45
C.3.a Measuring Capillary Bridge Balance
An important operating variable for the viscometer is the balance of the capillary bridge.
At the factory, the bridge is balanced to 1% or better. During use, the balance can deteriorate
due to partial plugging of one of the capillaries. The balance is determined by measuring the
pressures at baseline condition and using the ordinary formula to compute the
apparent
specific
viscosity due to the bridge imbalance. As seen earlier:
[A.2-1]
η
sp
DP
IP
DP
=
−
4
2
where
DP
= Pressure across middle of bridge =
( )
( )
P
P
2
2
+ −
−
IP
= Inlet pressure through bridge top to bottom
=
( )
( )
P
P
1
1
+ −
−
η
sp
= Specific viscosity of the solution =
η η
η
−
0
0
The procedure to check the balance is simple:
1)
Turn the flow on and allow to run a few minutes.
2)
Turn the flow off (disconnect at the inlet to the viscometer detector).
•
Zero the IP signal.
3)
Turn the flow back on again.
•
Record the initial jump of the DP signal and its settling value (Pa). An
initial jump value greater than 150 Pa indicates air is in the system.
•
Record the IP value (kPa). If air in the system has been ruled out as the
problem, compute the balance as below.
For example, if the inlet pressure IP is 30 kPa and DP reads 20 mV = 40Pa (with a
sensitivity of 0.5mV/Pa), the apparent specific viscosity with the flow through the balance is
easily determined:
(
)
η
sp
DP
IP
DP
=
−
=
×
−
=
4
2
4 0 040
30 2 0 040
0 00535
.
.
.
Guideline: If the balance gets worse than 3% (
η
sp
> 0.03 at baseline), steps
should be taken to clean the bridge, or replace it if necessary.
C.3.b Measuring Baseline Noise
The measurement of baseline noise can be done most rigorously using the actual digital
values of the signal to compute the rms or standard deviation about the mean. There may be
some applications where this degree of rigor is necessary, but in most chromatography
situations it is adequate to estimate the noise graphically as shown in
Figure 19
below.