AMI 1700 - Relationship between Calibration and Sensor Length; Variations in the Dielectric with Changing Density

40 Rev. 2
Calibration
Capacitance-Based Level Calibration : Relationship Between Cal and Sensor Length
4.2.2 Relationship between Calibration and Sensor Length
The capacitance-based method of measuring the liquid level operates by
measuring the frequency of an oscillator, which is contained in the
oscillator/transmitter unit. As the liquid level varies, the value of the
capacitance varies proportionally. Since the dielectric properties of liquids
vary and the component tolerances for the sensor and oscillator introduce
variations, a calibration is required to assure maximum accuracy for a
specific sensor immersed in the target liquid. The calibration minimum
and maximum settings correspond to the maximum and minimum
oscillation frequencies, respectively, for a given sensor and target liquid
configuration.
The length setting of the instrument is only provided as a means of scaling
the 0% (minimum calibration) to 100% (maximum calibration) range of the
measurement to meaningful units of length. During the calibration it is
important to accurately measure the distance between the physical
locations on the sensor corresponding to the maximum and minimum
calibration points. The measured value for the length will be used in
configuring the instrument for operation.
4.2.3 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 4-6 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 (see page 53) 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 49 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.
1. Data obtained from NIST Standard Reference Database 12.