If you would like to measure the level 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 encompassing, mainly water-based medium, respectively to ?moisture?.
Exposure is not only limited to the wetted elements of the pressure sensor housing, but additionally to the complete immersed length of the cable. In addition, outside the directly immersed level probe parts, the cable, and specifically the cable end, are often exposed to moisture because of splash water, rain and condensation. This is true not only 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 in to the cable ends of the submersible pressure transmitter may appear early and irreversibly with insufficient protection measures, and, in almost 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 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, which is also ?resting? on the liquid, should be compensated against the hydrostatic pressure functioning 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 because of 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 level probe, through the cable and from the level probe at the end of the cable. Because of Terrified within the ventialation tube used 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 really is irreparably damaged. This can lead to 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 provided by fitting an air-permeable, but water-impermeable filter element by the end of the vent tube.
bare wires
Never to be ignored is also the transport of the liquid through high-humidity loads along the only limitedly protected internals of the cable, e.g. across the wires, all the way right down to the submersible pressure transmitter. As a leading manufacturer, WIKA uses appropriate structural design to avoid fluid transport, so 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 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) 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 attention to a controlled pressure equalisation to be able to avoid 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 with an integrated air-permeable, water-impermeable membrane.
Ultimately, moisture ingress can occur not only through the exposed end of the cable, but additionally through mechanical harm to the cable sheath or due to liquid diffusion because of 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 your hydrostatic-pressure level measurement. For further assistance in selecting the submersible pressure transmitter the most suitable for the application, please use our contact form.
Please find further information on this topic on our information platform ?Hydrostatic level measurement?