General Product Description - Novasina LabTouch-aw Operating Instructions Manual

2.

General Product Description

2.1 System Overview
The LabTouch-aw system has been specifically developed for determining the
water activity in a test sample like food, cosmetics or pharmaceuticals. Water
activity is defined as the availability of "free" water in a sample and should not
be directly compared with the water content (g water/ g sample). The water
activity is given as the aw – value and ranges between 0 (absolute dryness)
and 1 (100% relative humidity) and is a relative, sample-mass independent
measurement. Only this free water takes an active part in the exchange of
moisture with the ambient air and can possibly form the ideal medium for
microbiological growth on the surface and influences the biological functions
of microorganisms which has an impact on the microbiological stability plus
chemical and physical properties of a product.
Special laboratory equipment is required for water activity (aw -value)
determination which quantifies the air humidity over a sample in a closed
chamber after reaching the humidity equilibrium between free water in the
sample and water in the surrounding air.
The measured air humidity is directly proportional to the aw -value. An accurate
and reproducible measurement is only possible if the sample temperature is
kept constant (i.e. 25°C) and if the measurement is taken after reaching the
humidity equilibrium. It is obvious that the measurement of water activity always
take some time, means is not finished within a few seconds. The establishment
of the equilibrium is sample-dependent and can not be shortened anyhow due
to the fact that it is physics and chemistry.
An inevitable pre-requisite for determining water activity with extreme precision
is the use of an excellent humidity sensor that provides exact, reproducible
response over a very wide range (humidity from 0....100% rH), any hysteresis and
a good chemical robustness. Novasina has been developing special electrolytic
sensors that stand out for such unique properties for more than 50 years.
The integrated, resistive electrolytic LabTouch-aw sensor is based on the new
chemical substance the Novasina „Novalyte", which achieves the required
accuracy and reproducibility for the daily sample measurement in the lab.
5