ESD problem on water tank?

Question

We have a water level monitor attached to a water tank. The water monitor has an hydro-static water sensor (output 0-5 V) that goes to the bottom of the the tank (see figure).

In most cases this works OK, but for some of our clients, the water sensor burns out repeatedly.

We think this is caused by an ESD issue. By analysing the situation of these water tanks, it seems that this occurs in zinc tanks that have an internal plastic coating in some really dry and dusty areas.

Our hypothesis is that wind blows dust onto the tank, which builds a potential with respect of the water (given that the water is separated from the metal by a plastic layer) and this difference of potential gets discharged into the water sensor.

To solve this problem, we were considering switching to 4-20 mA water sensors, with ESD protection in the sensor, but we are not sure if this will be enough to solve the problem.

Additionally, we are considering grounding our device to the water tank, to prevent electric charge from building between the tank and the water. The idea is that the charge would flow with low resistance through the ground of our device and the probe.

My question would be: would this approach be correct? Wouldn't conducting the electrostatic charge to the tank create galvanic corrosion in the tank to device contact areas?

Also, maybe it is not ESD; could it be that the probe cables are acting as inductors, and creating a big potential on changes of current to the probes?

Thanks in advance.

Answer 1

A guess:

Seems like there's water in contact with your sensor. The water is inside plastic tank and runs in plastic pipes. The sensor and its electronics is the only connection from the water to the local ground. But water inside the pipes connects the water in the tank to places at unknown distances which may well have potential jumps (=voltage surges) when compared to your local ground. All currents caused by those potential jumps go through your sensing system.

The water should be connected to your local ground by having a grounded stainless steel net around those parts of your sensor which are in the water. The shield must not have any direct contact to the sensor, only to the local ground and to the water.

Answer 2

Sounds like a bad design of the sensor. Static is always a potential problem so the sensor should be protected. If it’s a capacitive type for example (very common) it would include a proper series resistor(s) and a spark gap.
If the tank is floating, grounding it would be the first thing to try. Static should not cause corrosion.

Answer 3

Had you considered connecting the pressure sensor to a pipe above the level of the water, so that the water transferred the pressure to the air and thence the sensor. Then you'd have no contact between the water and the sensor.

Answer 4

One way to find out would be to put a voltage meter on the sensor and one end in the water and see if there is any significant voltage between the sensor and the water.

Probably the best thing would be a continuity check on the tank and the water. Get a bench supply and attach one end to a grounding rod nearby, then attach the other end to the top of the tank. Slowly ramp of the voltage until you see a noticeable current, then ramp up the voltage again until the current doubles, then again for when it triples. You should see a linear relationship which you could chart and then apply /$ V=I*R/$. The resistance should be low, in the ohms range. Do this again for the water, it should show the same thing.

Ideally it shouldn't take much voltage to get some kind of current, if you are applying more than 20V and you don't see any current (less than 1mA), then it's likely that the continuity is higher than 20kΩ.

You could also do this for the ground end of the sensor (only one side) to see if there is any continuity between the sensor and ground (there shouldn't be any).

I would bet that you find that the top of the tank is grounded (a few ohms or less to ground) and also the water. I would hope that the sensor is open with respect to ground (if it isn't then you have some big problems).

My guess is that its probably not normal ESD but lightning that is causing the sensor to burn out, if it is then you need to protect the sensor or the monitoring device inputs. Wind can also generate high voltages but I'll bet the tanks are grounded.