built on a silicon substrate with an IR sensitive area that releases free electrons when impacted by the photons. The flow of electrons produces electrical signals proportional to the incident energy. These detectors are often used as arrays in thermal imaging systems.

A detector needs protection from the environment, and the selected window material must allow the correct wavelength band to pass through with minimum attenuation. A zinc sulfide or germanium window is best for the long-wavelength detectors, glass is suitable for short-wavelength detectors, and quartz for the mid-wavelength spectrum. Some instruments use a fiber-optic light guide to direct the radiation to the detector.

Since all types of IR detectors produce signals in the microvolt range, a high-gain amplifier should follow the detector.  Detector output vs. temperature curves are not linear and fluctuate greatly with a change in ambient temperature. To remedy this a signal-conditioning circuit stabilizes the temperature and linearizes the signal. Many applications require an analog-to-digital converter (ADC) to convert the temperature reading to a digital format.

Hand-held and many other instrument types have a built-in display, while other devices connect to a computer, data acquisition system, or temperature control system via an RS232 or RS-485 cable.  Some instruments simulate a thermocouple output, others have a 0 – 20 mA or 4 – 20 mA current loop, or voltage output.

The key specifications and considerations for any IR thermometer application are field of view (FOV) and distance; spectral band; response time; accuracy and repeatability; emissivity of the object or media being measured; media between the object and IR thermometer, such as vacuum, air, steam, gas, glass, or other; object temperature range; mounted or hand-held application; and type of output signal or display.

The FOV characterizes the diameter of a circle (target) that the IR detector will “see” at a certain distance from the measured surface. However, there is always a minimum target diameter that depends on the optical system and detector size. The detector measures and averages temperatures of all objects within the target area.  FOV is commonly called distance-to-spot size ratio and is a ratio of the distance between the meter and the target to the diameter of the target.

For example, a 10:1 distance-to-spot size ratio means that if a measured surface is located 10 inches from the thermometer it will measure and average the temperature of a circle with 1 in.diameter. Move the thermometer away to 20 in. and the target will increase to 2 in., and so on. A thermometer with a 1:1 ratio will measure within a one-foot diameter circle when held one foot away from the target.

Thermometers designed for measurements of small areas have a very narrow FOV and measure temperatures of objects less than a tenth of an inch. For example, such a thermometer held near a component on a pc board will measure the temperature of just that component and ignore the
components around it.

Other optical systems allow accurate temperature measurement of a spot several inches in diameter at a distance of tens of feet. However, such measurements require accurate pointing. Though notches on top of the instrument provide some help, aiming lights and built-in laser pointers prove to be most helpful.

Unfortunately, a laser pointer may occasionally lead to erroneous measurements if the user is not familiar with the IR thermometer operation and the FOV concept. Some first-time users mistakenly think that the laser beam that they see has something to do with the process of measuring temperature.  They presume that the instrument displays the temperature of the tiny spot where the laser beam meets the surface. Such measurements will not yield satisfactory results.

Practical considerations

Here are some helpful tips:
• Avoid degrading measurement accuracy by environmental elements, such as dirt, dust, smoke, steam, other vapors, extremely high or low ambient temperatures, and electromagnetic interference from other devices.
• Select an IR thermometer with a wavelength band compatible with the measured object (especially high reflectivity objects) and with the media between the thermometer and measured object (especially glass, smoke, or steam).
• Select an instrument with a temperature range not much greater than the maximum application temperature. Wider than needed temperature ranges lead to lower accuracy or higher instrument cost.
• An IR thermometer averages the temperature of all objects within its field of view: Select the instrument with an appropriate FOV, and calculate the proper distance so that only the desired area is measured.
• Avoid hot objects near the measured object. They radiate energy that can be reflected or transmitted by the measured object into the thermometer FOV.

For more information go to:

www.omega.com

www.watlow.com

www.gesensing.com

www.raytek.com

www.murata.com

Digital temperature sensor line

Franklin Lakes, NJ — Digital electronics and precision engineering contribute to the small size of the latest addition to Process Sensor Corp’s IR thermometer line.

The DS-40N and DG-40N series of stainless steel pyrometers are 124.5mm long overall by M40×1.5 threaded diameter, operate in a current loop powered mode to produce a linear 4 to 20mA output over selected temperature ranges from an overall span of 250 to 2500°C in two models.

In a 4 wire mode, the sensor also offers green LED, illuminating the exact target size, or laser aiming.

Depending on the model, either DS-40N or DG-40N, and temperature range, selected optics provide small target, close focus e.g. 1.2mm diameter at 210mm distance, or extended focus out to 4m with a 20mm diameter target.

A USB connection gives access to PC adjustments of such parameters as emissivity, response time, temperature sub-range and peak picker, with supplied software.

These non-contact sensor will operate in ambient temperatures up to 70°C without cooling.  Adding the optional cooling and lens-purging jacket can extend ambient temperatures higher to 200°C .

The Model DS-40N has a spectral response of 0.8 to 1.1µm, and the DG-40N 1.5µm to 1.8µm, for minimum vulnerability to variable target emissivity, and to optimize the effective emissivity of low radiance targets.

The temperature ranges, optics, rugged construction and signal processing capability will appeal to users in the Steel, Ceramics and Glass Industries, Cement and Refractory manufacture, Induction Heating and Sintering operations.

