A digital hygrometer for PPM level moisture measurement for commercial application

Moisture is one of the most important parameters to be measured precisely and accurately for manufacturing different industrial products and condition monitoring of key electrical equipment [1]. The range of applications, where the humidity measurement is needed, is endless and is increasing with time for quality of products, safety, and comfort of human. Range of moisture measurement is extremely large. It can extend from very high value represented in %RH to extremely low represented in ppm to ppb level [2]. In some application, moisture in the range of parts per trillion (ppt) is to be measured. Some critical applications of ppm moisture sensor include transfer oil, SF6 gas used in gas insulated switch gear and protection system, electronic gases for semiconducting device fabrication, organic LEDs fabrication, plutonium pallets in nuclear power plants etc. [3-4]. So, it is extremely difficult to fabricate moisture sensor which has wide dynamic range. For ppm moisture measurement, the sensor should possess high sensitivity,low response time, and high reproducibility. In addition, the response should be linear and the cost should be low. A simple RH sensor, for humidity range from 10% to 98% is available for only few dollars but a trace moisture sensor, with measurement range from 0 to 1000 ppm, requires thousands of dollars. There are various technologies used for moisture measurement from ppm to ppb level. Popular technologies are chilled mirror hygrometer, cavity ring down spectroscopy (CRDS) [5], and anodized aluminum oxide capacitive sensors [4-5]. However, the chilled mirror hygrometer and the capacitive sensor show poor performances in ppb range. The CRDS is the most reliable technology for trace moisture measurement even in ppb level but such system requires skilled manpower, long measurement time and is a costly technology not suitable for field application. A low-cost portable moisture measurement system can be developed using solid state capacitive sensor. There is no sensor based Indian Technology to measure moisture in ppm concentration.