Level Sensors

Level may be measured by a wide variety of level devices. Probably the most common principle used today is still the measurement of product backpressure. If a clean liquid level is measured the backpressure measurement is usually made by a standard pressure or differential pressure transmitter. It is important to know if the product being measured is under pressure or at atmospheric pressure. It is also important to know the density of the product being measured. If the product density is changed, the level transmitter must be recalibrated.

When product viscosity is high or the product tends to plug or coat, a diaphragm level transmitter, a purge system, a capacitance probe, or ultrasonic level sensor or other types of sensors must be used to reduce maintenance or operating problems. Each type of level device has its good and bad features. An attempt will be made to cover most of the more common sensors and discuss ways to troubleshoot and maintain them.

There are quite a few types of electronic level sensors. A rough listing includes the following:

1. Capacitance

2. Radio frequency

3. Electromechanical

4. Float/displacer

5. Load cells

6. Microwave

7. Nuclear

8. Optical/laser

9. Resistance

10. Sonic/ultrasonic

Level can usually be checked visually, in open tanks, and inferentially, in closed tanks. Before removing a level device for calibration, try to check its reading at both a high level and a low level. Establish the true zero and make every effort to establish the 100% level indication to be the actual tank or vessel overflow...