AquaLab Vapor Sorption Analyzer
2. About the VSA
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determined using Decagon‘s patented chilled-mirror dewpoint
sensor. Drying of the sample is imposed by flowing dry air from a
desiccant tube across the sample. Wetting of the sample is
imposed by saturating the air with water before it enters the
chamber and flows across the sample. The water reservoir is
mounted to the temperature controlled lower block to ensure
humidity saturation and minimize temperature fluctuation.
The DVS method for static or equilibrium isotherms consists of
tracking sample weight change as the sample is exposed to
different controlled humidities. The sample is held at each
humidity for a preset time interval or until a steady state weight
change is achieved, the goal being to achieve equilibrium
between the sample water activity and the controlled humidity.
Typically, several humidity levels are chosen and preset during
the setup process. The instrument then tracks equilibrium
progress at each humidity level and automatically steps to the
next humidity when equilibrium requirements are achieved.
Weight change vs. time data is recorded allowing for determining
kinetics of sorption for each humidity level. Using less stringent
equilibrium settings for weight change or setting a reduced time
interval at each step will speed up the isotherm test, but may
give non-equilibrium results. In addition, setting smaller steps in
humidity will increase the data resolution of the isotherm, but at
the expense of much longer test times.
The VSA is a stand-alone instrument with an integrated pump
system that eliminates the need for an external gas cylinder,
however an external gas source can be used if desired. The
instrument consists of a case which houses the power supply, air
pump, balance, temperature controlled sample chamber, sensor
block, sensor and temperature control electronics, water
