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If you need 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 perhaps a so called level probe. The characteristic submersed application implicates a maximum contact with the surrounding, mainly water-based medium, respectively to ?moisture?.
Exposure is not only limited by the wetted parts 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, tend to be exposed to moisture as a result of 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 required. Irrespective of the mark application, whether in water and wastewater treatment or in tank monitoring, moisture ingress in to the cable ends of the submersible pressure transmitter can occur 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 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 contrary to the hydrostatic pressure acting on the pressure sensor (see article: hydrostatic level measurement).
Ventilation tube
Thus, it really 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 compensate 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 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 down to the sensor.
Thus not only air, but also moisture penetrates into the tube, hence the sensor in the probe and the electronics around it is usually irreparably damaged. เกจวัดแรงดัน 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 at the end of the vent tube.
bare wires
Not to be ignored can be the transport of the liquid through high-humidity loads across the only limitedly protected internals of the cable, e.g. along the wires, all the way 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 on 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) which is matched to the installation location. If this cable junction box is subjected 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 handle this technical requirement, as an accessory to a submersible pressure transmitter, it is possible to order a link box having 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 harm to the cable sheath or because of 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 at length.
WIKA offers comprehensive solutions for your 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 with this topic on our information platform ?Hydrostatic level measurement?
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