If you want 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 exposure to the surrounding, mainly water-based medium, respectively to “moisture”.
Exposure is not only limited to the wetted parts of the pressure sensor housing, but also to the entire immersed length of the cable. In addition, outside the directly immersed level probe parts, the cable, and in particular the cable end, are often 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 target application, whether in water and wastewater treatment or in tank monitoring, moisture ingress into 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 into the cable outlet and from there on downwards into 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, 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 threat 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 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 of the level probe at the end of the cable. Due to capillary action within the ventialation tube used for ambient pressure compensation, moisture can also be transported from the surrounding ambience down to the sensor.
Thus not only air, but also moisture penetrates into the tube, hence the sensor inside 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 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 provided by fitting an air-permeable, but water-impermeable filter element at the end of the vent tube.
bare wires
Not 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. along the wires, all the way 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. 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 appropriate 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, it is also recommended to pay attention to a controlled pressure equalisation in order 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, it is possible 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 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 in detail.
WIKA offers comprehensive solutions for your hydrostatic-pressure level measurement. For further assistance in selecting the submersible pressure transmitter most suitable for your application, please use our contact form.
Please find further information on this topic on our information platform “Hydrostatic level measurement”
