How to prevent the ingress of moisture through the cable of submersible pressure transmitters and level probes?

If you need to measure the degree 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 perhaps a so called level probe. The characteristic submersed application implicates a maximum contact with the encompassing, mainly water-based medium, respectively to ?moisture?.
Exposure is not only limited to the wetted parts of the pressure sensor housing, but also to the complete immersed amount of the cable. Furthermore, outside the directly immersed level probe parts, the cable, and in particular the cable end, tend to be exposed to moisture due to splash water, rain and condensation. This is true not only during operation, but a lot more during installation and commissioning, or when maintenance or retrofitting is required. Irrespective of the mark application, whether in water and wastewater treatment or in tank monitoring, moisture ingress into 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 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 measure the level with highest accuracy, the varying ambient pressure above the liquid media, that is also ?resting? on the liquid, must be compensated contrary to the hydrostatic pressure acting 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 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 useful for ambient pressure compensation, moisture can also be transported from the encompassing ambience right down to the sensor.
Thus not merely air, but also moisture penetrates into the tube, hence the sensor inside the probe and the electronics around it might 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 into 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 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, all the way right down to the submersible pressure transmitter. As a respected manufacturer, WIKA uses appropriate structural design to prevent fluid transport, as far as possible, in to the electronics of the submersible pressure transmitter. Due to molecular diffusion and capillary effects, a guaranteed one-hundred percent protection over the full lifetime 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) which 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 to be able 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, you’ll be able to order a link box with 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 harm to the cable sheath or because of liquid diffusion because of improper chemical resistance of the cable material. In this 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 Succeed in selecting the submersible pressure transmitter most suitable for your application, please use our contact form.
Please find further information on this topic on our information platform ?Hydrostatic level measurement?

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