AquaLab TDL 2 ABOUT AQUALAB
Data Communications: USB
Power: 110 to 220 VAC, 50/60 Hz
Warranty: One year parts and labor
2.2 AquaLab and Water Activity
Water activity (a
w
) is a measurement of the energy status of the
water in a system. The value indicates how tightly water is bound,
structurally or chemically, within a substance. Water activity is the
relative humidity of air in equilibrium with a sample in a sealed
measurement chamber. The concept of water activity is of partic-
ular importance in determining product quality and safety. Water
activity influences color, odor, flavor, texture and shelf-life of many
products. It predicts safety and stability with respect to microbial
growth, chemical and biochemical reaction rates, and physical prop-
erties. For a more detailed description of water activity as it pertains
to products, please refer to Section 3 of this manual, titled “Water
Activity Theory.”
2.3 How TDL Works
The TDL uses a tunable diode laser to measure the water activity of
a sample. The sample is equilibrated with the head-space of a sealed
chamber containing a tunable laser that shines light of a controlled
wavelength at a detector that receives the light from the laser. At
equilibrium, the relative humidity of the air in the chamber is the
same as the water activity of the sample. In the TDL, the vapor
pressure of the headspace in equilibrium is determined by the loss of
signal strength from the laser caused by the presence of water vapor
in the headspace. This vapor pressure is divided by the saturated
vapor pressure at the sample temperature, which is measured us-
ing an IR sensor, to give water activity. AquaLab then signals you
by beeping and displays the final water activity. Since the sample
temperature is directly measured and no sensor is being used to de-
tect humidity, all that is needed to make a measurement is vapor
equilibrium, which can happen in as little as two to three minutes.
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