How Aqualab Works; Aqualab And Temperature - Decagon Devices AquaLab 4TE Operator's Manual

AquaLab
2. About AquaLab

How AquaLab Works

AquaLab uses the chilled-mirror dewpoint technique to measure the
water activity of a sample. In an instrument that uses the dewpoint
technique, the sample is equilibrated with the head-space of a sealed
chamber that contains a mirror and a means of detecting condensa-
tion on the mirror. At equilibrium, the relative humidity of the air
in the chamber is the same as the water activity of the sample. In
the AquaLab, the mirror temperature is precisely controlled by a
thermoelectric (Peltier) cooler. Detection of the exact point at which
condensation fi rst appears on the mirror is observed with a photo-
electric cell. A beam of light is directed onto the mirror and refl ected
into a photo detector cell. Th e photo detector senses the change
in refl ectance when condensation occurs on the mirror. A thermo-
couple attached to the mirror then records the temperature at which
condensation occurs. AquaLab then signals you by beeping and dis-
plays the fi nal water activity and temperature.
In addition to the technique described above, AquaLab uses an in-
ternal fan that circulates the air within the sample chamber to reduce
equilibrium time. Since both dewpoint and sample surface tempera-
tures are simultaneously measured, the need for complete thermal
equilibrium is eliminated, which reduces measurement times to less
than fi ve minutes.

AquaLab and Temperature

Samples not read at room temperature during the read cycle will
equilibrate with the AquaLab's temperature before the water activity
is displayed. Large temperature diff erences will cause longer reading
times, since a complete and accurate reading will not be made until
the sample and the instrument are within 2°C of each other.
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