Micromeritics FlowSorb III 2305 Operator's Manual page 79

FlowSorb III
OTHER GASES AND COMPOSITIONS
Although nitrogen in helium (30/70) with liquid nitrogen as the cold bath pro-
vides the most frequently used conditions, there are occasions where it may be de-
sirable to employ a different set of circumstances. For example, some investi-
gators favor a 20/80 nitrogen-in-helium mixture when analyzing carbon blacks;
carbon dioxide in helium is sometimes preferred when surveying coals; and n-bu-
tane permits analysis at ice water temperature, a convenience at some sites.
Nitrogen mixtures with helium which differ by more than ± 1% from 30 mole %
nitrogen require that the instrument be calibrated to ensure linearity. This is ac-
complished by lowering the Dewar tray, depressing the DET. and X1 pushbut-
tons, and turning both the FINE and COARSE ZERO controls fully clockwise
until the stops are encountered. Establish a stable flow of the differing gas mix-
ture through the instrument and adjust the small screwdriver-adjusted potentiome-
ter on the right panel near the gas inlet until the display indicates the percentage
of nitrogen known to be contained in the mixture. Use equation 6 of Appendix A
to compute a new calibration factor for the mixture and then proceed otherwise
as with the 30% mixture.
The user should be aware that one-to-one surface area value correspondance with
nitrogen values or among the other gases is unlikely when using these other
gases. The area occupied by their molecules on solids is less investigated and ap-
parently varies more widely depending on the nature of the solid.
The proper concentration of a gas other than nitrogen for single-point surface
area measurement is selected on the basis of what concentration produces a rela-
tive pressure P/P
B.1 is to be used in combination with helium, the result is
since the vapor pressure of argon at liquid nitrogen temperature is 250 mmHg
and the saturation pressure of liquid nitrogen is taken to be 760 mmHg for pur-
poses of this illustration.
Table B.1 also lists basic physical property information as required in arriving at
the appropriate constant in equation 6 for a number of gases at selected tempera-
tures. By way of illustration, suppose 30 mole % n-butane mixed with 70 mole
% helium were to be used at ice water temperature to measure surface area. The
numerical constant in equation 7 would be
Aug 00
of approximately 0.3. For example, if argon as listed in Table
0
250 mmHg x 30.3
Ar concentration =
760 mmHg
(273.2/295.2) x 6.023 x 10
x [1-(0.3 x 760)/1060] = 9.15
x 100 = 9.9%
23 -20
x (46.9 x 10 /22414)
Appendix B
B-1