If you want to measure the degree 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 encompassing, mainly water-based medium, respectively to ?moisture?.
Exposure isn’t just limited to the wetted elements of the pressure sensor housing, but also to the entire immersed length of the cable. Furthermore, 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. This is true not only during operation, but even more during installation and commissioning, or when maintenance or retrofitting is necessary. 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, 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 must be actively eliminated by preventive actions by an individual. 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 risk of a moisture-related failure due to moisture ingress (both via the ventilation tube and through the specific 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 the level probe by the end of the cable. Psychological to capillary action within the ventialation tube used for ambient pressure compensation, moisture may also be transported from the surrounding ambience down to the sensor.
Thus not only air, but additionally moisture penetrates in to the tube, hence the sensor in the probe and the electronics around it might be irreparably damaged. This may lead to 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 must 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 along 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, 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.
Hence, it is recommended that the cable is always 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 attention to a controlled pressure equalisation as a way to avoid the formation of condensation or perspiration water and pumping effects. To address this technical requirement, as an accessory to a submersible pressure transmitter, you’ll be able to order a connection 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 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 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.
Please find further information on this topic on our information platform ?Hydrostatic level measurement?