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

If you need to measure the level 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. Approved submersed application implicates a maximum contact with the encompassing, mainly water-based medium, respectively to ?moisture?.
Exposure isn’t just limited to the wetted elements of the pressure sensor housing, but additionally to the complete immersed amount of the cable. In addition, beyond your directly immersed level probe parts, the cable, and in particular the cable end, are often exposed to moisture because of splash water, rain and condensation. That is true not merely during operation, but a lot 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 in to the cable ends of the submersible pressure transmitter can occur early and irreversibly with insufficient protection measures, and, in virtually all cases, result in 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 must be actively eliminated by preventive actions by the user. To gauge the level with highest accuracy, the varying ambient pressure above the liquid media, that is also ?resting? on the liquid, must be compensated against the hydrostatic pressure acting on the pressure sensor (see article: hydrostatic level measurement).
Ventilation tube
Thus, it really is logical that there 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 protective measures. To pay the ambient pressure ?resting? on the media, a ventilation tube runs from the sensor element within the level probe, through the cable and out from the level probe at the end of the cable. Because of capillary action within the ventialation tube used for ambient pressure compensation, moisture can also be transported from the surrounding ambience right down to the sensor.
Thus not only air, but additionally moisture penetrates in to the tube, hence the sensor in the probe and the electronics around it really is irreparably damaged. This can lead to measurement errors and, in the worst case, even to failure of the level probe. To avoid any premature failure, the ingress of moisture into the ventilation tube must be completely prevented. Additional protection against moisture penetration through the ventilation tube is supplied by fitting an air-permeable, but water-impermeable filter element by the end of the vent tube.
bare wires
Not to be ignored can be the transport of the liquid through high-humidity loads across the only limitedly protected internals of the cable, e.g. along the wires, completely right down to the submersible pressure transmitter. As a leading manufacturer, WIKA uses appropriate structural design to prevent fluid transport, as far as possible, into the electronics of the submersible pressure transmitter. Due to molecular diffusion and capillary effects, a guaranteed one-hundred percent protection over the full duration of the submersible pressure transmitter, however, is never achievable.
Hence, it is recommended that the cable is definitely terminated in a waterproof junction box with the correct IP protection (e.g. IP65) that is matched to the installation location. If this cable junction box is exposed to weather and varying temperature conditions, additionally it is recommended to pay focus on a controlled pressure equalisation as a way to prevent the formation of condensation or perspiration water and pumping effects. To address this technical requirement, being an accessory to a submersible pressure transmitter, it is possible to order a connection box having an integrated air-permeable, water-impermeable membrane.
Ultimately, moisture ingress can occur not only through the exposed end of the cable, but also through mechanical damage to the cable sheath or as a result of liquid diffusion due to improper chemical resistance of the cable material. In the article ?Selection criteria for preventing moisture-related failures of submersible pressure transmitters or level probes? this failure mode is described at length.
WIKA offers comprehensive solutions for your hydrostatic-pressure level measurement. For further assistance in selecting the submersible pressure transmitter most suitable for the application, please use our contact page.
Please find more info on this topic on our information platform ?Hydrostatic level measurement?

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