Capacitance level probes

[coldrush]

hi guys, does anyone know if contact with a metal (earthed stainless steel) will affect the operation of capacitance level probe?? i currently have a malfunctioning level probe and it is in permanent contact with metal strainer.

 

[Leftyretro]

Well it's always best to refer to the OEM manual and it's installation section. However the capacitance level probes I've used in the past have always been insulated from ground and should not contact any grounded material inside the vessel. I have worked with models that used two parallel probes that change capacitance as the liquid rises between the probes and changes dielectric seen by the twin probes. That kind didn't care if the vessel shell was grounded or not or even if the vessel was of a non-conducting material. Other types use a single probe insulated from ground (the vessel shell) but still see a change of capacitance between the probe and the shell as the liquid level rises and starts to cover the probe. That kind did require a grounded conducting vessel material.

By the way for those not familiar with capacitance probes they work by sensing a liquid level in contact with it's probe, changing it's capacitance depending on how much of the probe is in contact with the liquid. The electronics in the probe sensor is basically an oscillator that changes frequency as the probe changes it's capacitance. The electronics then uses a frequency to DC voltage converter to convert to a DC analog voltage change corresponding to level changes. In many industrial models this is then converted to a 4-20MA loop powered measurement circuit back to the control system.

There are 'point' type capacitance probes that can be mounted horizontally into the vessel that just respond to being in or out of liquid and are used as a on/off alarm signal rather then a continuous level measurement.

 

[coldrush]

thank you very kindly Lefty, i appreciate the help. the probe im using is a fork/prong type, it gives hi or lo signal to the plc depending on wether there is water in the sump tank.
i find your response very helpful in that i have always wondered how the fork type works seeing that the two forks are joined together i.e (a capacitor needs two plates and a di-electric to store charge). thanx to you i have just learned that the forks and earth function as the plates and the water as di-electric....