Level Sensors
There are a number of sensor options for automatically sensing liquid levels. Below we layout the fundamentals of each.
Submersible Pressure Level Sensor
A cabled sensor is immersed and sits at the bottom of the liquid. An internal diaphragm in the sensor is deformed in proportion to the pressure of liquid above.
Type
Pressure sensor
Accuracy
Fair. 10-20mm for 5m tank.
Suitable For
Most liquids including water, fuels, chemicals
Pros
Inexpensive, easy installation
Cons
Contacts liquid, has a cable, may require calibration for non standard liquid densities
Ultrasonic Sensor
Emit a sound beam which reflects off the liquid surface then measures the time taken for the echo to return to the sensor.
Type
Distance sensor
Accuracy
Excellent. 1-2mm for 5m probe
Suitable For
Most liquids, non enclosed tanks/water bodies, liquid surface must be clear of obstruction
Pros
Non contact, compact, low power, accurate, does not need calibration, no moving parts
Cons
More fiddly installation than pressure sensors, susceptible to condensation on the sensor surface in enclosed tanks, minimum distance from sensor to liquid
Radar Sensor
Emit a microwave beam which is reflected off the liquid surface subtly changing the frequency and this is used to measure the distance between the sensor and liquid surface.
Type
Distance Sensor
Accuracy
Excellent. 1mm for 5m probe
Suitable For
All liquids, all tank configurations, can handle surface foam and particulates in air
Pros
Non contact, accurate, insensitive to condensation or buildup on sensor, insensitive to liquid surface foam, no minimum sensor distance, can be installed near tank walls, no moving parts
Cons
Expensive, more power required compared to other sensor types, more fiddly installation than pressure sensors
Pressure Transducer
Works the same as the submersible level pressure sensor but installed into a liquid pressure pipe line after the tank offtake / shut off valve and not in the tank.
Type
Pressure Sensor
Accuracy
Fair, 10-20mm for 5m tank.
Suitable For
Most liquids including fuels, chemicals, pressure pipe installation only (tank offtakes)
Pros
Inexpensive, no cables in liquid, insensitive to condensation/liquid foam/turbulence/tank obstructions, no minimum sensor distance, not installed on top of tank
Cons
Requires access to tank pipe offtake