Non-Contact Photoelectric Level Sensors

Southington, Conn. – For a range of liquid level detection applications in laboratory automation, medical devices, pharmaceutical packaging, beverage processing and other industries, Baumer has introduced the FFDK 16 Photoelectric Level Sensors, compact sensors designed to be mounted onto transparent or half-transparent standpipes from 3 to 13 mm in diameter.

These easy to mount, non-contact sensors feature integrated electronics and require no adjustments to be made post-installation for accurate detection of fluid presence. A dark/light switch allows the user to select the appropriate setting for their specific application. These rugged sensors are made with PFI for chemical resistance.

Foam can cause measurement errors in standard optical level sensors. To prevent this problem, the optical version of the FFDK 16 sensor features fibers that are arranged in rows, which allow the sensor to suppress foam and air bubbles up to 3 mm in size.

Baumer
www.baumerelectric.com/usa

SICK’s W15 Photoelectric Sensors

Minneapolis, MN – SICK announces the launch of its W15 Photoelectric Sensors. These high-performance sensors use SICK’s third generation custom ASIC (application-specific integrated circuit) chip that incorporates OES3 technology to provide exceptional background suppression at an extended range. OES3 technology enables the W15 to ignore stray background reflections, detect multi-colored/shiny objects without false trips, and provide high immunity to ambient light.

The W15 provides flush mounting that reduces setup time and prevents the disruption of product flow. All W15s are assembled in the U.S. so customization options, such as cable cut to length, cable wrap with customer-specific part number, special connectivity, sensor pre-mounted to a bracket, kitted together with bracket and reflector, can help reduce material and labor costs. The W15’s advanced background suppression technology makes it ideal for a variety of applications in the packaging, material handling, and food and beverage markets.

SICK
www.sickusa.com

Energy West LLC Announces Representation of Janitza electronics GmbH in North America

Huntington Beach, CA – Energy West LLC announces that they have reached an agreement with Janitza electronics GmbH to represent their line of products, specializing in the measurement and management of energy.

Janitza electronics GmbH concentrates on the development and production of efficient systems for energy measurement, power quality, and cost savings. As a leader in the European market, Janitza has grown internationally as a producer of universal measuring devices, energy management systems, and power quality solutions.

With this partnership, Energy West LLC will be able to offer additional value added solutions to provide to the end user. This partnership allows Janitza to enter the North American market, while Energy West LLC enables their continued growth and international presence.

“We are extremely pleased with this latest agreement,” explained Dan Lent, Vice President of Energy West LLC. “This addition to our solutions portfolio, along with Janitza’s experience and German technology, opens additional markets for us. I truly believe that Janitza will become one of Energy West LLC anchor lines of application solutions in addition to contributing to Janitza’s successes.”

Energy West LLC, can immediately provide sales and service solutions for the Janitza line of products in the US and Mexico. For additional information on Energy West LLC or Janitza electronics GmbH, please contact Chris Sheetz, Director of Operations, Energy West LLC.

Energy West LLC
www.nrgwest.com

MicroStrain® Wins Silver and Gold at Sensors Expo

Williston, VT – MicroStrain won two more “Best of Sensors Expo” awards at Sensors Expo in Rosemont, IL. These Gold and Silver awards bring MicroStrain’s total to ten Gold awards and two Silver awards, accumulated over the last eight years.

^{Steve Arms, President of MicroStrain (right) and Mike Robinson, VP Sales and Marketing accept awards}

This year’s gold award was for HS-Link™ three channel High Speed Wireless Node with sample rates up to 100 kHz on all three channels. During user definable sampling sessions, HS-LINK™ stores bursts of data in a buffer; once sampling is complete data are then transferred to non-volatile flash memory. Each sensor channel has a dedicated 16 bit analog to digital converter (A/D) enabling the three distinct sensor inputs to be sampled simultaneously. HS-LINK™ also features a precision timekeeper, which can receive a high priority timing beacon, which enables multiple HS-Links™ in a star network to achieve a node-to-node data sampling synchronization of ±4 microseconds.

The silver award was for 3DM-GX3™-25, Miniature Attitude Heading Reference System. 3DM-GX3™-25 is amongst the smallest and lightest AHRS on the market today, with versions weighing as little as 11.5 grams. Improved performance in the face of ambient vibrations and oscillations is achieved by oversampling at 30 KHz and then digitally filtering and performing coning and sculling integrals at 1 kHz. Oversampling also greatly improves the resolution of the sensor readings. User adjustable output rates of up to 1000 Hz make the 3DM-GX3™-25 AHRS one of the fastest attitude heading reference systems available today. Each 3DM-GX3™-25 is individually calibrated to compensate for gyro g-sensitivity and sensor misalignment and includes routines for hard and soft iron field calibrations. Full temperature compensation ensures performance over a wide operating temperature range.

