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AQUALAB VSA
15. To choose to have the isotherm loop, highlight “Loop” and press the Enter button to
toggle between on and off.
Turning loop on causes the isotherm to automatically return to the start a
w
(with the
same settings) once the final a
w
has been achieved. This would create both an adsorption
and desorption curve (or vice versa depending on the starting direction).
16. When finished adjusting the settings for the stage, press the Save button to save and
return to the Edit Stage screen.
Press the Cancel button to return to Edit Stage screen without saving changes.
17. After setting up all stages on the Edit Stage screen, select the double Right arrow.
18. Follow the message on the screen to place an empty stainless steel cup in the chamber
and seal it. Press the double right arrow to tare the cup.
19. Follow the message on the screen to place the sample in the cup and select the double
right arrow to weigh the sample.
20. When filling the sample cup, remove the cup from the chamber to avoid spills in the
chamber area.
21. Press the Enter button to begin the test.
7.4 USING DDI TO INVESTIGATE GLASS TRANSITION
To investigate glass transition events using dynamic isotherms, run a DDI test with the
initial water activity set to 0.1 a
w
, final water activity > 0.80 a
w
, flow rate of 80 to 100 ml/min,
resolution of 0.01 a
w
and no timeout should be used.
7.5 CREATING A WORKING ISOTHERM
Using the VSA Users can generate working isotherms using the VSA, as described in
Section3, though it requires several steps.
1. Isotherm Tests: Generate two isotherm tests using the VSA, one for adsorption and
one for desorption, with both starting at the “current” water activity value. A working
isotherm is an analysis of the sorption characteristics of a sample starting from its
native state. Generating this data requires the analysis of two sub-samples in the same
condition as the original sample.
2. Adsorption Curve: Analyze one sub-sample for adsorption from its current state. Set up
one stage of the test with your desired settings for method (DDI or DVS), temperature,
step value, flow rate etc., but the starting water activity must be “current,” and the final
water activity must be a value higher than the current water activity of the sample.
3. Desorption Curve: Analyze a second sub-sample for desorption from its current state.
The test is again setup with one stage with desired settings for method (DDI or DVS),
temperature, step value, flow rate etc., but the starting water activity must be “current,” and
the final water activity must be a value lower than the current water activity of the sample.
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