Micromeritics TriStar II 3020 Operator's Manual page 127

TriStar II 3020
Field or Button
Target Pressures
Options button
(continued)
Dose Increments
option
302-42828-01 - Dec 2012
• Absolute / Relative pressure tolerance - values used to determine how
close the actual pressure must be to each target pressure from the
pressure table. At lower pressures, the relative tolerance value is lower.
At higher pressures, the absolute tolerance value is lower. For example:
Experiment 1: You have an absolute tolerance of 5 mmHg, a relative
tolerance of 5%, and a target pressure of 40 mmHg; 5% of 40 mmHg is 
2 mmHg. Since 2 mmHg (relative tolerance) is lower than 5 mmHg
(absolute tolerance), 2 mmHg is used. Therefore a minimum pressure of
38 mmHg (40 - 2) must be attained to collect data for a target pressure of
40 mmHg.
Experiment 2: You have an absolute tolerance of 5 mmHg, a relative
tolerance of 5%, and a target pressure of 200 mmHg; 5% of 200 mmHg
is 10 mmHg. Since 5 mmHg (absolute tolerance) is lower than 
10 mmHg (relative tolerance), 5 mmHg is used. Therefore a minimum
pressure of 195 mmHg (200 - 5) must be attained to collect data for a
target pressure of 200 mmHg.
Normally, surface area measurement points are widely spaced and the
resulting measurement is not very sensitive to the precise location of
points so wider tolerances may be used. Unnecessarily tight tolerances
lengthen the analysis.
The sample is dosed repeatedly with a fixed amount of gas and isotherm
points are collected after equilibrating each dose.
• Options button - each entry in the table defines how much to dose for a
segment of the isotherm. In this form of dosing, there is no predefined
pressure table. Define target ranges for up to 10 dose increments. The
ending relative pressure and the dose amount (entered or as the fraction
of the previous quantity adsorbed) for each target range are shown.
Description (continued)
Analysis Conditions Files
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