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4.2 Novasina Stable System
There are tons of literature about it, you can love it or hate it but it is proven by
physics and chemistry that a reliable, precise and accurate water activity measu-
rement requires an established humidity equilibrium between the free, availa-
ble water in the sample and the humidity of the air which is located above the
sample surface. A perfectly established humidity equilibrium requests endless
time. But no lab technician has endless time, thus a reasonable compromise
between sufficient accuracy and measurement time is needed. The Novasina
stable system is it exactly. By offering various settings, it can be perfectly adapted
to sample properties, taking different water-sample interactions (van der Waals
forces, hydrogen-bridges, coulomb forces etc) and layering effects (like liquid oil
which stays on top of the water layer) into the account. But how does it work?
4.2.1 Working principle of Novasina Stable System
The basic layout is quite simple. Once the water exchange between free availa
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ble water in the sample and humidity in the air move towards an eqilibrium, the
net amount of water which is exchanged becomes smaller and smaller. Therefo
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re, the change in air humidity /and water activity finally) becomes smaller in the
same way.
Now, monitoring the change of water activity by time tells something about the
degree of humidity equilibrium establishment.
Novasina‘s approach is to allow a change in value of 0.001aw (0.1%rH) within a
certain time frame. And the time frame is what you have to program in menu
„Chamber Settings“ -> Stable Mode. You can select from various presettings or
select a stability time manually. In this case, an additional submenu called „Stabi-
lity observation time“ is displayed.
For example, you program 2 minutes as stability observation time, then
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it does not mean that measurement is finished within 2 minutes
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it means, measurement is performed so long, until measurement value does
not change more than 0.001aw (0.1%rh) within 2 minutes.
Graphically, the stable system can be exaplined this way. Again, a stable time of 2
minutes has been programmed.
Measurement has started and instrument is always considerng the last two mi
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nutes and build the difference between entry and exit value:
The measurement continues until the change in water activity value is equal or
less than 0.001aw in 2 minutes: