Abstract  

A point by point comparison shows that there is no real advantage of instruments that use a chilled mirror over instruments that use a relative humidity sensor.

Water Activity 
Measuring Techniques

Commercially available water activity instruments commonly use one of the following humidity measurement techniques:

Water activity is defined under static conditions of equilibrium. Under such conditions, the partial pressure of water vapor at the surface of the product is equal to the partial pressure of water vapor in the immediate environment of the product. When placed in a sealed chamber, most products typically require at least 20 to 30 minutes to achieve full water vapor pressure equilibrium with the air volume inside the chamber. This assumes that the temperature of the product sample and the temperature of the air volume inside the chamber are the same from the beginning to the end of the process. Unless something is very wrong, the time that is required to reach equilibrium does not depend on the technique used to measure humidity in the air volume above the product sample.

Based on the above, getting a valid water activity reading in only a few minutes generally requires the use of a mathematical algorithm to project the final result ahead of the time when actual equilibrium occurs. In this regard, there is absolutely no difference between the chilled mirror technique and the relative humidity sensor technique.

Temperature stability is an important factor when measuring water activity. When measuring the water activity of a product, any temperature imbalance or temperature instability can change the partial pressure of water vapor generated by the product sample. Any type of sensor used to measure water activity, chilled mirror or relative humidity, is affected by this. When temperature is not stable, measurements typically take longer and are inaccurate.

The temperature of a product sample takes more than a few minutes to change from one value to another. Using a fast response instrument that features a temperature controlled sample holder makes sense only when each product sample is pre-conditioned to the temperature of measurement.

Instruments based on the chilled mirror technique typically claim an accuracy of
± 0.003 aw.

Instruments based on a relative humidity sensor claim a more modest ± 0.015 aw accuracy. Is the chilled mirror technique inherently more accurate when applied to the measurement of water activity? Here are three facts to consider:

# 1: A chilled mirror measures dew point as opposed to measuring water activity. Any instrument that uses a chilled mirror computes water activity from the value of both dew point and temperature. The resulting accuracy depends on the combination of the errors made on the measurement of both dew point and temperature.

# 2: At room temperature, the water activity of many foods is within the range of 0.800 to 1.000 aw. Within this range, measuring water activity with an accuracy of ± 0.003 aw would require dew point to be measured with an accuracy of ± 0.05°C, assuming  no error on the measurement of temperature.

Top-of-the-line chilled mirror instruments such as those used by national laboratories do not claim better than ± 0.10°C accuracy. A chilled mirror water activity instrument that claims an accuracy of ± 0.003 aw  effectively claims to be at least two times more accurate than the best laboratory chilled mirror instruments. Is this to be believed?

# 3: At room temperature and within the range of 0.800 to 1.000 aw, the ± 0.015 aw accuracy claimed by instruments based on a relative humidity sensor corresponds to a dew point accuracy of ± 0.15°C (similar to a good chilled mirror instrument). This makes sense because the conversion of both dew point and temperature into relative humidity is always less accurate at high humidity values than at low humidity values.

Considering the above, it is reasonable to suggest that the accuracy specification of
± 0.003 aw published by some manufacturers of chilled mirror water activity instruments possibly refers to the repeatability (or precision) of the instrument rather than to its actual accuracy. This kind or repeatability is matched by instruments using a relative humidity sensor.

The ability of a chilled mirror instrument to measure low water activity values depends both on the power available to cool the mirror and on the evacuation of heat away from the mirror. Chilled mirror based water activity instruments are usually severely limited and should not be used to measure products that are below 0.20 .. 0.30 aw. By contrast, a value of 0.01 Aw can be measured without problems with an instrument that uses a relative humidity sensor.

Many products contain volatile additives and some products are in the form of a fine powder. Because the chilled mirror operates at condensation, there is the tendency to trap airborne contaminants on the mirror. Since deposits on the mirror clearly affect measurement accuracy, frequent mirror cleaning is required.