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

If you would like to measure the degree 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 contact with the surrounding, mainly water-based medium, respectively to ?moisture?.
Exposure is not only limited by the wetted parts of the pressure sensor housing, but also to the entire 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. 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 target 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 almost all cases, lead to premature failure of the instrument.
The ingress of moisture into the cable outlet and from there on downwards into the electronics of the particular 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, must be compensated against 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 risk of a moisture-related failure because of moisture ingress (both via the ventilation tube and through the specific cable itself) if you can find 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 from the level probe at the end of the cable. Due to capillary action within the ventialation tube used for ambient pressure compensation, moisture can also be transported from the encompassing ambience right 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 is usually irreparably damaged. This may 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 in to 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 can be 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 leading manufacturer, WIKA uses appropriate structural design to prevent fluid transport, so far as possible, into the electronics of the submersible pressure transmitter. Because of 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 appropriate IP protection (e.g. IP65) which 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 so as to 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 link box having an integrated air-permeable, water-impermeable membrane.
Ultimately, moisture ingress can occur not merely 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 your application, please use our contact page.
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