Process Sensor “PSC Spot Software” supports sensor set-up, multi-tasking, graphic display of temperature trends, and data logging, and in general allows the user to customize the sensor to the application.

Process Sensors Corporation

Minco Chill-Out™ Combination Sensor

Minneapolis, MN — Minco has a new line of heating, ventilating and air conditioning (HVAC) temperature sensors designed to protect chillers in air handling equipment in commercial buildings. The new sensor line is called the Chill-Out™ Combination Sensor.

Visit Minco’s Total Cost of Ownership (TCO) Calculator to see your true savings if you install both a freeze stat low limit device and an averaging sensor.

Most schools, hospitals, office complexes and other commercial buildings use a device called a Freeze Stat, along with an averaging sensor to protect against freezing temperatures. The solid state, low temperature cut-out Chill-Out sensor has a number of advantages over traditional Freeze Stat/averaging sensor technology:

– Minco’s Chill-Out™ Combination Sensor replaces the Freeze Stat, but, unlike the Freeze Stat, it has two sensors in one easy-to-install package. One is a low-temp cut-out sensor and one is an independent averaging-temp sensor. This dual sensor package delivers accurate measurements and eliminates the need to install a separate averaging sensor.
– Saves $13K-$35K over a typical Freeze Stat/averaging sensor installation.
– There are no capillary tubes to break, leak or kink.
– Considered a “green” product by the industry as it is solid state and has no harmful gases or chemicals.
– Easy mounting in any direction. Horizontal orientation is not required.
– Relay changes state to deliver a failure detection signal in the event of a power loss.
– Low-temp sensitivity within a 12-inch segment, versus 18 inches with gas-filled capillary tubes.
– Easily formed aluminum or ultra flexible PVC coated galvanized armor sensor case.
– 4 to 20 mA temperature loop output available with optional Temptran™ transmitter.
– No diaphragm case needed.

The Chill-Out Sensor is described in the handy Sensors & Instruments Solutions Guide. The free 163-page guide explains various temp sensing solutions for process building automation and other commercial applications. The guide has complete technical data on sensor assemblies, probes, miniature sensors, sanitary sensors, stator RTDs, HVAC temperature and humidity sensors, flexible sensors, elements, instruments, transmitters and accessories. In addition to the Guide, a Non-Invasive Sensors Design Kit is also available.

Minco
www.minco.com

Designing with Semiconductor Temperature Sensors

By John R. Gyorki
Editorial Director

Semiconductor temperature sensors are easy to use if you do your homework when matching the sensor with the application.

Temperature sensors are designed into every heating, ventilation, and air conditioning system. Automotive systems rely on temperature sensors for engine control. Many industrial processes require highly accurate and stable temperature measurements. Today’s computers and portable electronic devices have extremely high circuit density with no easy way for heat removal, which makes temperature monitoring a must.

All temperature sensor applications fall into the temperature measurement or thermal management category.  The most widely used types of temperature sensors are thermocouples, resistance temperature detectors (RTDs), thermistors, and semiconductor temperature sensors.

Thermocouples are known for their wide temperature range and low drift. RTDs have the highest accuracy and stability, and good linearity.  Thermistors are fast and have high sensitivity. However, all these sensors require a modest amount of interface circuitry and are difficult to use. Relative newcomers, semiconductor temperature sensors offer several benefits that other sensors cannot match.

Though their temperature range of -50^oC to +150^oC is rather limited, semiconductor temperature sensors have several valuable features such as excellent linearity, high sensitivity, and extremely small size.  They are inexpensive; require no linearization, cold junction compensation, or signal conditioning; often combine several functions on one chip; and produce analog, logic, or digital outputs that can be interfaced directly to an analog measurement circuit, an analog-to-digital converter (ADC), a microprocessor, or a fan control. Measurement-system noise immunity is superior to other sensors since the output is already amplified or digitized inside the integrated circuit (IC).

Thanks to their small size, semiconductor sensors can be installed directly on PC boards, heat sinks, underneath high-power integrated circuits, and in small portable electronic devices. Typical applications include the fastest growing types of consumer products such as cell phones, PCs, PDAs, MP3 players, and automotive systems.

The sensing element
A semiconductor temperature sensor is an IC that combines a temperature-sensing element with signal conditioning, output, and other types of circuitry on one chip.  It relies on the change of voltage across a p-n junction, essentially a silicon diode, in response to a temperature change to determine the ambient temperature. The bipolar IC substrate is designed to build p-n-p and n-p-n transistors, so in practice, the sensing diode is usually formed using a transistor with the base and collector shorted. The following equation shows the effect of temperature on the forward voltage of a silicon p-n junction:

V_BE = V_G0 (1-T/T₀) + V_BE0(T/T₀) + (ηKT/q) ln (T₀/T) + (KT/q) ln (I_C/I_C0)

Where:
T = temperature, ^oK (Kelvin)
T₀ = reference temperature, ^oK
V_G0 = bandgap voltage at absolute zero, V
V_BE0 = forward voltage at current I_C0 and temperature T₀, V
K = Boltzmann constant, J/^oK
q = charge of an electron, C
η = constant associated with a specific device
I_C = forward current at temperature T, A
I_C0 = forward current at reference temperature T₀, A

However, determining temperature using this equation is not very practical since the voltage depends not only on the temperature, but also on the current and the device properties. To eliminate the effect of these variables, practical circuits switch the diode alternately to two constant current sources and determine the temperature based on the difference between the two forward voltages or their ratio:

Δ V_BE = (KT/q) ln (I_C1/I_C2)

Where:

I_C1 = forward current from first current source, A
I_C2 = forward current from second current source, A

Frequently I_C2 is selected to be 10 times greater than I_C1.  In this case, the thermal coefficient of the sensing element is approximately 200 µV/ ^oC.

