American Magnetics 188CPS Point-Sensing Instrument - Manual (Page 18)

17
Calibration
Calibration procedure
Variations in the dielectric with changing density
For cryogenic liquids, the dielectric of the liquid and/or gas will change with a change
in density. The amount of change is dependent on the properties of the specific
cryogen. Figure 2-1 illustrates the variations in dielectric for nitrogen vs. pressure
under saturated conditions.
1
Since the 188CPS uses a capacitance-based method for
determining liquid level, such a change in the dielectric of the liquid can result in a
shift in the point sensing reading of the instrument. The 188CPS instrument is most
accurate when applied in situations where the operating conditions of the cryo-vessel
are relatively constant, i.e. the operating pressure and temperature of the cryo-vessel
are relatively constant.
As can be seen in Figure 2-1, if the operating conditions of the cryo-vessel approach
the critical point (far right in graph), then the discrimination range of the instrument
can be significantly compressed. This extreme compression of the dielectric range
between the liquid and gas can result in false indications of an exposed or immersed
condition.
If the dielectric of the target liquid is relatively constant, and the gas used to pressurize
the vessel does not display a significant dielectric shift, then the probability of the
188CPS displaying false indications is significantly reduced. If any questions exist in
regard to calibration issues, contact AMI for assistance in determining the optimal
calibration strategy.
1. Data obtained from NIST Standard Reference Database 12.
0 50 100 150 200 250 300 350 400 450 500
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Legend
Diel-1 (sat. liquid) Diel-1 (sat. vapor)
Pressure (psi)
Diel - 1
Figure 2-1. Dielectric vs. pressure for nitrogen under saturated conditions.
(The center line indicates the discrimination point for immersed or exposed indication.)