If you wish 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 exposure to the encompassing, mainly water-based medium, respectively to ?moisture?.
Exposure isn’t just limited to the wetted elements of the pressure sensor housing, but additionally to the entire immersed length of the cable. In addition, outside the directly immersed level probe parts, the cable, and in particular the cable end, are often exposed to moisture as a result 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 target 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 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 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, which 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 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 particular level probe, through the cable and from the level probe by the end of the cable. Due to capillary action within the ventialation tube useful for ambient pressure compensation, moisture may also be transported from the surrounding ambience down to the sensor.
Thus not only air, but also moisture penetrates into the tube, hence the sensor inside 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 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 by 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, all the way down to the submersible pressure transmitter. As a respected 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 on 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. Unresponsive ) 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 attention to a controlled pressure equalisation so as to avoid the formation of condensation or perspiration water and pumping effects. To handle this technical requirement, being 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 merely through the exposed end of the cable, but also through mechanical damage to the cable sheath or as a result of liquid diffusion because of improper chemical resistance of the cable material. In Exquisite ?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 most suitable for your application, please use our contact page.
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