Sensor types
Semiconductor temperature sensors come in three types determined by the output signal they produce: analog, logic, and digital. They also measure temperature in one of two ways: internally or remotely.  When the measuring diode-connected transistor is built on the sensor IC substrate, the sensor measures the temperature of its own body. Sensors that have no measuring diode built-in connect to a remotely mounted sensing diode-connected transistor.

Analog Voltage and Current Output Sensors: The oldest type, analog output semiconductor temperature sensors, generates output voltage or current proportional to sensed temperature. They combine the sensing diode with the offset and amplification circuitry on one chip. The offset is needed because the output signal of the sensing diode is proportional to absolute temperature, ^oK. Since ^oC and ^oF scales are more common than ^oK, the signal needs to be offset to the new zero ^oC or zero ^oF point. Additional offset is needed to allow measuring negative temperature without using a negative power supply.

Since the voltage across the sensing diode changes only 200 µV per ^oC an internal amplifier is needed to boost the output signal in order to increase the system noise immunity. This makes the voltage output compatible with most analog temperature measuring and monitoring systems. Industrial applications that have high levels of electrical noise use current output IC sensors where output current is proportional to the measured temperature.

Logic Output Sensors: Many temperature control applications do not need to know what the current temperature is until it reaches a preset level–when some action needs to be taken. Logic output sensors designed for this type of applications work like a thermostat in our homes. In fact, these sensors are commonly referred to as thermostat sensors. They do not output the temperature value, but instead one or several logic outputs change their state when temperature exceeds (“hot” sensors) the preset value, or drops below (“cold” sensors) it.

Some devices have user-programmable trip temperature and hysteresis, which is needed to prevent chatter.  The programming is usually done with resistors. Yet other devices have both the temperature and the hysteresis fixed. Logic temperature sensors are simple, inexpensive and are extremely easy to use. Typical applications include turning on fans and alarms, or setting an interrupt to signal the processor that the temperature limit has been exceeded.

Digital or Serial Output Sensors: A digital temperature sensor is really an analog sensor that contains an ADC and a serial output port. Sensor output options include an I²C, SMBus, or SPI interface that connects directly to a compatible  microprocessor/microcontroller interface or a pulse-width modulation (PWM) output. In addition, many sensors have one or several comparators that drive either logic, open collector, or open drain outputs. These outputs are typically wired as an
interrupt for the microcontroller, an on/off line for a fan or alarm, or a hardware power shut-off control signal.

Digital sensors using the single-wire PWM format vary the ratio of high time to low time (duty cycle) of a square wave to represent the temperature value. The processor counts the duration of the high and low times and calculates the temperature based on the ratio. The clock frequency accuracy is not critical since PWM relies on the time ratio, not on the absolute duration.

The I²C, SMBus, and SPI interfaces allow bidirectional communications between the sensor and the microprocessor or microcontroller. The sensor sends the temperature value to the processor, and the processor uses the same interface to program sensor registers that control temperature limits and other functions.

The I²C and SMBus ports require two wires and are frequently used in computer applications. The SPI interface can be configured as a three-wire or four-wire port used most frequently with microcontrollers and in automotive systems.

Temperature sensing can be done locally or remotely. Most semiconductor temperature sensors have the diode-connected transistor on board and thus measure the temperature of their own body. Remote-sensing sensors have no sensing element on board and instead connect to a remotely mounted diode-connected p-n-p or n-p-n transistor. The temperature-sensing transistor can be located tens of feet away from the remote sensor IC.  However, wires leading to the remotely installed sensing transistor require good shielding due to very low signal levels susceptible to noise.

PC thermal protection example
As computers (especially laptops and notepads) pack more computing power, thermal management becomes one of the most critical design issues. CPUs, graphics processors, and other highly integrated chips contain thousands of transistors, and their high clock rates increase dissipated heat due to switching losses. Simply running a fan is not a good option since fans produce audible noise and their service life is shortened when run continuously.

Today’s dense computing and portable electronic devices require sophisticated thermal management, and semiconductor temperature sensors provide this level of sophistication. These ICs combine a temperature sensor with additional functional circuitry and are frequently installed on a heat sink, directly under the microprocessor or a graphics chip. Some CPU ICs have a temperature-sensing transistor made on its own die that can be connected to the external remote-diode temperature sensor for monitoring.

An effective computer thermal management system can be built around a logic output temperature sensor with Low, High, and Control outputs. If the CPU temperature reaches the first preset point above the normal operating temperature due to high ambient temperature, exposure to heat, component failure, or blocked ventilation ports, the Low sensor output changes state and sends a signal to the CPU to reduce the clock rate to lower switching losses and heat dissipation.

