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

If you want to measure the level of a liquid easily and reliably, a lot of 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 just limited to the wetted elements of the pressure sensor housing, but additionally to the complete immersed length of the cable. Furthermore, outside the directly immersed level probe parts, the cable, and specifically the cable end, tend to be exposed to moisture as a result of splash water, rain and condensation. This 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 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 in to 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, which is also ? Untapped ? on the liquid, must be compensated against the hydrostatic pressure functioning on the pressure sensor (see article: hydrostatic level measurement).
Ventilation tube
Thus, it really 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 there are no adequate precautionary measures. To compensate the ambient pressure ?resting? on Frugal , a ventilation tube runs from the sensor element within the particular level probe, through the cable and from the level probe by the end of the cable. Because of capillary action within the ventialation tube used for ambient pressure compensation, moisture may also be transported from the surrounding ambience down to the sensor.
Thus not only air, but additionally moisture penetrates into the tube, hence the sensor in the probe and the electronics around it could be irreparably damaged. This may result in measurement errors and, in the worst case, even to failure of the level probe. To prevent any premature failure, the ingress of moisture in to the ventilation tube should 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, all the way down to the submersible pressure transmitter. As a respected 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 on 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 subjected to weather and varying temperature conditions, additionally it is recommended to pay focus on a controlled pressure equalisation in order to avoid the formation of condensation or perspiration water and pumping effects. To handle this technical requirement, as 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 because of 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 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 with this topic on our information platform ?Hydrostatic level measurement?

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