Conductivity level switches
They do so by sending a small current through their probe tip. Current flowing between the sensor and some reference (often the metal tank holding the liquid) is compared inside the sensor to current through reference resistors to determine whether or not liquid is present.
Modern types contain electronics that generate an alternating voltage that is present on a stainless-steel tip that touches the liquid. The presence of an electrically conductive liquid completes the circuit which, in turn, changes the condition of the transistor output. Output options can be used to actuate relays, indicator lights, or interface with controllers.
Conductivity sensors can work in either single-point or differential mode. In single-point mode, sensor current (usually about 1.5 mA at 12 V) flows between the sensor and a metal tank (or to an additional sensor when the tank is nonmetallic) if there is liquid present. Differential mode is often used when filling or emptying tanks and employs sensors at the top and bottom. The metal tank wall generally serves as a reference. The sensors connect to a latching relay which controls filling or emptying operations.
Modern devices send a 50 or 60-Hz signal through the liquid to thwart erosion of the probe from electrolysis. Any metal from the probe tip that erodes in the first half cycle is replaced in the second half cycle so there is little material loss. Some sensors use dc signals and may be more prone to loss of probe material.
The operating point of these sensors depends somewhat on the resistivity of the fluid. And fluid resistivity can depend on temperature. Thus designers need to know details about how fluid resistivity behaves in the environment the switch will see. The typical fluid in which conductance sensors are applied is tap water with a resistivity of about 3 k/cm. For comparison, condensate can have a resistivity of 18 k/cm and distilled water will be 450 k/cm. The sensitivity of the switch must be above the resistivity of the fluid sensed, but not so high that false triggering (as, say, from condensate) is a problem.
One of the largest applications for these devices is in the control of boilers for generating steam. Thus the sensors tend to have high operating temperatures (above 250°F). For the same reason they also are constructed to withstand significant pressures (2,500 psi typical).
Gems Sensors provided information for this article (gemssensors.com).