Rising Pressures; Nonreproducible Results - Micromeritics 1305 Operator's Manual

Nonreproducible Results

Many samples such as organic materials or closed cell foam plastics allow helium
or other gases in the initial gas charge to slowly diffuse into the interior of the
sample. To determine whether the sample is responsible for any discrepancy, re-
move the sample and re-test with the chamber empty. While some such samples
may be run and useful information produced, they may require the development
of special procedures.
Samples colder than the instrument, especially those that are massive or non-po-
rous can cause falling pressures. Always try to store samples in the vicinity of
the instrument for a time sufficient to produce thermal equilibration before testing.

Rising Pressures

Pressures which rise after initial charging with gas are most frequently caused by
the presence of condensible vapors (usually water) on the sample. Continued purg-
ing of the sample, vacuum treatment, or repeated runs usually clear up this prob-
lem for most samples.
A leak through the FILL valve may cause rising pressures. Removal of inlet pres-
sure would then cause the direction of the leak to reverse, confirming the source.
Samples warmer than the instrument, especially those that are massive or non-po-
rous, can cause rising pressures until temperature equilibration takes place. Al-
ways try to store samples near the instrument for a sufficient period of time to
allow thermal equilibration.
Nonreproducible Results
Unstable pressures are the most common cause of lack of reproducibility. Pres-
sures should stabilize to better than 0.005 psig per minute before readings are
taken if the specified accuracy is to be achieved. Refer to the previous sections
for assistance in stabilizing pressures.
Timing of pressure readings is important. The charging pressure, P1 should be re-
corded just before the PREP/TEST valve is placed in the TEST position. The ex-
pansion pressure, P2 should be recorded as soon afterward as the pressure has
stabilized; should it not stabilize within 30 seconds, some difficulty exists.
Particles on the rim of the sample chamber or on the mating surface of the cham-
ber cap cause a volume change of the chamber and a corresponding error in the
sample result. Inspect to be sure that firm metal-to-metal contact is being made,
and that the cap has been fully seated.
4-2
Model 1305 Multivolume Pycnometer
Jun 99