How to prevent 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, most people can 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 contact with the surrounding, mainly water-based medium, respectively to ?moisture?.
Exposure isn’t only limited to the wetted parts of the pressure sensor housing, but also to the complete immersed amount of the cable. Furthermore, beyond your directly immersed level probe parts, the cable, and in particular the cable end, are often exposed to moisture as a result 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 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 virtually all cases, result in 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 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 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 from the level probe by the end of the cable. Because of capillary action within the ventialation tube useful for ambient pressure compensation, moisture can be transported from the encompassing ambience right down to the sensor.
Thus not merely air, but additionally 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 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 provided by fitting an air-permeable, but water-impermeable filter element by the end of the vent tube.
bare wires
Never 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, all the way down to the submersible pressure transmitter. As a respected manufacturer, WIKA uses appropriate structural design to prevent fluid transport, so far as possible, into the electronics of the submersible pressure transmitter. Because of 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 always terminated in a waterproof junction box with the correct IP protection (e.g. IP65) which is matched to the installation location. If Courageous is subjected to weather and varying temperature conditions, it is also recommended to pay focus on a controlled pressure equalisation so as 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 link 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 because of 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 in detail.
WIKA offers comprehensive solutions for the 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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