Hygrometer, Chilled Mirror, Dual Detector
Saturday, September 09, 2006
A Dual Detector Chilled Mirror Hygrometer has been designed and developed with automatic compensation for reading variations caused by mirror contamination or component drift. The Hygrometer has an accuracy of +/- 0.5 °C and dew point range of -40 °C to +60 °C. The Hygrometer consists of a sensor probe and electronic control unit.
The Hygrometer contains a gold plated or rhodium plated copper mirror with a surface temperature controlled by a Thermoelectric Cooler. A collimated Laser Diode illuminates the mirror and a photodetector detects the reflected light beam. The incident radiation stops reflecting light when the mirror surface temperature is at dew / frost temperature. The incident light is refracted in all directions by the dew droplets or frost crystals that have formed on the mirror surface. The reflected energy can be reduced by particulate contaminants on the mirror surface.
The Hygrometer uses two photodetectors: 1) The direct light photodetector located in the center of the base of the illumination cone, same plane as the illumination source; 2) The scattered light photodetector located on the lateral surface of the illumination cone, receiving only a small background portion of the reflected radiation.
A technique of differential signal balance between the two photodetectors is used to control the thermoelectric cooler circuitry to stabilize mirror temperature at the dew / frost temperature. This information is also used for particulate contaminant accumulation, ambient temperature compensation, illumination source fluctuations, or to monitor any other variations in the Hygrometer.
Source: Sensors, July 1998 and Sensors, July 1992
Reference:
1) DewTrak (GTE Device Name)
2) Wayne Cole, EG&G, Burlington, MA
Nhan Matthews, GTE Sylvania, Waldoboro, ME
Industrial Products
posted by JD52 @ 1:29 PM,
