How to carry out a Water Activity (aw) Measurement

1. As a qualitative measurement it is important that variables are eliminated when performing measurements. This includes temperature and sample preparation.

2. Place a sample of the product to be measured into a Rotronic sample cup (14 or 40 mm deep).
Recommendation: fill up to within 3 mm of the rim if possible. The less air in the container, the faster the time to achieve equilibrium.
Important: Do not allow the sample to touch the measurement probe head! Contamination of the measurement head will falsify all subsequent measurements made with other product samples.

3. Place the sample cup into the sample holder.

4. Close the lid or place the measurement head on the sample holder.
Important: the probe and sample holder must form a tight seal. Only then is the system closed and equilibrium can be achieved. A clamp is available.

5. Water activity can be measured in two ways: either via a predictive model or by waiting until the water vapor pressure and the temperature have reached equilibrium within the measurement chamber. Rotronic devices have embedded Aw Quick software to perform this process quickly and repeatedly.

Temperature control

Water activity is sensitive to temperature. Measurements can only be made when the product sample, sample holder and measurement sensors are at stable temperatures. Many standards require measurements to be made at a specific temperature.

In situations where ambient temperature cannot be controlled or where you wish to test samples at temperatures away from ambient, a temperature controlled system should be used. Rotronic provides measurement solutions for both applications. In each case, Rotronic instruments are designed to maintain stable temperature through the use of large thermal mass or active thermal control for the AwTherm.

What measurement principle is used?

Rotronic measures water activity with a thin film capacitive sensor. The capacitance of the sensor changes based upon the number of water molecules in the air. Rotronic uses the same principle for relative humidity measurement as
0…1 aw = 0…100 %RH. The HygroMer WA1 humidity sensor has excellent long-term stability and accuracy.

How can I be sure that my measurements are correct?

All Rotronic probes are delivered with a factory calibration certificate. The probes are adjusted at three relative humidity points and at one temperature point. They are then calibrated at one humidity and one temperature point. The calibration/adjustment reference is ISO 17025 certified. Further calibration can be requested pre-delivery to ISO 17025 standards.

How long is the calibration certificate valid?

Rotronic recommends calibrating probes every 12 months, but depending on customer-specific standard operating procedures (SOP) each customer will define their own calibration interval. ISO 21807 states that a calibration is required before each measurement or series of measurements, at least once a day.

Calibration and adjustment options

In order to carry out calibrations and adjustments (if found to be necessary) during usage, Rotronic offers various solutions:

• EAxx-SCS Salt Solutions, available at the following relative humidity values: 0.5, 5, 10, 11, 20, 35, 50, 60, 65, 75, 80 & 95 %RH. The Rotronic salt solutions are all delivered with an ISO 17025 calibration certificate.



• HG2-S & XL temperature and humidity generators, which can generate temperatures between 0…60 °C and relative humidity values between 5…95 %RH. The HG2-S & XL is delivered with an ISO 17025 accredited HC2-S reference probe. The major advantage of the HG2-S & XL is that you have the possibility to calibrate your probe at the exact temperature and relative humidity levels that you require.
• ISO 17025 calibration: We can provide ISO 17025 accredited calibration for temperature and relative humidity. ISO 17025 certificates are often required by regulatory bodies and for company Quality systems.

The Rotronic range of water activity analysers can be calibrated and adjusted via various display units (bench top, handheld, PC).

Is there any way to accelerate the measurement if it takes too long?

Rotronic AW measuring devices offer two integrated modes: AwE and Aw Quick

AWE mode is the conventional aw measurement with automatic detection of equilibrium. The natural (or static) equilibration of most products typically varies from 30 to 60 minutes.

The Aw Quick function allows water activity measurement results in typically four to six minutes by using an algorithm to predict the full equilibrium value (water activity) of the measured product. The function is very nearly as precise as the regular measurement method, typically within 0.005 aw of the value obtained conventionally.

Both methods require a stable temperature.

Want to learn more? Read our blog about the importance of water activity measurement