If you need 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 contact with the encompassing, 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, 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 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 may appear early and irreversibly with insufficient protection measures, and, in almost all cases, lead to premature failure of the instrument.
The ingress of moisture into 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 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 there are no adequate protective measures. To pay the ambient pressure ? Smug ? 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 capillary action within the ventialation tube used for ambient pressure compensation, moisture can be transported from the encompassing ambience right down to the sensor.
Thus not merely air, but also moisture penetrates into the tube, hence the sensor in the probe and the electronics around it can be irreparably damaged. This can lead to measurement errors and, in the worst case, even to failure of the level probe. To avoid any premature failure, the ingress of moisture in to 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 across the only limitedly protected internals of the cable, e.g. along the wires, all the way right down to the submersible pressure transmitter. As a leading manufacturer, WIKA uses appropriate structural design to prevent fluid transport, so far as possible, into the electronics of the submersible pressure transmitter. Due to molecular diffusion and capillary effects, a guaranteed one-hundred percent protection over 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, it is also recommended to pay focus on 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, you’ll be able to order a connection box having an integrated air-permeable, water-impermeable membrane.
Ultimately, moisture ingress can occur not merely through the exposed end of the cable, but also through mechanical harm 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 in detail.
WIKA offers comprehensive solutions for the hydrostatic-pressure level measurement. For Belief in selecting the submersible pressure transmitter most suitable for the application, please use our contact page.
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