American Magnetics 185 & 186 Liquid Level Instruments (CE-Marked) - Manual (Page 27)

Rev. 3 17
3 Calibration
Model 185/186 instruments are calibrated at the factory for a specific
length sensor for use in a specific liquid. The calibration length and
calibration liquid are listed on the calibration sticker on the bottom of the
instrument. If the factory calibration method utilized was approximate,
the calibration length will be noted as an approximate value.
3.1 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 MIN and
MAX 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% (MIN) to 100% (MAX) 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 MAX and MIN calibration points. The
measured value for the length will be used in configuring the instrument
for operation.
3.2 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 3-1 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.
1. Data obtained from NIST Standard Reference Database 12.