48
[ ]
η
=
KM
a
a
K
PS
0.71
1.28 x 10
-4
PEO/PEG
0.65
5.88 x 10
-4
Remember that the molecular weights of standards are often in error by 5 - 10 % and in
some cases can be off more than that. So run more than one standard before concluding that
the viscometer is in error.
The sensitivity/response of RI detectors varies too much to specify exactly what
response should be obtained for a standard sample. It is best to reference to the standard
sample run upon installation.
The viscometer sensitivity factor can be checked by using the PS280K standard, which
has an IV value of 0.87.
D. TROUBLESHOOTING
D.1 PROBLEMS IN THE VISCOMETER BASELINE
Noise in the viscometer (DP) baseline can be characterized as either
random
,
pulsation
,
or
sporadic
type.
D.1.a Pulsation Noise in the DP Baseline
Pulsation noise in the DP baseline is a common problem. In most cases it arises from
the pump pulsation, but it may possibly come from electronic sources. To eliminate the latter
possibility, turn off the pump flow. If the pulsation noise on the DP goes away, electronic
sources are clearly eliminated. Assuming it is pump-related, there are three possible causes to
consider:
1.
Air bubbles in the DP transducer and/or the delay column.
Any air in the DP transducer cavities or in the delay column will amplify normal pump
pulsation response to unacceptable levels. To test for air bubble in DP or column:
a) Shut off flow and remove the pneumatic pulse dampener from its tee, inserting a plug
instead.
b) Restore flow to viscometer and wait until baseline is reestablished.
c) Open the plug on the pulse dampener tee while observing the DP baseline. If the DP
returns to near zero with less than 50 Pa overshoot, there is no air in the DP transducer
or in the column. If more than 50 Pa of overshoot is observed, an air bubble may be
present.
To eliminate air bubbles, simply open the detector purge ports and allow at least one
column volume of solvent to flow through
.
