Degassing Considerations; Sample Considerations - Micromeritics FlowSorb III 2305 Operator's Manual

Degassing the Sample

Degassing Considerations

Heating to the highest temperature consistent with the thermal stability of the sam-
ple gives the most rapid degassing. The upper limit of both a glass-fiber heating
mantle and a borosilicate glass sample tube is about 400 °C. Degassing at 200 to
250 °C for 15 to 20 minutes usually is adequate. Many materials degas well at
120 to 150 °C.
Repetitively adsorbing and desorbing nitrogen at the Test position can be em-
ployed as a degassing means for those materials that cannot tolerate elevated tem-
perature degassing.
There is only one sure way to establish degassing requirements. A sample is ade-
quately degassed when further treatment results in no increase in measured sur-
face area. With unfamiliar materials, you may wish to perform a series of tests
varying either time of treatment or temperature or, perhaps, both to establish de-
gassing conditions. (For further insight on establishing degassing parameters, con-
tact your local Micromeritics sales engineer and request Application Note No.
121, Establishing Sample Degassing Conditions for the FlowSorb.)

Sample Considerations

A portion of some very fine, fluffy powders may be carried from the sample tube
by the gas passing over the sample. This problem can be corrected by one of the
following:
•
Reduce the quantity of sample; such a fine powder is likely to exhibit a
comparatively great surface area. This increases the free space above the
sample and reduces the gas velocity over it.
•
Insert a bit of glass wool into the exit stem of the sample tube to confine
the sample. Facing the installed sample tube, the exit stem is to the right.
•
Reduce the gas flow rate; the only significant effect here is a lengthening
of analysis time. Select a number on the flowmeter below the standard
marked level, say 30, and adopt this as the standard mark. Note that both
calibration and testing must be carried out at the same flowmeter setting.
Make the test in all other respects as before.
The level of the liquid nitrogen in the Cold Trap during surface area measure-
ments should be held more or less constant by replenishing it as necessary every
20 to 30 minutes, preferably between analyses. Avoid adding liquid nitrogen
while an analysis is in progress. The small, temporary flow rate change thereby
introduced may slightly alter results. Rapid and disruptive detector drift may oc-
cur if the liquid nitrogen in the cold trap is allowed to evaporate below the level
of the cold trap tube.
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FlowSorb III
Mar 2012