Water Activity
Water activity is the water in a material available to support the growth of microorganisms. 
Consequently, the measurement of water activity is important in areas such as food processing, pharmaceutical production, products affected by mold, and a host of products that change characteristics when exposed to moisture.
The basic idea of water activity measurement is
checking changes in relativity humidity over time. This is done by placing a test sample in a container closed to the atmosphere. Relative humidity is measured in the space above the sample.
Several measurement or relative humidity detection techniques are used. Most of these require many minutes to reach an endpoint.
Water Activity Test Results
To accelerate a test result, estimates of endpoints are made based on projections of the rate of change during the first few minutes of the test. These calculated results are sometimes referred to as Relative or Theoretical Water Activity. The difference between these forecasts and real end-point water activity can be significant.
Douglas Wright is President of Scigiene, a Toronto-based supplier of Water Activity Instruments. The following is a summary of his thoughtful dissertation on these differences. Here is Douglas's article.
Water Activity (aw) “REAL” versus Theoretical
What is REAL water activity?
Water activity or aw is the partial vapor 
pressure of water in a substance divided by the standard state partial vapor pressure of water. In simplified terms, aw is the measure of “free” water in food or other product of interest that is available to support the growth of microorganisms including spoilage organisms.
Time Required for Water Activity Equilibrium
The dilemma faced by many companies is that many foods take quite some time (sometimes hours) to actually reach this equilibrium. Factors such as particle size, food makeup (fats, proteins etc.) temperature, salt content, etc. can all affect how long it takes a sample to reach the “TRUE” equilibrium and therefore the TRUE water activity.
Software Forecasts Incur Errors
Many water activity meter suppliers based on customer demands, have developed software that takes several readings, and then projects the final water activity. These projections might give results in 5minutes or less, but they are not “REAL” water activity readings. As soon as you start doing software projections you introduce significant error.
In the case of Water Activity, the error of such projections is no less than +/-0.015 aw. This conflicts directly with many claims of accuracies of as good as +/-0.001 aw to +/-0.003 aw. As per the FDA, even 0.003 aw is “theoretical.”
More On Water Activity Measurement Errors
The use of a chilled mirror makes this accuracy theoretically possible. But as per the FDA studies, water activity changes by +/-0.005 for every 0.1°C variance between the product and the headspace being measured1. Yet, the best of these meters only claims to control temperature by +/-0.2°C or +/-0.01 aw.
Most aw meters use software that actually projects the aw number. When using the QUICK mode, even these companies state that you should only expect an accuracy of +/-0.015 aw not the theoretical
+/-0.003 aw! These facts throw the entire food industry into a conundrum if you are using 3 decimals. You can no longer trust the accuracy claims of your expensive water activity meters.
Adding the errors from software projections, and temperature variances alone (we have not even discussed other variables yet), your ultra-precise (?) water activity meter could be off by over +/-
0.025 aw! The regulatory bodies know this already and for this reason their requirements only show 2 decimals. Based on this, it could take you hours to get “REAL” water activity.
One Answer to Test Time And Test Accuracy Issues
GOOD NEWS. Scigiene comes to the rescue with our NEW Humimeter AW3 water activity kit.
In order to speed up true aw readings, we pr
operly homogenize a larger sample. The larger sample reduces variance seen by taking too small a sample, and the finer grind gives a much greater surface area for moisture transfer, for faster and more accurate TRUE water activity in under 5 minutes. This will give true accuracy and not software generated guessing!
Our Humimeter AW3 contains a small fan that speeds up the true equilibrium and prevents RH stratification that occurs in other makes.
Finally, to eliminate the temperature variance we use a thermally insulated sample holder to reduce/eliminate temperature variance, and thereby increasing our accuracy even further.
About Calibration
Another critical issue is the lack of frequent calibration due to the cost of single use standards.
Scigiene has developed a reusable saturated salt standard to encourage frequent and easy checks of your Aw meter. If this indicates drift, then our certified single use Aw standards can be used to further check and calibrate your device. This may be the single most important part of our program!
Theoretical 3 decimal accuracy claims would require a new meter with a clean sensor and just calibrated at precise temperatures. Chilled mirrors RH sensors are extremely delicate, and most people do not clean them and definitely not often enough. This results in serious drift over the year until the meter is sent to the supplier for servicing for cleaning and recertification. In the interim, drift over and above those discussed is occurring. In these instances, daily calibration checks are paramount.
By: Douglas Wright, President, Scigiene
Balancing Water Activity Test Time and Accuracy
In many areas of moisture measurement, there is a trade-off issue between production requirements for short test time and test results precision. These are usually resolved by concluding a balance of repeatability of tests on known quality acceptance samples.
Douglas Wright takes on these issues by dealing with sample preparation and sample environment in closing the test-time/precision gap. Check out the Scigiene solution here.
Another example of the conundrums I get myself involved within these rants, raves, and monologues. I hope this is helpful. If it is, please share it with your associates.
You might find the CSC Water Activity Resources. page of interest.
Art
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P.P.S. You might also like this video, Water Activity the Basics.
P.P.P.S. Click on this image to get into more on Water Activity and to see the instrument Douglas was talking about. The instrument that produces quick tests and accurate results.
