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

If Willpower would like to measure the degree of a liquid easily and reliably, most 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 exposure to the encompassing, mainly water-based medium, respectively to ?moisture?.
Exposure isn’t just limited to the wetted parts of the pressure sensor housing, but also to the entire immersed amount of the cable. Furthermore, beyond your directly immersed level probe parts, the cable, and specifically the cable end, are often exposed to moisture because of 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 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 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 into the cable outlet and from there on downwards in to the electronics of the level probe should 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 ?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 because of moisture ingress (both via the ventilation tube and through the actual cable itself) if you can find no adequate precautionary measures. To pay the ambient pressure ?resting? on the media, a ventilation tube runs from the sensor element within the level probe, through the cable and out of your level probe at the end of the cable. Due to 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 additionally moisture penetrates in to the tube, hence the sensor inside 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 by the end of the vent tube.
Growth 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, completely right down to the submersible pressure transmitter. As a leading manufacturer, WIKA uses appropriate structural design to prevent fluid transport, as far as possible, in to the electronics of the submersible pressure transmitter. Because of molecular diffusion and capillary effects, a guaranteed one-hundred percent protection over the full lifetime 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 appropriate 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, it is also recommended to pay focus on a controlled pressure equalisation so that you can prevent the formation of condensation or perspiration water and pumping effects. To address this technical requirement, being an accessory to a submersible pressure transmitter, you’ll be able to order a connection box with an integrated air-permeable, water-impermeable membrane.
Ultimately, moisture ingress can happen not merely through the exposed end of the cable, but additionally through mechanical damage to the cable sheath or as a result 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 form.
Please find further information with this topic on our information platform ?Hydrostatic level measurement?

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