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

If you want 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 perhaps a so called level probe. The characteristic submersed application implicates a maximum exposure to the encompassing, mainly water-based medium, respectively to ?moisture?.
Exposure isn’t just limited by the wetted parts of the pressure sensor housing, but also to the entire 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 merely during operation, but even more during installation and commissioning, or when maintenance or retrofitting is necessary. Irrespective of the prospective application, whether in water and wastewater treatment or in tank monitoring, moisture ingress into the cable ends of the submersible pressure transmitter may appear early and irreversibly with insufficient protection measures, and, in virtually all cases, lead to premature failure of the instrument.
The ingress of moisture into the cable outlet and from there on downwards in to the electronics of the level probe should be actively eliminated by preventive actions by the user. To measure the level with highest accuracy, the varying ambient pressure above the liquid media, that is also ?resting? on the liquid, should be compensated contrary to the hydrostatic pressure functioning on the pressure sensor (see article: hydrostatic level measurement).
Ventilation tube
Thus, it is logical that there surely is a constant threat of a moisture-related failure because of moisture ingress (both via the ventilation tube and through the actual cable itself) if there are 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 out of the level probe at the end of the cable. Because of capillary action within the ventialation tube useful for ambient pressure compensation, moisture can be transported from the surrounding ambience right down to the sensor.
Thus not merely air, but also moisture penetrates in to the tube, hence the sensor in the probe and the electronics around it is usually irreparably damaged. This can lead to measurement errors and, in the worst case, even to failure of the particular level probe. To avoid 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 by the end of the vent tube.
Inspired to be ignored can be the transport of the liquid through high-humidity loads along the only limitedly protected internals of the cable, e.g. along the wires, all the way right 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. Because of molecular diffusion and capillary effects, a guaranteed one-hundred percent protection on 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 correct IP protection (e.g. IP65) which is matched to the installation location. If this cable junction box is subjected to weather and varying temperature conditions, additionally it is recommended to pay attention to a controlled pressure equalisation so as to avoid the formation of condensation or perspiration water and pumping effects. To handle 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 merely through the exposed end of the cable, but additionally through mechanical damage to the cable sheath or because of liquid diffusion because of improper chemical resistance of the cable material. In the 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 your hydrostatic-pressure level measurement. For further assistance in selecting the submersible pressure transmitter most suitable for your application, please use our contact page.
Please find more info with this topic on our information platform ?Hydrostatic level measurement?

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