One of the main causes of failure in submersible applications of pressure transducers and transmitters is moisture ingress, either via liquid seeping in through the interface between the cable and the pressure sensor or down the inside of the cable from the cable termination at the surface.
In order to minimise the risk of moisture penetration some design features and installation practises need to be considered to ensure the submersible pressure sensor lasts as long as possible.
The first consideration is to make sure the cable seal is IP68 rated to a depth greater than the one required. There are basically 2 methods for sealing the cable: one is to fill the electrical interface with a permanent sealant and the other is to use O ring seals. Both methods are able to achieve very high IP ratings but the more permanent sealant method is arguably the most reliable and most resilient to variations in temperature over time.
The O ring gasket approach provides a very convenient seal which is easily assembled by the manufacturer without waiting for sealant compounds to cure. The O ring gasket method also makes it possible to produce a detachable cable design so that a pressure sensor or cable can be replaced without having to bare the cost of replacing the whole assembly.
If the cable assembly is detachable it will provide the specified IP rating from new, however it is very important to follow the manufacturer’s recommended instructions for disconnecting and re-connecting the cable, otherwise the seals IP rating maybe compromised if they are not free from dirt & dust or not seated correctly.
A submersible pressure transducer or transmitter is mostly used for measuring the hydrostatic head of a liquid and it is necessary to vent the reverse side of the sensing diagram in order to cancel out the atmospheric pressure component which is the pressure acting on the liquid at the top of a vented tank or open reservoir. The vent path can easily lead to premature failure of the submersible pressure sensor if certain precautions are not implemented.
One of the main causes of failure for a submersible pressure sensor is moisture damage from condensation forming or making its way down the vent tube or between the wires. The condensation is created by the difference in temperature between the location where the cable is terminated and the liquid temperature. Generally the liquid temperature is lower especially during the summer months and if the relative humidity is high enough it will condense on the cooler surfaces that are submerged.
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There are two ways to protect a submersible pressure sensor from condensation damage, one relates to its installation and the other to its construction: The first and most important is to ensure the enclosure where the other end of the cable is terminated has adequate desiccant to ensure a dry environment. If a submersible pressure sensor does not incorporate all of the features in the proceeding sections of this article it is even more important to vent into a dry atmosphere.
The other way to prevent moisture damage is to ensure that any moisture that does manage to collect at the back of the submersible pressure sensor cannot cause any damage.
Moisture can enter the back of the submersible pressure sensor via the vent tube or between the wires. If the vent tube is connected all the way to the back of the pressure sensing diaphragm there will be no chance for any moisture to cause damage to the electronics.
If the pressure sensor is completely filled inside with silicone or similar potting compound any moisture entering between the wires will not be able to reach the electrical connections or electronic components.
Some types of pressure sensor technology have exposed electrical connections to the strain gauges on the reference side of the sensing diaphragm. These types of connections will need to be protected by a thin water resistant coating. However these coatings can affect the sensors accuracy performance introducing more pressure hysteresis particularly on low pressure range sensors.
These product enhancing features should only be treated as a back up protection for when a dry vent is compromised and should not be relied on completely to protect the sensor from internal moisture damage. Generally it is not good practice to allow moisture to collect in the vent tube, no matter how well protected the sensor is from internal moisture damage, since this will also lead to a liquid head offset or meniscus surface tension effects on trapped air behind the sensing diaphragm causing a calibration error. The accuracy of lower range submersible pressure sensors are particularly sensitive to moisture trapped behind the sensing diaphragm.
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