15
Calibration
Calibration methods
unit from AMI, then the factory calibration will be adequate in most cases. However,
for the exceptions noted in the previous paragraphs (which are
approximate
calibrations), the customer should perform a more accurate open dewar or closed
dewar calibration. A customer performed calibration is also required for sensors that
are purchased as a separate item from the instrument, since the instrument and sensor
were not both available for calibration at AMI facilities.
Variations in the dielectric with changing density
For cryogenic liquids the dielectric of the liquid will change with a change in density.
The amount of change is dependent on the properties of the specific liquid. Figure 2-2
illustrates the variations in dielectric for nitrogen vs. pressure under
saturated
conditions.
1
Since the instrument uses a capacitance-based method for determining
liquid level, such a change in the dielectric of the liquid will result in a shift in the
level reading of the instrument. The calibration procedures described herein are 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.
To minimize the effects of shifts in the dielectric of the target liquid, perform a closed
dewar calibration at the expected operating condition of the cryo-vessel. If this is not
feasible, then perform an open dewar calibration at atmospheric pressure and then use
the approximate calibration method to compensate for the shift of the dielectric when
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
Diel-1 (saturated liquid)
Diel-1 (saturated vapor)
Pressure (psi)
Di
e
l
-
1
Figure 2-2.
Dielectric vs. pressure for nitrogen under saturated conditions.
Содержание 186
Страница 3: ...2 Introduction ...
Страница 13: ...12 Installation Configuring power ...
Страница 23: ...22 Calibration Approximate Calibration ...
Страница 29: ...28 Operation Sensor contamination ...
Страница 37: ...36 RS 232 Communication Data Logger Option Error Codes ...
Страница 47: ...46 IEEE 488 Communication Option Serial Poll Status Byte ...
Страница 55: ...54 Virtual Instrument Operation Running multiple GPIB devices ...
Страница 65: ...64 Index ...