What are Level Measurement Sensors?
An electronic device that measures the height of material-typically liquid-inside a tank or other vessel.
Integral to process control in many industries, level measurement sensors fall into two main types. Point level measurement sensors are used to mark a single discrete liquid height-a preset level condition. Generally, this type of sensor functions as a high alarm, signaling an overfill condition, or as a marker for a low alarm condition. Continuous level sensors are more sophisticated and can provide level monitoring of an entire system. They measure fluid level within a range, rather than at a one point, producing an analog output that directly correlates to the level in the vessel. To create a level management system, the output signal is linked to a process control loop and to a visual indicator.
Level Measurement Sensor Selection
Key questions to ask before selecting a level measurement sensor:
Are you measuring a liquid or solid?
What are the application's temperature and pressure ranges?
Is point level or continuous measurement required?
What level measurement range do you need?
Is the measured material electrically conductive?
Will the material coat or build up on surfaces?
Does turbulence, foam, or vapor occur at the surface of the liquid?
Will you need contact or non-contact level measurement?
What kind of output do you need-analog, relay, digital display, etc.?
Level Sensing Technologies:
In these point level sensors, a magnetic float moves with the liquid surface, actuating a hermetically sealed "reed switch" in the stem. The simple, low-maintenance design installs readily; minimizes shock, vibration, and pressure; and works with a variety of media. The reed switch can be single pole, single throw (SPST) or single pole, double throw (SPDT).
Non-Contact Ultrasonic Sensors
These sensors incorporate an analog signal processor, a microprocessor, binary coded decimal (BCD) range switches, and an output driver circuit. Transmit pulses and a gate signal from the microprocessor route through the analog signal processor to the sensor, which sends an ultrasonic beam to the liquid surface. The sensor detects the echo from the surface and routes it back to the microprocessor for a digital representation of the distance between the sensor and the surface level. Through constant updating of received signals, the microprocessor calculates averaged values to measure liquid level.
Contact Ultrasonic Sensors
A low-energy ultrasonic device within these sensors measures liquid level at a certain point. Consisting of a field-mounted sensor and integral solid state amplifier, contact ultrasonic sensors have no moving parts and require no calibration. Typically, they are equipped with terminal blocks for connection of a power source and external control devices. The ultrasonic signal crosses a one-half inch gap in the sensor, controlling relay switches when the gap contains liquid. The sensing level is midway along the gap for horizontally mounted sensors, at the top of the gap for vertically mounted sensors. As liquid falls below this level, the ultrasonic signal attenuates and ultimately switches the relay to its prior state.
Capacitance Level Sensors
Like ultrasonic sensors, capacitance sensors can handle point or continuous level measurement. They use a probe to monitor liquid level changes in the tank, electronically conditioning the output to capacitive and resistive values, which are converted to analog signals. The probe and the vessel wall equate to two plates of a capacitor, the liquid to the dielectric medium. Because the signal emanates from level changes alone, material build-up on the probe has no effect. Non-conductive fluid vessels may dictate dual probes or an external conducting strip.