Using conductive electrode as input

kallileo · Apr 2, 2012

There is a milk cooling tank where a conductive electrode used as water level switch is connected to a PLC Digital Input.
The electrode grounds the PLC input when the water level reaches the electrode to activate the water circulation pump.
The problem is that having the PLC input grounded to the metal body of the tank doesn't seem like a good idea to me.
I have seen in many cases some SMDs located on the PCB near the DI where destroyed.
The design of this system has a flaw.
I believe the source of problem is this electrode and the grounding damages the DI.

Is correct to use this electrode this way?

I have attached photo of the electrode.

BobB · Apr 2, 2012

The device should have a controller with a digital (relay) output probably.
I dislike conductive probes but this one may be OK in milk with a proper controller.
I also do not like grounding the PLC input power supply at all - always let it float.

Mark Snodgrass · Apr 2, 2012

Here is a link to the type of sensor we use in our boilers. It isolates the probe circuit from the plc side.

http://www.gemssensors.com/en/Produ...d-Liquid-Level-Control/Relays/Model-DC-Series

Good luck

nonuke · Apr 2, 2012

We always use BobB solution. Everytime we have conductive probes for low/high level or both, we always use a level probe relay from Carlo Gavazzi: CLP2ET1CM24

shooter · Apr 2, 2012

It is illegal use As you may not use any electrics in foodstuff, so conduction of the milk is a NONO.

Doug_Adam · Apr 2, 2012

You could run the conductive probe through a level sensing relay. Then it is isolated from the PLC.

nehpets · Apr 3, 2012

You could do away with level sensing or conductive probes and use load cells under the tank legs milk is approx. 1.06Kg to a Litre

just a thought

Steve

iant · Apr 3, 2012

there are always alternative non contact - Capacitive - ultrasonic.

the actual tank cleaning process will cause damage so the sensor will not work for long, with any conductive probe.
- Too hughly caustic.

@BobB - i have seen many M/C's with the 0Vdc rail tied to Ground.
I see your point but there has been very little problems.

I concure that nothing should touch food products especially milk
(excelent Growth media)

kallileo · Apr 4, 2012

The probes are located outside the tank in the automated washing system and never come in contact with milk.
The probes just measure the level of water that will be used to clean the tank and activate the water circulation pump to start the cleaning of the vessel.
The chemicals used in the cleaning process are not going to damage the probes the are made of stainless steel 304 (used in food industry) because they are mixed with water.

I have attached a photo with the damaged DIs on the PLC.

iant · Apr 4, 2012

that is a sign of a high voltage other than DC
I would suggest the point Bobb made is the cause
an earth leakage fault from some where else has fed back through the water to the nearest ground point.
i.e. the PLC input to the grounded 0V

it is time to use a different method

run the electrode to a relay coil tie the other coil terminal to a 'fused' 24V+
then use the relay contact to switch the input
(isolate it)

iant · Apr 5, 2012

I have figured out what caused this blow out.
it would have been someone welding on the machine,
with a poorly connected welding electrode.

Using conductive electrode as input

kallileo

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