Hydrostatic level sensors (generally known as submersible pressure transmitter) have become often installed in outdoor applications, primarily in the water and wastewater industries, where open bodies of water, deep wells or boreholes are monitored. A good connection to ground is vital when installing hydrostatic level sensors, since no grounding or poor grounding can lead to destruction or damage to the level sensor.
Common failures from improper grounding
Hydrostatic level sensors in outdoor applications are connected via cables to the PLC or routed to local telemetric systems. These cables can act like an antenna, conducting electromagnetic or aerial voltage spikes down the wires to the sensor, causing an overload in the electronics and thereby premature failure. The media that is measured may itself store energy such as a capacitor. Amazing is caused by lightning strikes, electrical surges or simply static electricity. When there is not just a sufficiently low impedance ground connection for the particular level sensor, this might cause voltage surges that flash through the electronics causing them to overload and burn up.
Even though the voltage difference is too low to cause an overload of the electronics, it could cause electrolytic action as a result of difference in voltage potential. This electrolytic action causes the metal housing material of the hydrostatic level sensor to be ?eaten away? over long-term operation. Electrolytic corrosion pits will form in the material which can cause the diaphragm or housing to perforate, ultimately causing premature failure of the level sensor. This may be mistaken for chemical corrosion but is, actually, caused by the difference in voltage potential between your sensor and the encompassing liquid. Without a good ground the sensor becomes a sacrificial anode and is inevitably eaten away.
How to properly ground and protect a hydrostatic level sensor
WIKA level sensors can be found with optionally integrated lightning protection that acts on dangerous differences in the voltage potential between electronics, cabling and transmitter body, and routes any harmful voltages to ground before they are able to damage the inner circuitry. However, if the grounding for the transducer is poor, it will still have nowhere else to go but in to the electronics or body of the sensor, causing a premature failure.
In the case of metallic and plastic tanks, any isolated metal parts ought to be connected to a standard ground with an impedance of < 100 Ohms. In applications onboard ships, where, in essence metal tanks are always present, all of the different ground potentials ought to be linked to the ship?s main ground point during docking. In lakes and reservoirs, a low impedance link with ground could be hard to attain, but will be well worth the effort since it saves the hydrostatic level sensor electronics from failure. On artificial constructions or rock sites, even long copper spikes driven into the ground may not give a low-enough impedance to ground, so earthing grids may be embedded in to the ground to achieve the right resistance to ground.
Grounding of hydrostatic level sensors is a basic requirement of the reliable operation of level sensors, especially in outdoor applications, where overvoltage spikes and surges because of lightning strikes may appear regularly. Failure to supply adequate grounding can result in the failure of the level sensor.
Take a look at the profiles of WIKAs submersible pressure transmitters LH-20 and LS-10.
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