Comparing Two Measuring Techniques

Which water activity system is right for you? The majority of instruments on the market today will provide equally accurate readings. When comparing relative humidity (RH) sensor-based systems, the most important consideration is the long term stability and repeatability of the RH sensor. When comparing RH based systems with chilled mirror based systems, there is no accuracy advantage. Therefore, base your decision on all the factors important to your use. Considerations such as instrument flexibility, ease of use, maintenance requirements, cost, and system soft-ware all play an important role. Measuring Techniques, Speed Most laboratories measure water activity with an instrument that either uses a relative humidity sensor or uses a chilled mirror. Instruments based on relative humidity sensors are typically less expensive than the chilled-mirror instruments. This raises the question of whether the chilled mirror Aw meters have enough inherent advantages such as speed, accuracy and measuring range to offset the disadvantage of both a higher price and higher maintenance requirements. The measurement of water activity is an indirect measurement. All AW instruments measure the amount of water vapor in the air surrounding the product sample. The time required for the water vapor in the product sample to equilibrate with the air in the chamber varies significantly with the composition of the product and temperature stability between the product sample and the air. Typically, the full equilibrium time requires 30 minutes or more. Using a variety of methods to speed the measurement most Aw instruments available today pro-vide a reading of Aw in approximately 5 minutes.