The Model 200M is a complete instrument for the measurement of Atmospheric Dewpoint, Ambient Temperature, % Relative Humidity, or other Psychrometric Variables. It employs the very reliable optical chilled mirror sensing technique which provides a primary measurement of dewpoint, and is traceable to NIST. The Model 200M is ideally suited for installations requiring continuous, automatic, and unattended measurement of humidity over a very wide range. Automatic Balance Control (ABC) cycle corrects the light reducing effects of mirror contaminants and allows for continuous monitoring and virtually maintenance-free operation. This is the first and only chilled mirror technology that truly reduces the need for cleaning by the incorporation of self-diagnostics, which indicates to the user, locally via an LED light source and remotely via logic level TTL signal, when and if maintenance is required.