Should temperature continue to rise and cross the second set point just below the maximum allowable CPU temperature, the sensor’s High and Control logic outputs change state. The Control  output turns on the cooling fan and the High output commands the CPU to further reduce the clock rate. Should the temperature remain above the High set point after a certain amount of time, the computer will be powered down to prevent permanent damage. However, the most reliable shutdown mode bypasses the CPU since at this point the software might not be running correctly. Instead, an output from the sensor is wired directly to the shutdown input of the power supply.

An alternative approach uses a serial output temperature sensor where the CPU periodically polls the sensor and reads the actual temperature through a serial port. If the temperature reaches excessive levels, the processor will attempt to reduce the heat.  If the temperature exceeds the absolute maximum limit, the sensor’s comparator output will directly shut down the power supply.

Things to watch
IC temperature sensors are not free from self-heating. Some devices have relatively high operating current, the main cause of self-heating, and this might become an accuracy concern when measuring the temperature of still air, for example. The thermal transfer between the sensor plastic housing and still air is not very efficient, so self-heating can cause the sensor-body temperature to rise above the temperature of the air. Since that sensor actually measures the temperature of its own body, the readings will be higher and thus reduce the accuracy.

Heat radiated or conducted through the printed circuit board by other components near the sensor can also affect the measurement accuracy, so when finding the best position for the sensor, the whole system needs to be carefully reviewed.

For more information go to:

Analog Devices, Inc.
http://www.analog.com

Maxim Integrated Products
http://www.maxim-ic.com

Microchip Technology, Inc.
http://www.microchip.com

National Semiconductor Corp.
http://www.national.com

Texas Instruments, Inc.
http://www.ti.com

Designing with RTD temperature sensors

By John R. Gyorki
Editorial Director

Analyze the RTD’s strengths and weaknesses with respect to the application before making a selection.  The application ultimately determines the RTD’s specifications.

Watlow’s RTD sensors are designed to ensure precise and repeatable measurements as well as meet environmental requirements for each application. A high signal-to-noise ratio output increases the accuracy of data transmission and permits greater distances between the sensor and the measuring equipment. These resistance-wire RTDs have a positive temperature coefficient; the resistance change is proportional to the temperature measured.

Resistance Temperature Detectors (RTD) typically operate within a broad temperature range of -200°C to +850°C, are fairly linear, and have excellent long-term stability. Unlike thermocouples, cold junction compensation is not needed, and their temperature range and linearity are superior to both thermistors and thermocouples. When applied correctly, RTDs exhibit extremely low drift, so they do not require recalibration. On the other hand, their comparative weaknesses include lower sensitivity, slower response time, and susceptibility to self-heating. All these qualities make them application-specific.

Principle of operation
RTDs are passive components that require an excitation current to produce an output signal. Similar to thermistors, their resistance “varies” in direct proportion to changes in temperature. The temperature-sensitive element is made of metal or a metal alloy, which gives them the positive temperature coefficient.

Platinum, gold, silver, tungsten, nickel, and copper have been successfully used in different RTD devices, however, platinum is superior to the other metals. It has the highest resistivity, 59 ohms per circular mil foot (Ω/cmf), a wide temperature range, good linearity, and low long-term drift. Nickel or nickel alloy RTDs are more economical, but have a narrower temperature range, poorer linearity, and a greater long-term drift.

There are two major types of RTDs: wire-wound and thin film and each has some advantages and disadvantages. For example, a typical wire-wound RTD uses platinum wire wound around a ceramic or glass bobbin in one of two configurations: birdcage and helix. The birdcage winding construction keeps the platinum wire loose and lets it expand and contract freely with a change in temperature. This minimizes long-term stress-induced resistance change, but has very poor resistance to vibration and is primarily limited to lab use. In a sealed helix-constructed, wire-wound RTD, the bifilar winding is wound around the bobbin and then sealed with molten glass, ceramic cement, or another high-temperature, non-conductive coating. This construction helps protect the wire from vibration, but it is prone to long-term stress induced resistance change when the bobbin and platinum wire have different temperature coefficients of expansion.

In the simplest wire-wound RTD construction (top), thin platinum wire is wound around an insulator bobbin. The wire ends are spot welded or high-temperature soldered to the lead wires. A non-conductive protection coat with good thermal transfer properties covers the whole RTD element assembly. In the thin-film type, the sensing element is formed by depositing a thin layer of platinum onto a ceramic substrate and attaching the leads to the connecting pads. A glass coat (not shown) encapsulates the element.

Newer thin-film type RTDs are made by depositing platinum or another metal alloy film onto a substrate, etching the shape of the resistive element, and then sealing the sensor. The thin film devices are smaller, faster, and considerably less expensive than the wire-wound parts. Thin-film platinum RTDs have virtually linear resistance versus temperature curves and provide a low cost alternative to high-accuracy, wire-wound devices. The drawbacks of film-type RTDs include poor long-term stability and narrower temperature range.

