by Matt Burns, Technical Marketing Director, Sensors, Avnet
There are multiple ways for engineers to measure and keep track of liquids in a container, and one will fit your application.
Anyone who lives near a river or creek knows the anxiety that can happen during a thunderstorm, when rising water levels can cause high stress levels. A common sight in flood-prone areas are high water marks. Used since at least the time of the Egyptians, high water marks help residents take action for their safety. They are also used by planners to place new developments above the flood plain.
Whether man-made or the physical impression left from a flood, high water marks were among the first “sensing” solutions for monitoring the level of liquid. Another common technique is visual, passive water level gages. Placed at known locations where the water depth is known, it visually tells people the level of water in that particular lake or river.
But what solutions are available for liquids in a container? Typical industries requiring liquid level sensing in a container range from automotive and oil and gas to food processing and pharmaceutical. Given the various types of liquids that need measured, we’ll review basic liquid level sensing techniques to help engineers pick the best solution for their application, along with a quick overview of some of the latest products to hit the market.
Floats work exactly how they sound. A device buoyant in the liquid level under test will float on top of the liquid. As the liquid goes up and down, the float interacts with electronics—typically a series of resistors or reed switches embedded in the float stem—to provide liquid levels changes in discrete steps. Floats can be a single-point application, like a switch, or a multi-point system, providing many liquid levels.
While floats are easily implemented and fairly common, they cannot discriminate level values between steps. Additionally, floats have mechanical limitations when the shape of the tank is irregular (something other than a cylinder or rectangle), so they are not suitable for all container shapes.
A load cell can be used to measure changes in liquid level by measuring the change in force applied by the liquid on the container. To measure level, the load cell must be incorporated into a container’s overall structure. As the liquid fills the container, the force on the load cell increases. Combining the container’s cross-sectional area with the liquid’s specific gravity provides a calculated liquid level based on the load cell’s output.
One challenge to using load cells are the mechanical system design considerations. The mechanical support structure of the container must be designed to fit the specifications of the load cell. The container and its support structure must also be weighed by the load cell while empty and full for proper calibration. If these steps are feasible, load cells are an attractive option for sensing levels of many liquids—especially corrosive liquid found in industrial and process control applications—because they require no direct liquid contact.
Another approach to sensing liquids leverages the conductive property of liquids to mimic a variable capacitor. Customized electrodes are embedded either inside of or on the side of a container. Typically, they are placed parallel to each other. One electrode is tied to electrical ground. The other electrode—the level electrode—forms the second plate of the capacitor. The electrodes are excited by external circuitry. They are then calibrated to detect the minimum liquid level height which corresponds to a minimum capacitance between the electrodes. As the liquid level rises (or falls), the changing capacitance can be measured and conditioned to produce the appropriate output for the system.
One option for capacitive liquid level sensing is Molex’s capacitive fluid level sensors, which mount to the outside of a container and measure capacitance through almost any non-metallic material. They can be customized and optimized for a range of applications. A design can use a traditional printed circuit board for a flat surface, or a thin, flexible circuit to accommodate curved surfaces or space-constrained applications. The sensors include customized embedded software which can be configured for auto-calibration for easy installation, or manual calibration to maximize accuracy.
Ultrasonic sensors measure liquid levels using time of flight (ToF) principles. The ultrasonic transducer emits an ultrasound pulse—usually in the tens of kilohertz frequency ranges. The ultrasonic transducer then “listens” for the reflected pulse off of the liquid surface. Because the speed of sound in air at specific temperatures and gas mixtures is known, measuring the travel time of the ultrasound pulse from a transducer to the liquid surface and back provides calculated liquid level measurement.
In this arrangement, the ultrasonic transducer would sit at the top of the container and use air as the transmitting medium. As an example, Murata makes a series of waterproof ultrasonic sensors featuring a hermetically sealed structure in which the piezoelectric ceramics are attached to the metal case and the case opening is filled with resin. This structure protects the sensor from water droplets in level sensing applications.
A new approach to ultrasonic liquid level sensing has been released by Texas Instruments. Instead of using air as the medium, the TI solution uses the liquid as the transmitting medium by placing the ultrasonic transducer at the bottom of the container. This allows ultrasonic liquid level sensing to be used with many applications, especially with corrosive liquids that would harm a sensor inside the container. Another advantage is that the speed of sound in various liquids is readily available in the public domain.
TI’s newly released TDC1011 ultrasonic analog front end (AFE) makes ToF measurements simple by exciting the transducer and receiving the echo. The TDC1011 creates a start and stop pulse which can be timed by the system microcontroller unit, or MCU. This acts like a stopwatch to measure ToF and achieve 1 mm height accuracy.
Laser liquid level measurement uses ToF principles similar to ultrasonic liquid level measurement previously mentioned. A laser mounted at the top of the container emits a laser beam toward the liquid. It then measures the remnants of the laser reflected off the liquid. The travel time of the laser is then measured. The liquid level in the container can be calculated as one-half of the measured ToF multiplied by the speed of the laser beam.
Lasers are often used in non-transparent or opaque liquids that provide a clear reflection of the laser beam. Because lasers have a small beam pattern, they can be targeted in narrow spaces, such as down a metal tube of a chute in an irregular container. One challenge with lasers is that constant maintenance is required. Dust, dirt and other particles coating the laser transmitter or receiver can cause signal degradation. Foam build-up for certain liquids can also “fool” the laser, so constant system maintenance is required.
The types of liquid level sensors continue to proliferate. Continuous level sensing techniques (capacitive, ultrasonic, laser and so on) are becoming more popular. As support electronics increase in functionality and decrease in cost, active liquid level sensing is becoming more commonplace.