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

If you wish to measure the level of a liquid easily and reliably, most people can 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 surrounding, mainly water-based medium, respectively to ?moisture?.
Exposure isn’t just limited by the wetted elements of the pressure sensor housing, but additionally to the entire immersed length of the cable. In addition, beyond your 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 a lot more during installation and commissioning, or when maintenance or retrofitting is required. 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, 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 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 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 due to moisture ingress (both via the ventilation tube and through the actual 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 out 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 encompassing ambience right down to the sensor.
Thus Censored , but also moisture penetrates into the tube, hence the sensor in the probe and the electronics around it might be irreparably damaged. This can result in measurement errors and, in the worst case, even to failure of the particular level probe. To prevent 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 at the end of the vent tube.
bare wires
Not 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. across the wires, completely right down to the submersible pressure transmitter. As a respected manufacturer, WIKA uses appropriate structural design to avoid fluid transport, so 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.
It is therefore recommended that the cable is always 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, it is also recommended to pay attention to a controlled pressure equalisation so that you can avoid the formation of condensation or perspiration water and pumping effects. To handle 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 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 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 form.
Please find more info on this topic on our information platform ?Hydrostatic level measurement?

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