MicroStrain
www.microstrain.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

Sensors for Clean-in-place Applications

U-GAGE® M25U Ultrasonic Sensors specifically suit sanitary environments. The sensors, rated IP69K, IP67 (NEMA 6), are constructed of heavy-duty 316 stainless steel to withstand high-pressure washdowns, severe temperatures, and aggressive cleaning chemicals common in food and beverage applications.

The smooth barrel housing enables thorough cleanup with minimal effort. The units also include IP68-rated washdown cordsets and FDA compliant brackets. M25U sensors can be wired for either normal or high speed. Normal speed offers a longer sensing range, while high speed provides a shorter response time, ideal for high-speed counting applications.

Banner Engineering Corp.
www.bannerengineering.com

Long Range Diffuse Photoelectric Sensor

Buffalo Grove, IL – Carlo Gavazzi launches a new long range diffuse photoelectric sensor with background suppression technology. The new PD112 will detect black objects at distances up to 2 meters, and white and grey objects at up to 2.5 meters. Using triangulation technology, the PD112 can be easily and precisely adjusted, with a 28 turn potentiometer. The user can fine tune the sensor to detect the target at the desired distance, while completely ignoring reflective objects only millimeters beyond.

The PD112 is packed with features that make it simple to use, and highly versatile. Each model has both NPN and PNP output, which is automatically detected by the sensor when wired. Normally open or normally closed output can be selected via a dip switch. Two adjustable timers (1 – 16 seconds) for ON and OFF delay allow for a customized solution.

A truly innovative feature of the PD112 is the mode operation selection. The sensor can be operated in a traditional industrial mode, which is ideal for applications such as material handling, packaging and wrapping, and wood processing. By making a simple dip switch selection, the PD112 can also be operated in a door mode, which automatically optimizes the performance of the sensor for detecting people and objects around automatic doors. This mode allows the sensor to be remotely tested by a door controller, making the sensor compliant with door industry safety standards.

Carlo Gavazzi
www.GavazziOnline.com

Extended Range Inductive Proximity Sensors

TWINSBURG, OH – Pepperl+Fuchs introduces X-Series Pile Driver™ Extended Range Inductive Sensors. These robust sensors are available in 12mm, 18mm and 30mm diameters to deliver sensing ranges of 5mm, 10mm or 15mm – up to 2.5 times longer range than traditional inductive sensors. 100% stainless steel housings deliver durability more than 20 times that of the competition’s brass and plastic housings. Black Armor™ coated weld-immune models are also available.

X-Series Pile Driver’s extended sensing field is a big benefit in automotive manufacturing applications, as it enables reliable detection of irregularly shaped contours, and increased sensor-to-target distances help eliminate contact-related scuffing of “Class A” hood, door and trunk surfaces. Weld-immune models are uniquely capable of providing long range part detection with full immunity to both AC and DC weld fields, and repelling weld splatter.

X-Series Pile Driver’s can detect all metals. Advanced circuitry enables restriction-free mounting while delivering the highest available immunity to the effects of industrial noise and temperature extremes. Unlike traditional inductive sensors that are limited to IP67/68 protection, X-Series Pile Driver sensors are IP69k-rated to withstand high pressure washdown cleaning.

Pepperl+Fuchs
www.pepperl-fuchs.com

Dual Axis, Gravity-Referenced Servo Inclinometers

Sherborne Sensors Limited has announced the North American debut of its Series T233/5, a family of compact, dual axis (x and y), gravity referenced servo inclinometers, designed for use in aggressive military, commercial and industrial applications where a high level of accuracy and resolution is required.

Series T233/5 offers a ± 15 VDC input voltage, and ± 5 VDC output signal, with self-test on both axes. Units are capable to a resolution of 0.1 arc second. Each axis is fully conditioned, with total electrical isolation between axes. Models are silicone oil and electric damped, and available in ranges from ±1° to ±90°. Series T233/5 offers either solder pin leads or a connector as standard, and delivers a complete, cost effective, high-accuracy measurement system.

Sherborne Sensors
www.sherbornesensors.com