The RTD temperature coefficient represents the sensors’ sensitivity to temperature change. The larger the temperature coefficient (α), the larger the resistance change (ΔR) in response to an ambient temperature change (ΔT):

ΔR = αR_o ΔT,

Where:

α = temperature coefficient, Ω/Ω/°C
R_o = nominal sensor resistance at 0°C, Ω
ΔT = temperature change from 0°C, °C

According to the DIN 43760 standard, the resistance-temperature coefficient of platinum wire typically used in RTD manufacturing is 0.00385 Ω/Ω/°C at 0°C. Another frequently mentioned value, 0.00392 Ω/Ω/°C at 0°C, is the resistance-temperature coefficient of chemically pure platinum wire used for standards. To illustrate using the equation above, consider an ideal 100 Ω RTD that has a resistance of 100.000 Ω at 0°C. Therefore, at +1°C the RTD resistance will be:

R_T = [R_o + (αR_oΔT)] = 100 + (0.00385)(100)(1) = 100.385 Ω.

A problem here is that the RTD temperature coefficient changes over the temperature range, so to obtain an accurate value at any given temperature, a curve-fitting process is required. Use the Callender-Van Dusen equation to calculate the RTD resistance over the entire temperature range:

R_T = R_o + R_o α  [T – δ(T/100 – 1) (T/100) – β(T/100 – 1)(T/100)3]

Where:

R_T = resistance at temperature T, Ω
R_o = nominal RTD resistance at 0oC, Ω
α = temperature coefficient, Ω/Ω/oC
δ = 1.49 for pure platinum
β = 0 if T > 0
β = 0.11 if T < 0

Excitation current vs. resistance
Because RTDs are resistors, they need an excitation current to produce an output voltage. At a given current value, an RTD with higher resistance will have higher voltage resulting in lower required amplification and higher signal-to-noise ratio. However, increasing the RTD’s resistance considerably slows its response, which might be unacceptable for many measurements.

An RTD does not produce any voltage by itself. A source of voltage and an excitation resistor, Rex are needed to make the RTD work. The excitation resistor and the RTD form a voltage divider. Voltage drop across the RTD is proportional to its resistance, so when RTD resistance changes with temperature, the change in voltage represents the change in temperature.

Selecting a low-resistance RTD for remote sensor installations can also present a problem. Long lead wires can add significant error to the temperature measurement. For example, if an RTD with a nominal resistance of 100 Ω is installed at a distance of 200 ft from the signal conditioning circuit, and the two lead wires are made of 24 AWG stranded (7 strands) tinned copper wire, the output voltage seen by the voltmeter will be the sum of the voltage drops across the RTD and both lead wires.

The wire selected for this example has a resistance of 0.023Ω/ft, so the total wire resistance is calculated as:  R_wire = (0.023)(200 + 200) = 9.2 Ω. The total RTD resistance plus the wire resistance is 109.2 Ω, which adds a 9% error to the measurement. The problem stems from the fact that the same two wires are used to supply the excitation current and make the measurement. To make matters worse, copper resistance changes with temperature, making compensation difficult. Therefore, to obtain an accurate temperature reading, the excitation current wires and the measuring connection wires are separated in a 4-wire, Kelvin connection.  One pair of wires supplies constant excitation current, while the other pair connects the voltmeter (or signal conditioning circuit) directly across the RTD. Here, the wire resistance does not affect the voltage drop across the RTD, because the excitation current is constant, and the resistance of the measurement wires has no effect on accuracy since it is negligible compared to the high input impedance of the voltmeter.

One pair of wires in a two-wire connection supplies both the excitation current and measures the RTD output, so the voltage drop due to the resistance of the excitation current lead wires can add a significant error to the output signal. On the other hand, the four-wire or Kelvin connection separates the excitation and measurement wires. Here, the excitation current flows only through the RTD and excitation wires and the voltage drop does not appear in the measured variable. This makes the measurement more
accurate, even in remote installations.

A compromise between the 2-wire and the 4-wire connection is the most widely used 3-wire connection.  One end of the RTD connects to one wire, and the other end connects to two wires: a wire for power and a wire for signal.

Another frequently used circuit that avoids the effect of lead-wire resistance on the accuracy of a measurement is a four-resistor Wheatstone bridge. Two lead wires apply excitation power to the bridge. The bridge output is connected to a voltmeter, an operational or instrumentation amplifier, or a high-resolution analog to a digital converter. Ideally, the three bridge resistors should have a zero temperature coefficient, so only the RTD resistance depends on temperature. The output voltage depends only on the bridge resistance unbalance — it is not affected by the resistance of the lead wires.

The Wheatstone Bridge circuit using an RTD is similar to the circuit used with thermistors. The values and temperature coefficients of resistors R1, R2, and R3 should be selected in accordance with the accuracy needed for the application.

When selecting the RTD resistance and the excitation current, maintain a balance between the resolution and response time. It might be tempting to select a low resistance RTD for faster response, but since V_out = (R_RTD)(I_ex), a lower resistance requires higher excitation current to maintain the same output voltage and high system resolution. Higher excitation current generates more heat and raises the sensor temperature above the temperature of the object being measured, which produces a significant error. As a rule, the excitation current should be kept as low as possible to reduce the self-heating error. Typically, self-heating errors can be kept below 0.5°C, which is considered acceptable.

Another measurement error, thermal shunting, might creep in when measuring the temperature of a small object. Due to its relatively large size, the RTD might act as a heat sink and alter the temperature of an object that is similar in size or smaller than the sensor itself.

Maintenance
Interchangeability is critical when replacing worn or failed RTD elements. Knowing the allowed variance of readings between two sensors, allows equipment maintenance without recalibration. The American Society for Testing and Materials (ASTM), American Scientific Apparatus Manufacturers Association (SAMA), International Electrotechnical Commission (IEC), and Japanese Standard (JIS) have all developed several standards for platinum RTD elements. These standards guarantee element interchangeability when used within the specified temperature range.

