If you want to measure the level 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 surrounding, mainly water-based medium, respectively to ?moisture?.
Exposure isn’t only limited by the wetted parts of the pressure sensor housing, but also to the complete immersed length of the cable. In addition, beyond your directly immersed level probe parts, the cable, and in particular the cable end, are often exposed to moisture due to 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 prospective 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 in to the cable outlet and from there on downwards into the electronics of the level probe must be actively eliminated by preventive actions by the user. To measure the level with highest accuracy, the varying ambient pressure above the liquid media, that is also ? Unleashed ? on the liquid, should be compensated contrary to 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 protective 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 at the end of the cable. Because of capillary action within the ventialation tube used for ambient pressure compensation, moisture can be transported from the surrounding ambience right 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 really is 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 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 is also the transport of the liquid through high-humidity loads across the only limitedly protected internals of the cable, e.g. across the wires, completely right down to the submersible pressure transmitter. As a leading manufacturer, WIKA uses appropriate structural design to prevent fluid transport, as 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.
It is therefore 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 this cable junction box is exposed to weather and varying temperature conditions, it is also 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, 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, Efficient can occur not only through the exposed end of the cable, but also through mechanical harm to the cable sheath or as a result 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 form.
Please find further information with this topic on our information platform ?Hydrostatic level measurement?

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