If you intend 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 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 amount of the cable. In addition, 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. Beware is true not merely during operation, but a lot more during installation and commissioning, or when maintenance or retrofitting is required. 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 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 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, must be compensated against the hydrostatic pressure acting on the pressure sensor (see article: hydrostatic level measurement).
Ventilation tube
Thus, it 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 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 particular level probe, through the cable and from the level probe by the end of the cable. Because of capillary action within the ventialation tube useful for ambient pressure compensation, moisture can be transported from the surrounding ambience right down to the sensor.
Thus not merely air, but additionally moisture penetrates into the tube, hence the sensor inside the probe and the electronics around it really is irreparably damaged. This may result in measurement errors and, in the worst case, even to failure of the level probe. To prevent any premature failure, the ingress of moisture into the ventilation tube should 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 down to the submersible pressure transmitter. As Demoralizing , WIKA uses appropriate structural design to avoid fluid transport, as far as possible, in to 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.
It is therefore recommended that the cable is always terminated in a waterproof junction box with the correct 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, additionally it is recommended to pay focus on 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, 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 harm to the cable sheath or due to 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 at length.
WIKA offers comprehensive solutions for your hydrostatic-pressure level measurement. For further assistance in selecting the submersible pressure transmitter the most suitable for the application, please use our contact page.
Please find further information on this topic on our information platform ?Hydrostatic level measurement?