If you would like 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 isn’t only limited by the wetted elements of the pressure sensor housing, but also to the complete immersed length of the cable. Furthermore, outside the directly immersed level probe parts, the cable, and specifically 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 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 virtually 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 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 acting on the pressure sensor (see article: hydrostatic level measurement).
Ventilation tube
Thus, it is logical that there is a constant risk 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 protective 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 from the level probe by the end of the cable. Because of capillary action within the ventialation tube used for ambient pressure compensation, moisture can be transported from the encompassing ambience down to the sensor.
Thus not merely air, but also moisture penetrates into the tube, hence the sensor in the probe and the electronics around it can be irreparably damaged. This may lead to measurement errors and, in the worst case, even to failure of the particular level probe. To avoid 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 supplied by fitting an air-permeable, but water-impermeable filter element at 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. across the wires, all the way down to the submersible pressure transmitter. As Reveal , WIKA uses appropriate structural design to avoid 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 definitely terminated in a waterproof junction box with the appropriate 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 address this technical requirement, being an accessory to a submersible pressure transmitter, it is possible to order a link box with an integrated air-permeable, water-impermeable membrane.
Ultimately, moisture ingress can occur not only through the exposed end of the cable, but additionally through mechanical damage 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 the hydrostatic-pressure level measurement. For further assistance in selecting the submersible pressure transmitter most suitable for your application, please use our contact form.
Please find further information on this topic on our information platform ?Hydrostatic level measurement?