Aircraft Condensation Hygrometer, ACH, is intended for measurements of air humidity (dew/frost-point temperature) from board an airplane. This instrument has been developed under the guidance of Dr. M.Yu. Mezrin of the Central Aerological Observatory, Russia, and used in a number of projects: Arctic HAZE (the Northern Hemisphere, 1994), BASE (Beaufort and Arctic Storm Experiment, Canada, 1994), APE (Airborne Polar Experiment, Finland, 1997), APE-THESEO (Third European Stratospheric Experiment on Ozone, the Indian Ocean, the Seychelles, 1999), etc. The instrument was installed aboard the Russian aircraft weather labs IL-18 and AN-30, on the Canadian aircraft Convair-580, and lately, has been adapted to the Russian high-altitude aircraft M-55 «Geophysika». When on board IL-18 or Convair-580, it was complemented by ultra-violet hygrometer that additionally provided data on turbulent humidity fluctuations.
The condensation hygrometer is known to operate on a principle of maintaining phase equilibrium between condensate on a cooled mirror and air water vapor. This equilibrium is maintained due to sophisticated feedback between an optical condensate detector and refrigerator control, which involves processes of heat exchange, droplet and crystal growth, and radiation scattering on these particles. The mirror temperature is a measure of dew/frost point.
The instrument includes a fairing, a control box and connecting cables. The fairing houses a sensor, an analogue-to-digital converter, (ADC) and a processor. The control box includes a power supply unit, interface circuits and a panel. The ACH electric circuit is made of elements resistant to temperature and pressure. The processor and ADC also satisfy these conditions. The ADC measurements are handles by the processor and passed by an RS 232 sequential code to the data acquisition system, DAS. Parallel connection of a portable computer is possible. The panel reflects the instrument status.

The mirror temperature is measured with a miniature semiconductor thermistor fixed under the mirror surface. The thermistor resistance is measured with an analogue-to-digital converter and passed to the data acquisition system. It must be converted to the mirror temperature while processing the data obtained. To do this, universal functions and coefficients obtained in calibrating the instrument are employed.
Upon the instrument activation, the processor provides the heating of the mirror and measurement of background photocurrent rates. This permits making allowance for the contamination of the mirror in the course of the instrument operation.

Technical specification: