If you 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 contact with the surrounding, mainly water-based medium, respectively to ?moisture?.
Exposure isn’t just limited to the wetted parts of the pressure sensor housing, but additionally to the entire immersed amount of the cable. Furthermore, outside the directly immersed level probe parts, the cable, and specifically the cable end, tend to be exposed to moisture as a result of 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 almost 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 particular level probe should be actively eliminated by preventive actions by the user. 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 really is logical that there is a constant threat 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 precautionary measures. To pay the ambient pressure ? Gargantuan ? 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. Because of capillary action within the ventialation tube useful for ambient pressure compensation, moisture can be transported from the encompassing ambience right 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 really is 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 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
Never 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, completely right down to the submersible pressure transmitter. As a respected manufacturer, WIKA uses appropriate structural design to avoid 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 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 subjected 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 address 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 damage to the cable sheath or because of liquid diffusion because of improper chemical resistance of the cable material. In Backbone ?Selection criteria for the prevention of moisture-related failures of submersible pressure transmitters or level probes? this failure mode is described at length.
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?