For example, European standards IEC751 and DIN 43760 contain identical accuracy and tolerance parameters. They are collectively referred to as DIN IEC 60751, or just IEC751 and specify resistance values at various temperatures for platinum RTD sensors. Standard compliant devices have a resistance of 100.00Ω at 0°C and a temperature coefficient of resistance of 0.00385 Ω/Ω/°C from 0°C to 100°C. Specified overall temperature range and tolerance depend on the class.

The most accurate, DIN Class A has a ±0.06% tolerance at 0°C that spreads to ±0.24% at -200°C and ±0.46% at 650°C. DIN Class B has a ±0.12% tolerance at 0°C with a wider spread of ±0.56% at -200°C and ±1.34% at 850°C. Manufacturers frequently express tolerance specifications in Ω or in °C instead of %.

Most American and European manufacturers produce Class A and B IEC751-compatible elements. In addition, some RTD manufacturers offer less accurate class C and D elements with a ±0.2% and a ±0.5% tolerance at 0°C, respectively.

Most Japanese and some American manufacturers use Japanese Standard JIS C 1604. This standard specifies the same base resistance of 100.00Ω at 0°C, a temperature range of –200 to +650°C, and tolerances as IEC751, except the temperature coefficient of resistance is 0.003916 Ω/Ω/°C.  JIS C 1604 J and K standard tolerance classes correspond to the A and B classes of the European standard.

The environment
Many industrial and some lab applications require the RTD element to be protected from the environment. Moisture, corrosive environments, mechanical impact, and vibration can quickly degrade the sensor if not taken into consideration. Selecting the type of RTD or RTD probe appropriate for the application and compatible with the environment is the key for reliable service.

RTDs for industrial applications are typically built into a probe with a stainless steel or Inconel sheath protecting the sensing element from the environment and mechanical impact. This protection allows the measuring end of the probe to be inserted directly into the measurement area. Termination wires or a connector installed on the opposite end connect probes to the measuring instrument.

Low temperature range, thin-film RTD probes are typically safe to use in the -40°C to +200°C range.  Low temperature range wire-wound probes extend further into the -200°C to +200°C range. Inside the probe, the RTD element uses silver-plated copper lead wires with plastic insulation, such as Teflon® rated at 260 °C. By comparison, fiberglass insulation is usable up to 480 °C, while PVC insulated wires are limited to only 105 °C. Other considerations include:

• Welding — the preferred method of wire termination.
• Empty space inside the probe is typically packed with aluminum oxide powder that has good heat-transfer characteristics and acts as a shock and vibration absorber.
• To protect the element from moisture, the probe is sealed on the lead-wire side using epoxy or other potting compound.

High-temperature probes designed to work in the -200°C to +600°C range typically have internal lead wires made of nickel held in place with magnesium oxide insulators. Empty space inside the probe is typically packed with magnesium oxide powder, and the lead-wire end is sealed with epoxy. In both designs, the lead wires are connected in a 2-wire, 3-wire, or 4-wire configuration and are brought out with several options of insulated wires, terminated wires, and male and female connectors.

Surface, gas, and liquid measurements require different sensor configurations. Probes most suitable for liquid measurements are typically encased in a stainless steel sheath that has excellent corrosion resistance. Inconel sheaths provide superior protection against corrosion and oxidation at high temperatures. Outside diameters are typically 1/8 in., 3/16 in., ¼ in., and 3/8 in.

Surface sensors vary greatly depending on application and method of attachment. They can be touched, screwed, bolted, or glued to the measured surface. Gas and air measurements require free access of the gas to the RTD element to facilitate the heat transfer.

Certain precautions should be observed during installation and operation. RTDs, especially the wire-wound type, are susceptible to mechanical damage and should be installed with care and protected during use. In addition, minimizing mechanical and thermal stresses is essential for long service life. Electromagnetic interference can be a serious problem because the signal level is quite low. Use proper shielding and twisted-pair wires to keep electrical noise at an acceptable level. Increasing the gauge of lead wires minimizes their resistance, which is especially important for minimizing measurement error in two-wire RTD connections.

For more information:

Omega Engineering, Inc.
http://www.omega.com

Thermo Sensors Corp. (TSC)
http://www.thermosensors.com

U.S. Sensor Corp.
http://www.ussensor.com

JUMO Process Control, Inc.
http://www.jumoplus.com

Watlow Electric Manufacturing Co.
http://www.watlow.com

Pyromation, Inc.
http://www.pyromation.com

Minco
http://www.minco.com

Combination Liquid Level and Temperature Switch

A liquid level and temperature switch was designed into one assembly to reduce the cost of a fluid system. Tempco can design a custom sensor to meet many applications such as industrial and commercial degreasers and hot liquid dispensing machines.

Liquid Level Sensor Options
The liquid level sensing can be obtained using magnetic floats and switches, or the conductivity method. The conductivity method uses the simple concept of the conductive property of liquids to complete a circuit and cause an external control relay to actuate.

Temperature Sensor Options
Thermocouples, RTD’s, thermistors and thermostat switches with an assortment of accuracies can be incorporated into the assembly.

