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

water tank diagram

In most cases this works OK, but for some of our clients, the water sensor seem to be getting burned out repeatedly.

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

Our hypothesis is that wind blows dust on the tank, which makes it build 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 4-20mA 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,

  • \$\begingroup\$ It depends on your schematic of the "water sensor". Is it protected with a series R and clamp TVS? \$\endgroup\$ – Tony Stewart EE75 Dec 7 '18 at 0:36
  • \$\begingroup\$ On the old sensors, there was no clamp TVS... we did have some BAT54 diodes for discharge on the device side... \$\endgroup\$ – Alex Mantaut Dec 7 '18 at 1:09
  • \$\begingroup\$ The new sensors will have TVS. Also, I'm not sure about the series resistance... I am trying to figure it out... \$\endgroup\$ – Alex Mantaut Dec 7 '18 at 1:11
  • \$\begingroup\$ The probe is the HPT604 with the 0.5-4.5v, but without the lightning protection \$\endgroup\$ – Alex Mantaut Dec 7 '18 at 1:25
  • \$\begingroup\$ Does it have the optional lightning protection? Keep in mind ESD HBM is only 100pF and water having Dk of 80 and a large volume makes the Joules of storage much greater than the HBM of 1/2CV^2=1/2* 100pF* 4kV^2. The dielectric breakdown threshold is unknown nor is the tribelectric buildup. I suggest a semiconductor insulation shunt or a more detailed analysis of the problem looking for partial discharge.(PD) \$\endgroup\$ – Tony Stewart EE75 Dec 7 '18 at 1:38

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.

  • \$\begingroup\$ Yes, the model of sensor we were using does not seem to be really prepared for ESD.. In regards to grounding, would it be grounding the tank directly to earth ? or to provide a path between the water and the metal tank? \$\endgroup\$ – Alex Mantaut Dec 7 '18 at 1:13
  • \$\begingroup\$ Not sure. I was told that static was mostly a problem when working with dry powders, so we made our sensors well protected. If it is static there are 3 possibilities...zinc to earth, zinc to device circuit low, all 3 tied together. I would try zinc to earth first and zinc to device circuit low second. \$\endgroup\$ – Robert Endl Dec 7 '18 at 2:18

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.

  • \$\begingroup\$ Mmm, I am not sure I understand how that would work... With this probes, the probes need to be in the water, as they measure the pressure exerted by the column of water above them (barometric probes) \$\endgroup\$ – Alex Mantaut Jan 10 '19 at 22:20

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