Conductive devices for monitoring liquid level and opening and closing switch contacts at predetermined liquid level thresholds.
A conductive liquid level switch is an instrument for monitoring the height of a liquid and tripping a relay contact switch or transmitting solid state relay switch output when a maximum, minimum or intermediate level has been reached. The liquid level switch output can then be utilised by other instrumentation to open a valve, illuminate a warning lamp, activate an audible alarm, or switch on a pump.
Conductive liquid level switches are threshold contact trip devices which typically consist of one or two metal rods extending out from the switch probe housing.
One of the metal rods has a custom length to reach the required liquid height.
The other metal rod extends down to a lower position, or alternatively the outer wall of a metal tank is used to complete the circuit.
When the liquid touches the custom length metal rod, the circuit is closed between the two electrodes.
Both electrodes are wired to an electrode controller module which maybe fitted within the probe housing or connected remotely. The controller will monitor the two electrodes and when the resistance changes, due to the liquid touching both electrodes, it will activate the switch contacts.
Additional liquid level switching thresholds can be achieved by incorporating more metals rods into the switch assembly, each one providing an additional level switching channel.
A conductive liquid level switch probe makes direct contact with the liquid to be measured, and therefore material compatibility has to be a consideration, but for most applications a stainless construction is more than adequate for most fluids.
Unlike capacitive type liquid level sensing probes, the conductive type works best with conductive liquid media.
Since the conductive level switch probe is essentially a stainless steel rod fitted to a tank, it is possible to construct the device to withstand high pressures so that it can be used on sealed pressurised tanks.
Also, since there is no sensitive components immersed in the fluid it is possible to use them on higher temperature media that would not be tolerated by other types of liquid level switch.
In comparison to other liquid level switch technology the conductive type is much easier to produce, and is often a much more economic option.
Conductive liquid level sensing probes are generally mounted through the top of a tank with the probe protruding into the tank.
- Inexpensive product design, lowest cost type of liquid level switch
- Multiple switch points can be incorporated into one device
- No moving parts requiring regular inspection and maintenance, or leading to wear and tear
- Simple mechanical construction
- Works well with conductive media
- High temperature fluid compatibility, since there are no sensitive components making contact with the fluid
- Sealed pressurised or evacuated vessels will not affect performance
- SG & density insensitive
- Some types may produce sparks when probe is in proximity to fluid
- Deterioration of probe tips due to residue, electrolytic effect or chemical corrosion leading to pitting, plating, or fouling
- Thick or sticky media will leave a coating on the probe creating an insulation layer causing malfunction
- Requires a higher voltage level across probes when used with less conductive liquids
- Probe protruding into liquid causing obstruction, attracting fouling and hygienic cleanliness issues