Fundamentals of Water Activity: Methods of Measurement
Two Methods of Water Activity Instruments are Available Image
There is no device that can be put into a product that directly measures the water activity. Rather, aw
is measured with an indirect method. Water activity is measured by equilibrating the liquid phase water
in the sample with the vapor phase water in the headspace of a closed chamber and measuring the relative
humidity of the headspace. Methods for water activity determinations are detailed in the Official Methods of
Analysis of AOAC International (1995). New instrument technologies have vastly improved speed, accuracy and
reliability of measurements. Reliable laboratory instrumentation is required to guarantee the safety of products
and enforce government regulations. Two different types of water activity instruments are commercially
available. One uses chilled mirror dewpoint technology while the other utilizes relative humidity sensors that
change electrical resistance or capacitance. Each has advantages and disadvantages. The methods vary in
accuracy, repeatability, speed of measurement, stability in calibration, linearity, and convenience of use.
Chilled Mirror Dewpoint
In a chilled mirror dewpoint system, a sample is placed in a sample cup which is sealed against a sensor block.
Inside the sensor block is a dewpoint sensor, an infrared thermometer, and a fan. The dewpoint sensor measures
the dewpoint temperature of the air, and the infrared thermometer measures the sample temperature. From these
measurements the relative humidity of the headspace is computed as the ratio of dewpoint temperature saturation
vapor pressure to saturation vapor pressure at the sample temperature. When the water activity of the sample and
the relative humidity of the air are in equilibrium, the measurement of the headspace humidity gives the water
activity of the sample. The fan is to speed equilibrium and to control the boundary layer conductance of the
dewpoint sensor.
Water Activity Measurement in Less than 5 Minutes
The major advantages of the chilled mirror dewpoint method are speed and accuracy. Chilled mirror dewpoint is a
primary approach to measurement of relative humidity based on fundamental thermodynamic principles. Chilled
mirror instruments make accurate (± 0.003 aw) measurements in less than 5 minutes. Since the
measurement is based on temperature determination, calibration is unnecessary, but running a standard salt
solution checks proper functioning of the instrument. If there is a problem, the mirror is easily accessible and
can be cleaned in a few minutes. For some applications, fast readings allow manufacturers to perform at-line
monitoring of a product's water activity.
Electric Hygrometers
Other water activity instruments use resistance or capacitance sensors to measure relative humidity. These
sensors are made from a hygroscopic polymer and associated circuitry that gives a signal relative to the
equilibrium relative humidity (ERH). Commercially available instruments measure over the entire water activity
range with an accuracy of ± 0.015 aw. Since these instruments relate an electrical signal to
relative humidity, the sensor must be calibrated with known salt standards. In addition, the ERH is equal to the
sample water activity only as long as the sample and sensor temperatures are the same. Accurate measurements
require good temperature control or measurement. Advantages of capacitance sensors include simple design and
inexpensive implementation.