Applications for the Vigilant dew point hygrometers range from heat treating furnaces and endothermic generators to pharmaceutical and semiconductor plants that need accurate reliable measurement of dewpoint, %RH, PPM, and gr/lb. The instrument is capable of displaying up to three parameters simultaneously on its backlit graphical LCD display, as well as transmitting this data through its RS232 serial port and up to 3 scaleable analog outputs. Technologies include PRT amplifier self-calibration and improved speed of response through ServoLock-its digital control servo. It also incorporates an Automatic Balance Ciruit that corrects for mirror contamination.