There are only two major types of instruments on the market that measure water activity: dew point instruments, and electrical properties instruments. Both instruments include a sealed chamber in which the sample equilibrates prior to taking a reading. The difference between them is how the humidity reading is taken.
- Dew point instruments use a chilled mirror to measure the temperature at which dew forms on a mirror. This information, combined with other sensor data, is used to calculate the water activity of the sample. Dew point instruments are very fast and accurate. This is because dew point is a primary method for measuring humidity (i.e., if you know the dew point, you know the humidity). Dew point sensors can give incorrect readings on samples that contain ethanol and propylene glycol, something for pet food and soy sauce manufacturers to keep in mind.
- Electrical properties measure humidity in a closed chamber, but do it through water binding to a particular substrate. The instrument measures the electrical properties of the substrate. Unlike dew point sensors, this is a secondary measurement. Electrical properties sensors are classified as secondary because they are calibrated to relate a particular parameter (e.g. capacitance) to a specific humidity. This yields less accuracy, but is suitable for many applications.