Tempco Electric Heater Corporation designs and manufactures electric heating elements,
temperature sensors, temperature controllers and turnkey process heating systems.

Tempco
www.tempco.com

Designing with Thermistors

John R. Gyorki, Editorial Director 

Temperature sensor applications usually fall into one of three general categories; monitoring, control, or circuit compensation, and four sensor types; thermocouples, thermistors, resistance-temperature detectors (RTD), and semiconductor temperature sensors. When selecting a sensor, some key characteristics to consider include temperature range, accuracy, response time, minimal temperature effect on the measured object, and the type of signal conditioning required.  Other factors are long-term stability, mechanical ruggedness, and cost. 

Unleaded NTC thermistor discs are frequently used in numerous automobile engine sensors to measure air and coolant temperature. The discs are located inside the tip of the housing, usually under a spring-load to maintain contact pressure. 

The table compares thermistor characteristics with other types of temperature sensors and shows that thermistor devices are essentially passive variable resistors and require excitation current to produce an output signal. In other words, you cannot just connect a voltmeter across the leads of a thermistor, touch the sensor to a hot object, and expect to see a voltage.   

Thermistors have a considerably higher sensitivity than most other sensors, but they are also much less linear. Although special high-temperature sensors, such as chromium oxide ceramic thermistors made by GE Sensing can operate up to 1000^oC, conventional devices have a relatively narrow temperature range and are not an optimal choice when long-term accuracy is required. However, thermistors are usually less expensive than the other sensors and react faster to temperature changes.

All sensors require linearization, but each to a different degree. Also, to achieve high accuracy the circuit must be calibrated with the actual thermistor sensor connected. These two tasks can be accomplished with analog conditioners and calibration circuits, but they can be quite complex and require manual calibration. If a digital design is used instead, the sensor signal is digitized by an analog-to-digital converter (ADC) and the linearization and calibration are done in software with 
minimum operator involvement.

Thermistor Fundamentals    
Thermistors are solid-state, temperature-sensitive resistors that come in two types: negative temperature coefficient (NTC) and positive temperature coefficient (PTC). As the names imply, the resistance of an NTC thermistor is inversely proportional to temperature, whereas the resistance of a PTC thermistor is directly proportional. The sensors’ terminal resistance changes with the temperature change of the thermistor body, which can come from ambient heat, self-heating due to excitation current, or both.

PTC thermistors are used most often for circuit-overload protection, compared to NTC devices that are used primarily for temperature measurement and compensation. This article focuses on temperature measurement devices, so only NTC thermistors are discussed.  

The resistance of an NTC thermistor decreases with an increase in its body temperature, however, the rate of resistance change is not linear. It is greatest at the lower temperature limit and gradually diminishes as the temperature increases.

NTC thermistors are a sintered mixture of metallic oxides, which include nickel, cobalt, manganese, and sometimes other oxides. The elements are formed as beads, chips, discs, rods, or thin-films. Bead thermistors are drops of semiconductor paste deposited on two platinum alloy wires, sintered at a high temperature. The wires are then cut to make individual thermistors. Chip and disc thermistors are fabricated as a thin sheet of material (wafer), and sintered at high temperature. The sides are silvered for attaching leads, and the wafers are cut into discs or chips. Rod thermistors are simply extruded.

Thermistor elements can be glass encapsulated, epoxy coated, or remain uncoated (bare). Bare thermistors respond faster, are smaller, and cost less, but they have no provisions for protection from the environment and mechanical impact. An epoxy coating can protect the device from the environment, but it slightly slows the response time and increases the cost.  Glass encapsulation ensures a hermetic seal, high-voltage insulation, and resistance to corrosive atmospheres. Long-term stability of glass encapsulated parts is typically ten times better than the stability of epoxy coated parts.

Mounting features include unleaded discs that require spring-loaded contacts, silver or gold electrodes for wire bonding, and surface mounting provisions such as those for SMD chips.  The leads can be axial or radial, bare or insulated, and straight or kinked. Axial lead and SMD parts are intended for automatic PCB insertion and pick-and-place equipment. Radial-lead devices and unleaded discs are well suited for temperature probe assemblies.

An unleaded NTC disc thermistor (a.) is commonly found in temperature probes. Adding radial leads to an uncoated disc thermistor (b.) lets it mount on a printed circuit board. Coating the disc thermistor with epoxy (c.) protects it from the environment. Epoxy-coated chip thermistors with flexible insulated leads are ideal for installations with limited space.
(Photos courtesy of GE Sensing & Inspection Technologies, Billerica, MA.)

One special type, thin-film thermistors, are deposited on a ceramic or flexible Kapton® base, only several tens of thousands of an inch thick. They have low dissipation values and fast reaction times due to their small mass. For example, TF series of thin-film NTC thermistors from Selco Products Company, are suitable for a -50^oC to +90^oC temperature range and have a dissipation value of 0.7 mW/^oC with a thermal time constant of 2 s, both in still air. They are ideal for air and other gas temperature measurements as well as probe assemblies.

Accuracy ratings vary greatly between different devices, depending on the application. For example, general-purpose disc thermistors typically have tolerances that range from ± 20% to ± 2%, and interchangeable thermistors can have accuracies as high as ± 0.05^oC in a narrow temperature range. They are available for probe replacement without system recalibration. For example, U.S. Sensor’s PR103J2 ultra-precision, interchangeable 10-kΩ thermistor is a highly accurate and stable sensor that matches the J-type NTC thermistor’s R-T curve with ± 0.05^oC accuracy from 0^oC to 50^oC. Other resistance values from 2 kΩ to 50 kΩ are also available.

