A water activity meter directly measures the water activity of a solution. A sample of a solution is placed in a small, closed chamber. In the chamber, an equilibrium between the sample and the gas phase is created. At equilibrium, the water activity of the sample is equal to the relative humidity in the gas phase above the solution. By a small mirror, the dew point temperature is determined, i.e. the temperature where the gas phase is saturated with water vapour. From the dew point temperature and the sample temperature, which is also measured, the water activity may be derived. 

A freezing point depression osmometer is another method to determine water activity. With this instrument, you utilise the fact that the freezing point of an aqueous solution is lowered by adding, for example, salt to the solution. A small sample is placed in the instrument and the temperature is decreased to undercool the solution. Using a vibration, the solution freezes instantly and thereby the freezing point temperature is determined. Pure water has a freezing point of 0 °C, while salty solutions will have a lower freezing point. From the decrease in the freezing point, the water activity can be determined.