I have a liquid level sensor with 3 probes, one us common, the other is for low level and the third is for high level. On the liquid level control relay diagram, I've noticed that the common probe is grounded. Why is this so?

Any information would really be appreciated. enter image description hereenter image description here

  • 2
    \$\begingroup\$ Please edit to add a link to the datasheet. \$\endgroup\$
    – Transistor
    Jun 11, 2019 at 6:13
  • \$\begingroup\$ I have added wiring diagrams \$\endgroup\$
    – Ph3ng
    Jun 11, 2019 at 15:42
  • \$\begingroup\$ @Ph3ng Thanks for the diagrams, but that might be not enough to answer your question. Please post a link to the datasheet or the manual for the sensor. (These diagrams came from somewhere.) \$\endgroup\$ Jun 19, 2019 at 20:49

1 Answer 1


The datasheet doesn't explain it but does say

Level sensor probe volts 5 VAC @ 1.5 mA.

The AC part is to be expected as DC would cause plating of the probes due to electrolysis. 5 V @ 1.5 mA suggests a source impedance of 5 / 1.5m = 3k3.

It then goes on to say

Adjustable sensitivity: 0 - 50 kV

which doesn't make any sense to me so I'd be interested in learning what this means - if anything.

Meanwhile, back at the earth termination:

  • The diagram is suggesting to me that the tank must be earthed (obviously relevant only if metallic) and that the probe should be connected to the tank. I suspect that the tank connection avoids problems with people trying to use painted, galvanized, corroded or limescaled tanks as the common electrode. The diagram on the controller contradicts this by showing only a ground connection to the tank. Who knows?
  • The datasheet says that the output is isolated from the supply. Ground reference will prevent the output floating and this may be significant to the sensing circuit to avoid stray triggering.

A further confusion with the diagram is that the common probe is higher than the low-level probe. That means that with a plastic tank the low-level indication will be given when the common probe is dry rather than when the low-level probe is dry. All-in-all, it's not very satisfactory documentation.

Note that the supplier has been inconsistent


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