Water Activity Measuring Techniques
Most laboratories measure water activity either with an instrument that uses a relative humidity sensor or with one based on dew point that uses a chilled mirror. Until recently, chilled-mirror meters were the only ones to feature measuring times of only a few minutes per product sample. However, equally fastand typically less expensiveinstruments that use a relative humidity sensor are now available, which raises the question of whether the chilled mirror Aw meters have enough inherent advantages (speed, accuracy and measuring range) to offset the higher price and higher maintenance requirements. Is the chilled mirror format by nature faster? Typically, chilled-mirror Aw meters require less than five minutes to determine the water activity of a product sample. Many of today's relative humidity sensors can achieve an acceptable reading within that same time frame.
In both cases, the readings are a projection of what the true reading will be when the sample and the air surrounding the sample are at equilibriuma process which would otherwise take 30 to 90 minutes. Each projection is reached via a series of calculationsin one case based on the temperature of the air, in the other based on its relative humidity. Neither is moreor lessvalid than the other. And both are recognized as official methods analysis by AOAC International.
Temperature Stability Temperature stability always matters to Aw measurements. A temperature imbalance, or lack of temperature stability, can change the partial pressure of water vapor generated by the product sample. All sensorschilled-mirror as well as relative humidityare equally affected by this. When temperature is not stable, measurements take longer and/or are innaccurate. In this regard, using a temperature controlled sample holder does not help much, unless the product samples are also pre-conditioned to the temperature of measurement. Hense, neither method is favored.
Accuracy A chilled-mirror instrument does not provide a direct measurement of water activity. Rather, water activity is computed using both the value of dew point and the value of temperature. The accuracy of this computation is dependant upon the accuracy of the measurement of both of these parameters.
In the food industry, many water activity measurements fall in the range of 0.800 to 1.000 Aw. At room temperature, a good relative humidity sensor can measure a condition of 0.950 Aw with an accuracy of ±0.01 Aw or better. In terms of dew point, the equivalent accuracy is ±0.175°C. In both cases this is close to the accuracy available from a good laboratory chilled-mirror instrument. Aw meters that use a chilled mirror typically claim an accuracy of ±0.003 Awa figure which is argueable at best. Assuming absolutely no error in the measurement of temperature, this means that dew point must be measured with an accuracy of ±0.05°C, which clearly exceeds the accuracy of the best commercially available laboratory chilled-mirror instruments. Possibly, ±0.003 Aw refers to the repeatability (or precision) of the instrument rather than to its actual accuracy.
Range of Measurement The ability of a chilled-mirror meter to measure low humidity values depends both on the power available to cool the mirror and the evacuation of heat away from the mirror. Typical chilled-mirror meters are limited in these two areas and should not be used to measure products that are below 0.20 or 0.30 Aw. By contrast, a value of 0.01 Aw can be measured without difficulty using a relative humidity sensor.
Maintenance Under some conditions, chilled-mirror instruments can be high-maintenance. Many test samples contain volatile additives, often in the form of a fine powder. Because the chilled mirror operates at condensation, there is the tendency for the mirror to trap airborne contaminants, hindering the instrument's accuracy, requiring the operator to frequently clean the mirror.
Clearly, water activity meters using relative humidity sensors offer a viable alternative to their oft-touted, chilled-mirror cousins.
