The article presents theoretical principles of the methods and devices for water activity (aw) measurement. The purpose of the research is the increase of the validity of water activity indicator measurement on the basis of a direct method of material test; the optimization of the structure of an automated measuring device to provide the possibilities for a simultaneous study of several samples (for example, food materials) with various water activity levels. A developed measuring device allows a simultaneous determination of indicators in several samples (8) located circumferentially and includes thermal sensors. The sensors in this device are represented by induction coils. They are installed on both leg pipes (made of nonmagnet materials) of a liquid level manometer. The device contains a special silicon organic oil with the pressure of partial steams within 10-6 Pa (at normal conditions) used as a manometric liquid. While measuring the device is vacuumized and thermostated. The method and device approbation resulted in making calibration charts at various temperatures, approximated with the help of software. The device provides the validity of measurements in water activity range from 0.45 to 1.0.