American Magnetics 286 Multi-Sensor Liquid Level Instrument (CE-Marked) - Manual (Page 32)

18
Calibration
Effects of dieletric shifts
To minimize the effects of shifts in the dielectric of the target liquid,
perform a closed dewar calibration (see page 22) at the expected operating
condition of the cryo-vessel. If this is not feasible, then calibrate the sensor
at atmospheric pressure and use the approximate calibration method to
compensate for the shift of the dielectric when the cryogenic liquid is
under pressure. For this type of approximate calibration, the reference
liquid will be the target liquid at atmospheric pressure — see page 26 for a
detailed discussion of the approximate calibration method. If any
questions exist in regard to calibration issues, contact AMI for assistance
in determining the optimal calibration strategy.
If higher accuracy is required, please contact an Authorized AMI Technical
Representative with your requirements. AMI can supply a self-
compensating, capacitance-based, liquid level sensing system (e.g. the
Model 187) for applications in cryo-vessels with a wide range of operating
conditions, or for applications where increased accuracy over a range of
operating conditions is desired. The self-compensating design also
possesses the advantage of factory calibration for all cryogens, i.e. no
calibration is required of the user for most applications.
Note
All references to “dielectric constant” herein refer to the unitless
relative dielectric to
ε
0
(
ε
0
is the dielectric constant of a vacuum).
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
Diel-1 (saturated liquid) Diel-1 (saturated vapor)
Pressure (psi)
Die
l -
1
Figure 3-1. Dielectric vs. pressure for nitrogen under saturated conditions.