Preventing the ingress of moisture through the cable of submersible pressure transmitters and level probes?

If pressure gauge octa intend to measure the degree of a liquid easily and reliably, a lot of people will do this using hydrostatic pressure measurement, e.g. with a submersible pressure transmitter or a so called level probe. The characteristic submersed application implicates a maximum exposure to the surrounding, mainly water-based medium, respectively to ?moisture?.
Exposure isn’t just limited by the wetted elements of the pressure sensor housing, but also to the complete immersed amount of the cable. In addition, outside the directly immersed level probe parts, the cable, and specifically the cable end, tend to be exposed to moisture due to splash water, rain and condensation. That is true not only during operation, but even more during installation and commissioning, or when maintenance or retrofitting is necessary. Irrespective of the target application, whether in water and wastewater treatment or in tank monitoring, moisture ingress in to the cable ends of the submersible pressure transmitter may appear early and irreversibly with insufficient protection measures, and, in almost all cases, lead to premature failure of the instrument.
The ingress of moisture into the cable outlet and from there on downwards into the electronics of the level probe should be actively eliminated by preventive actions by the user. To gauge the level with highest accuracy, the varying ambient pressure above the liquid media, which is also ?resting? on the liquid, should be compensated against the hydrostatic pressure acting on the pressure sensor (see article: hydrostatic level measurement).
Ventilation tube
Thus, it is logical that there surely is a constant risk of a moisture-related failure because of moisture ingress (both via the ventilation tube and through the specific cable itself) if you can find no adequate precautionary measures. To compensate the ambient pressure ?resting? on the media, a ventilation tube runs from the sensor element within the level probe, through the cable and from the level probe by the end of the cable. Because of capillary action within the ventialation tube useful for ambient pressure compensation, moisture may also be transported from the encompassing ambience right down to the sensor.
Thus not merely air, but additionally moisture penetrates into the tube, hence the sensor inside the probe and the electronics around it usually is irreparably damaged. This can lead to measurement errors and, in the worst case, even to failure of the level probe. To prevent any premature failure, the ingress of moisture into the ventilation tube should be completely prevented. Additional protection against moisture penetration through the ventilation tube is provided by fitting an air-permeable, but water-impermeable filter element at the end of the vent tube.
bare wires
Never to be ignored is also the transport of the liquid through high-humidity loads across the only limitedly protected internals of the cable, e.g. along the wires, all the way down to the submersible pressure transmitter. As a leading manufacturer, WIKA uses appropriate structural design to avoid fluid transport, so far as possible, in to the electronics of the submersible pressure transmitter. Due to molecular diffusion and capillary effects, a guaranteed one-hundred percent protection over the full lifetime of the submersible pressure transmitter, however, is never achievable.
Hence, it is recommended that the cable is always terminated in a waterproof junction box with the appropriate IP protection (e.g. IP65) which is matched to the installation location. If this cable junction box is exposed to weather and varying temperature conditions, it is also recommended to pay attention to a controlled pressure equalisation as a way to avoid the formation of condensation or perspiration water and pumping effects. To address this technical requirement, being an accessory to a submersible pressure transmitter, you’ll be able to order a connection box having an integrated air-permeable, water-impermeable membrane.
Ultimately, moisture ingress can happen not only through the exposed end of the cable, but also through mechanical damage to the cable sheath or due to liquid diffusion due to improper chemical resistance of the cable material. In this article ?Selection criteria for the prevention of moisture-related failures of submersible pressure transmitters or level probes? this failure mode is described in detail.
WIKA offers comprehensive solutions for the hydrostatic-pressure level measurement. For further assistance in selecting the submersible pressure transmitter the most suitable for the application, please use our contact page.
Please find further information on this topic on our information platform ?Hydrostatic level measurement?

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