46
Inlet Pressure (mv)
(
)
S
N
=
−
=
606 5
607 5 6055
303
.
.
.
603
604
605
606
607
608
609
610
Time
Figure 19 Method of Estimating Baseline Noise
Observe that the noise is estimated not by the peak-to-peak method, which is
inappropriate for random noise, but by a visual average of the highest and lowest excursions.
Specifications
:
DP baseline noise should be < 0.5 mV
LS baseline noise should be < 1.0 mV
C.3.c Computing Pump Pulsation Noise
The pulsation of the pump is an important factor in the viscometer baseline noise.
Remember that the bridge cancels most of the flow rate variations, but it cannot cancel all of
them. The actual pulsation noise of the pump would have to be measured at the pump head
itself using the pressure transducer normally provided with the pump. This may be a very
desirable thing to do, but it is not necessarily a good measure of the pulsation that is transmitted
to the detectors. The columns absorb some of the pump pulsation, for example. If the
pneumatic pulse dampener is installed in front of the viscometer, it will absorb a good deal
more.
For general purposes, the best measurement of the actual pulsation noise experienced
by the viscometer detector comes from the noise on the IP transducer signal. At the baseline
condition (with solvent in all four capillaries) this signal is directly proportional to the flow rate
through the viscometer bridge. A typical IP signal at baseline is shown in
Figure 20
.
