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 perhaps a so called level probe. Perfect submersed application implicates a maximum exposure to the surrounding, mainly water-based medium, respectively to ?moisture?.
Exposure isn’t just limited to the wetted elements of the pressure sensor housing, but additionally 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 due to splash water, rain and condensation. This is true not merely during operation, but even more during installation and commissioning, or when maintenance or retrofitting is required. Irrespective of the prospective 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 almost all cases, lead to 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, that is also ?resting? on the liquid, must be compensated contrary to the hydrostatic pressure functioning on the pressure sensor (see article: hydrostatic level measurement).
Ventilation tube
Thus, it is logical that there is a constant threat of a moisture-related failure due to moisture ingress (both via the ventilation tube and through the specific cable itself) if there are 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 out from the level probe at the end of the cable. Due to capillary action within the ventialation tube used for ambient pressure compensation, moisture may also be transported from the encompassing ambience down to the sensor.
Thus not only air, but also moisture penetrates in to the tube, hence the sensor inside the probe and the electronics around it usually is irreparably damaged. This can result in 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
Never 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 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 over the full lifetime 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 correct IP protection (e.g. IP65) which 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 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 only through the exposed end of the cable, but also through mechanical damage to the cable sheath or as a result of liquid diffusion because of 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 the hydrostatic-pressure level measurement. For further assistance in selecting the submersible pressure transmitter most suitable for the application, please use our contact page.
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