C Free-Space Correction
The Dewar is raised and the sample tube is cooled to cryogenic temperature. Again pressure
drops; when pressure has equilibrated, the value is recorded. Ambient and analysis free spaces
are calculated from (1) system volume, (2) system, ambient, and bath temperatures, and (3)
measured pressures. From these, the value of the portion of analysis free space at cryogenic
temperature which requires correction for nonideality can be determined.
This method may be undesirable if:
n
Helium is unavailable. Free space determination by the analyzer requires the use of helium.
n
Analysis speed is a major factor. A helium free space measurement of 10 to 15 minutes is
required.
n
The sample tends to absorb and retain helium for a prolonged period of time or if it adsorbs
helium.
CALCULATE
This method is the most rapid and efficient way of compensating for free space. Ensure the
following is accomplished:
n
Perform a blank analysis on the sample tube.
n
Load the blank analysis file data into the sample tube file.
n
Enter the analysis bath temperature (found on the p° and Temperature window).
n
Enter the sample mass and density (found on the Sample Description tab).
ENTER
This method allows for entering predetermined values for the ambient and analysis free spaces.
The values to enter may be obtained in one of two ways:
n
A pre-analysis free space calibration of the sample tube containing sample.
n
The total free space of an empty sample tube is measured and the displacement of the sample
calculated from its mass and density and subtracted from the total free space.
In either procedure, ensure that the level (or, in cases where the Isothermal Jacket is used, the
effective level) of the cryogen bath on the sample tube is the same when the analysis is performed
as it was when gathering data for free space calculations.
C - 2
TriStar II Plus Operator Manual
303-42800-01 (Rev M ) — Sep 2023
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