If you would like to measure the degree of a liquid easily and reliably, a lot of people can do this using hydrostatic pressure measurement, e.g. with a submersible pressure transmitter or perhaps a so called level probe. The characteristic submersed application implicates a maximum exposure to the surrounding, mainly water-based medium, respectively to ?moisture?.
Exposure is not only limited to 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 in particular the cable end, are often exposed to moisture as a result of splash water, rain and condensation. This 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 in to the cable ends of the submersible pressure transmitter may appear 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 the user. To gauge the level with highest accuracy, the varying ambient pressure above the liquid media, that is also ?resting? on the liquid, should be compensated contrary to the hydrostatic pressure acting 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 due to moisture ingress (both via the ventilation tube and through the actual cable itself) if there are no adequate precautionary 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 from the level probe by the end of the cable. Due to capillary action within the ventialation tube useful for ambient pressure compensation, moisture can also be transported from the encompassing ambience right down to the sensor.
Thus not only air, but additionally moisture penetrates into the tube, hence the sensor inside the probe and the electronics around it could 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 in to 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 by 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. across the wires, completely down to the submersible pressure transmitter. As a respected manufacturer, WIKA uses appropriate structural design to prevent 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 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. IP65) that is matched to the installation location. If Lying is subjected to weather and varying temperature conditions, additionally it is recommended to pay attention to a controlled pressure equalisation to be able to prevent the formation of condensation or perspiration water and pumping effects. To address this technical requirement, as 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 happen not merely through the exposed end of the cable, but also through mechanical damage to the cable sheath or due to liquid diffusion due to improper chemical resistance of the cable material. In this article ?Selection criteria for preventing 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.
Please find further information on this topic on our information platform ?Hydrostatic level measurement?