When it comes to monitoring SF6 gas moisture content, there are 4 types of moisture sensors that are commonly used. These sensors are commonly found in SF6 analyzers and other monitoring equipment.
Aluminum Oxide Sensor:
This may be an older design type, but AL2O3 sensors are still commonly used today. These legacy sensors are used in a temperature range of +20°C to -70°C with an accuracy rating of +/- 3°C atm. When not being used, the sensors are kept in a chamber filled with desiccant (a hygroscopic substance used as a drying agent). When it’s time to take a measurement, the desiccant is removed which in turn exposes the sensor to the gas. This process is referred to as “wet-down testing” because the SF6 has higher moisture levels compared to the desiccant. Aluminum Oxide sensors usually perform better on very dry gas. Unfortunately the sensor is known for trapping SF6 on its surface, leading to false dry readings. AL203 sensors need to be calibrated more often than others, and should be acclimated to ambient humidity and temperature for a few hours if the sensor hasn’t been used recently.
Capacitive Polymers:
These sensors are popular choice because of their robust nature, repeatability, and lower cost. The accurate measuring range is roughly +20°C to -60°C, with an accuracy rating of +/- 2°C atm. These sensors work very well within the expected moisture range of most outdoor SF6 circuit breakers, but can be inaccurate at times when measuring very dry SF6 (typically below 50 ppmv). Because the sensor is exposed to ambient moisture when in storage, it needs to be dried with new SF6 gas before using it (a process referred to as “dry-up testing”). If excess sensor drift is suspected because of excessively wet readings or erratic results, devices can be cross checked in the field using a gas of known quantity. Otherwise, it is best practice to send the unit back to the manufacturer for repair/calibration.
Chilled Mirror
The Chilled Mirror sensor is highly regarded as one of the most accurate sensors. The typical measuring range is +20°C to -50°C, with an accuracy rating of +/- 0.2°C. atm. During operation, a polished mirror surface is cooled to the point at which condensation forms on the mirror surface. A temperature reading is then measured. Since this condensation temperature is specific to water vapor concentration, extremely precise results are achieved without the use of humidity sensors. Unfortunately, chilled mirrors require a very clean environment, which translates to a lot of cleaning for a unit that is used in the field often. Cleaning the mirror requires a quick disassembly and the use of a sterile cotton swab.
NDIR (Non-Dispersive InfraRed)
The NDIR is known for having an extremely fast response time and precise readings. The ideal measuring range is +20°C to -50°C, with an accuracy rating of +/- 0.5°C. atm. During measurement, a broadband IR emitter passes light through a small chamber with a sample of gas inside. An optical filter in front of the detector eliminates all wavelengths of IR except the one absorbed by the gas. The gas concentration is then measured by the absorption of a specific wavelength. The great things about the NDIR are that it has no consumables, no drift from sensor degradation, and is impervious to arc byproducts. The downside of this technology is that the operator has to pay very close attention to the handling and test procedures due to the very small amount of gas that is used for each measurement.
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