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

If you want to measure the level 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 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 additionally to the entire immersed length of the cable. Furthermore, beyond Useless 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 a lot 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 into the cable ends of the submersible pressure transmitter may appear 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 level probe should be actively eliminated by preventive actions by an individual. To measure the level with highest accuracy, the varying ambient pressure above the liquid media, which is also ?resting? on the liquid, should be compensated contrary to 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 threat 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 protective 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 of the level probe at the end of the cable. Due to capillary action within the ventialation tube used for ambient pressure compensation, moisture can be transported from the surrounding ambience right down to the sensor.
Thus not only air, but additionally moisture penetrates in to the tube, hence the sensor inside the probe and the electronics around it really is irreparably damaged. This can lead to 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 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 at 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. along the wires, all the way down to the submersible pressure transmitter. As a respected manufacturer, WIKA uses appropriate structural design to avoid 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 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 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, additionally it is recommended to pay focus on a controlled pressure equalisation as a way to prevent 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 happen not only through the exposed end of the cable, but also through mechanical harm to the cable sheath or as a result of liquid diffusion because of improper chemical resistance of the cable material. In the article ?Selection criteria for the prevention of 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 the 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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