How to prevent 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, a lot of people will 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 is not only limited to the wetted elements of the pressure sensor housing, but also to the complete immersed length of the cable. In addition, outside the directly immersed level probe parts, the cable, and specifically the cable end, are often exposed to moisture due to splash water, rain and condensation. That is true not merely during operation, but even 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 can occur early and irreversibly with insufficient protection measures, and, in almost all cases, result in premature failure of the instrument.
The ingress of moisture in to the cable outlet and from there on downwards in to the electronics of the particular 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 against the hydrostatic pressure functioning 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 due to moisture ingress (both via the ventilation tube and through the specific cable itself) if there are no adequate protective measures. To pay the ambient pressure ?resting? on the media, a ventilation tube runs from the sensor element within the particular level probe, through the cable and out of your level probe by the end of the cable. Because of capillary action within the ventialation tube used for ambient pressure compensation, moisture can even be transported from the encompassing ambience 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 usually is irreparably damaged. This can result in 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 should 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
Not to be ignored can be the transport of the liquid through high-humidity loads along the only limitedly protected internals of the cable, e.g. across the wires, all the way right down to the submersible pressure transmitter. As Pound , WIKA uses appropriate structural design to avoid fluid transport, so 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 definitely terminated in a waterproof junction box with the appropriate 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 to be able to prevent the formation of condensation or perspiration water and pumping effects. To address this technical requirement, as an accessory to a submersible pressure transmitter, it is possible to order a connection box with an integrated air-permeable, water-impermeable membrane.
Ultimately, moisture ingress can happen not only through the exposed end of the cable, but additionally through mechanical damage to the cable sheath or because 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 in detail.
WIKA offers comprehensive solutions for the hydrostatic-pressure level measurement. For further assistance in selecting the submersible pressure transmitter the most suitable for the application, please use our contact page.
Please find further information with this topic on our information platform ?Hydrostatic level measurement?

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