Hydrostatic pressure measurement using submersible pressure transducers is a popular and effective method for determining liquid levels. However, moisture and corrosion plague the reliability and service life of submersible pressure transducers.
One option that helps prevent moisture from entering and corroding the transducer is the desiccant-filled vent filter, a transparent cylinder filled with an indicating desiccant and installed on the open end of the vent tube where the cable is terminated - most often in the junction box or control panel. As air enters the vent tube it passes through the filter that absorbs water vapor from the air. These filters are highly effective but must be monitored regularly and replaced when saturated. The desiccant filter material changes color as it absorbs moisture to provide a clear indication of its status.
If an application does not permit the type of regular maintenance required by a desiccant-filled vent filter, the aneroid bellows is an easily field-installable alternative. An aneroid bellows is a closed-end rubber bellows that connects to the cable vent tube via a flexible extension, provided with the bellows, and is mounted to the junction box or control panel with a DIN-3 instrument rail.
The bellows "telegraphs" changes in atmospheric pressure into the vent tube. This action provides the necessary atmospheric reference, while effectively protecting the transducer from moisture incursion. Bellows may not be a suitable replacement for the desiccant cartridge in applications where extremely high accuracy (0.1% or better) is required.
Another method of eliminating the possibility of water incursion is to specify that the transducer be constructed in a "sealed gage" configuration. In this case, the reference side of the sensor is not vented and the cable vent tube is plugged. The downside to this approach is that the accuracy of the transducer will be affected by changes in atmospheric pressure. For this reason, this technique is usually applied only when the range of measurement is large in proportion to typical barometric variation, approximately 100 psi or more.
Prevent Harm to the Cable
Harm to the cable almost surely results in moisture incursion into the transducer. Although the cable is quite flexible, do not bend the cable tighter than a 1-inch radius or around rough or sharp edges, and always use a cable reel during transport. When possible, suspend the unit in a perforated 2-3 inch stilling well and thread the cable through protective conduits to the nearest junction box. If a stilling well is not practical, an alternative approach is to purchase the level transducer with a conduit attachment, then using appropriate adapters, install a garden type hose over the cable. If the cable is cut, it can be repaired rather inexpensively, but consult the manufacturer to determine the most efficient and cost-effective solution.
Transducer Materials
Before purchasing a transducer, it is mandatory to determine compatibility between the media and the transducers materials of construction. 316 stainless steel offers good corrosion resistance at a reasonable cost. For extremely corrosive environments, other metals such as titanium and hastelloy may be appropriate.
Exercise caution and the submersible transducer will provide years of reliable and accurate service. Consult the manufacturer for specific questions or applications.
About the Author:
Mark Miller is a Senior Applications Specialist for PSI.
