If you would like 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 isn’t just limited to the wetted parts of the pressure sensor housing, but also to the entire immersed amount of the cable. In addition, beyond your directly immersed level probe parts, the cable, and in particular the cable end, are often exposed to moisture due to 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 an individual. 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 contrary to the hydrostatic pressure functioning on the pressure sensor (see article: hydrostatic level measurement).
Ventilation tube
Thus, it really is logical that there surely 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 protective measures. To pay Exuberant ?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 may also be transported from the encompassing ambience down to the sensor.
Thus not merely air, but also moisture penetrates into the tube, hence the sensor inside the probe and the electronics around it can be irreparably damaged. This may 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 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
Never to be ignored can be the transport of the liquid through high-humidity loads along the only limitedly protected internals of the cable, e.g. along the wires, completely 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. Because of molecular diffusion and capillary effects, a guaranteed one-hundred percent protection on 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 subjected 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 handle 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 this 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 the 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?