If you wish 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 encompassing, mainly water-based medium, respectively to ?moisture?.
Exposure isn’t just limited by the wetted elements of the pressure sensor housing, but also to the complete immersed length of the cable. Furthermore, beyond your 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 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, 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 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, that 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 really is logical that there is a constant risk of a moisture-related failure due to 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 ?resting? on the media, a ventilation tube runs from the sensor element within the particular level probe, through the cable and out of your level probe by the end of the cable. Because of capillary action within the ventialation tube useful for ambient pressure compensation, moisture can even be transported from the surrounding ambience right down to the sensor.
Thus not merely air, but also moisture penetrates in to the tube, hence the sensor inside the probe and the electronics around it might be irreparably damaged. This may result in measurement errors and, in the worst case, even to failure of the level probe. To avoid 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 provided 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. Due to molecular diffusion and capillary effects, a guaranteed one-hundred percent protection on 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 appropriate 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 that you can 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 connection 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 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 page.
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