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AQUALAB VSA
2.2 MEASUREMENT METHOD
The VSA is the only automatic isotherm generator that can create isotherms using both
dynamic and static methods. The VSA uses the Dynamic Dew Point Isotherm (DDI) method
to produce dynamic isotherms and a controlled humidity and balance system, commonly
referred to as Dynamic Vapor Sorption (DVS), to produce static or equilibrium isotherms.
The DDI method for dynamic isotherms is a water activity and gravimetric analysis method
that controls neither water content nor water activity, but dries or wets the sample and
measures water activity and water content during the wetting or drying process. The VSA
determines water content by weighing the sample using a high precision magnetic force
balance and water activity with METER’s patented chilled-mirror dew point sensor. The VSA
imposes drying of the sample by flowing dry air from a desiccant tube across the sample
and wetting of the sample by saturating the air with water before it enters the chamber to
flow across the sample. The VSA water reservoir mounts 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. Customers typically choose several humidity levels to preset during the set
up 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 versus 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 can speed up the isotherm test, but may give
nonequilibrium results. In addition, setting smaller steps in humidity 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 you can use an external gas source 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
reservoir, and desiccant supply. Setup is as simple as plugging the instrument in, installing the
desiccant tubes, and filling the water reservoir. Test parameters can be set using an on-board
interface or using a connected computer and VSA software package.
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