Several thermistor-related terms that are listed in catalogs and data sheets can help you select parts:
• The zero-power resistance, R_o, is a dc resistance specified at a particular temperature and an excitation current so small that the self-heating produced by power dissipation can be neglected. This special temperature is called the Standard Reference Temperature, and is typically 25^oC. 
• The resistance ratio characteristic is a ratio of zero-power resistance measurements made at two specific temperatures. It is typically the ratio of resistance at 25^oC to the resistance at 125^oC.
• The thermal time constant, τ, is the time in seconds required for a thermistor that dissipates zero power to change its body temperature 63.2% of the total temperature change in response to a step-function change in ambient temperature. This parameter characterizes the speed with which a thermistor can react to fast temperature changes and helps compare the response time of different devices. 
• The dissipation constant, δ, is a ratio of the change in thermistor power dissipation to the change of thermistor body temperature. It is measured in mW/ ^oC and is specified at a certain temperature. Both τ and δ depend strongly on the measured object or media.  For example, the dissipation constant of a GE type DC95 interchangeable chip thermistor is 8 mW/^oC in stirred oil, but is only 1 mW/^oC in still air. The thermal time constant is 1 second in stirred oil, but is ten times longer in still air.
• The maximum power rating is another characteristic related to power dissipation. It is the maximum power in mW at an ambient temperature of 25^oC that a thermistor can dissipate for an extended period of time without degrading its characteristics. This rating must be derated based on the ambient temperature.
• The zero-power temperature coefficient of resistance (TCR), α, is the ratio of the rate of change of zero-power resistance at any temperature point, T, to the zero-power resistance at that point: 

α_T = 1/R_T (dR_T)/(dT)

Where:
α_T = temperature coefficient of resistance at temperature T, 
Ω / Ω / ^oC, or %/ ^oC
R_T = resistance at temperature T, Ω
dR_T = change of resistance, Ω
dT = change of temperature, oC

Another way to express the temperature coefficient is:

α_T = – B/T²

Where:

B = material constant, ^oK
T = temperature, ^oK

Unfortunately, thermistor temperature coefficients are highly non-linear over their operating range, which means that the coefficient itself varies somewhat with temperature. A coefficient is at its highest value at its lowest temperature limit and gradually decreases as temperature increases. One value of a particular coefficient might work for a narrow temperature range, but most often, thermistor measurement circuits must be linearized to cover large temperature swings.

By simply adding one resistor in series with the thermistor, the output voltage vs. temperature curve can be linearized. When resistance vs. temperature linearization is desired, the resistor should be connected in parallel with the thermistor.

Circuits for linearizing thermistor outputs can be comprised of series, parallel, and series-parallel combinations of fixed resistors and additional thermistors. The simplest circuit is a parallel resistor, the value of which can be calculated from the following equation:

R = [R_TM(R_TL + R_TH) – 2R_TLR_TH] / [R_TL + R_TH – 2R_TM]

Where:

R = value of parallel resistor, Ω
R_TL = thermistor resistance at the lowest temperature T_L, ?
R_TH = thermistor resistance at the highest temperature T_H, Ω
R_TM = thermistor resistance at the midpoint temperature T_M, Ω 
Midpoint temperature TM = (TL + TH) / 2, °C

Simple on/off temperature control circuits and applications with a narrow temperature range and relaxed accuracy requirements usually do not need linearization. A simple Wheatstone bridge circuit is usually quite adequate. Another example that does not require hardware linearization is a digital temperature circuit where the linearization is handled in software. 

A simple on/off temperature control circuit can be designed using a thermistor in one leg of a Wheatstone bridge. Resistors R1, R2, and R3 must have a low temperature coefficient and be exactly matched to guarantee accuracy.

Operating Conditions
Certain operating conditions can significantly lower measurement accuracy or reliability and should be avoided. For instance, self-heating might become a hidden accuracy error. Thermistors generate their own heat when their excitation current is too high. The power it develops from the excitation current and its own resistance (P = I²R) can noticeably elevate the temperature of the thermistor body above the environment. Parts with a large dissipation constant, d, a low thermal resistance mounting, and other means of superior heat dissipation will have a lower temperature rise. But the primary way to avoid excessive self-heating is to keep the excitation current as low as possible.

Most measurement errors and premature failures often come from careless installation and operation. For example, although thermistors are considered to be rugged devices, take care not to crack a case, separate a bond, or exceed the upper temperature limit.

Lastly, aging is a phenomenon that is often overlooked and if not considered in the maintenance schedule, can lead to loss of calibration accuracy after extended periods of use. It manifests as an effective thermistor terminal resistance drift over time due to slowly changing resistances in the bulk material and in the contact areas between the leads and the thermistor material. 

For more information: 
Contact John Gyorki at the Engineering Exchange, 
www.engineeringexchange.com

www.omega.com
www.gesensing.com
industrial.panasonic.com
www.murata.com
www.ussensor.com
www.vishay.com
www.selcoproducts.com
www.thermosensors.com
www.jumoplus.com

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