American Magnetics 188CPS Point-Sensing Instrument - Manual (Page 16)

15
2 Calibration
Principle of operation
The capacitance-based method of measuring a point operates by measuring the
frequency of an oscillator, which is contained in the oscillator/transmitter unit. The
calibration MIN and MAX settings correspond to the maximum and minimum
oscillation frequencies, respectively, for a given gas/liquid configuration. As the liquid
level varies in relation to the location of a plate sensor, the value of the plate sensor
capacitance varies within a minimum and maximum value. The minimum capacitance
value occurs when the plate is fully exposed to gas. The maximum capacitance value
occurs when the plate is fully immersed in liquid.
Since the dielectric properties of gases and liquids can vary and the component
tolerances for the sensor and oscillator introduce variations, a calibration is required to
assure maximum accuracy for a specific plate sensor configuration installed in a
specific dewar.
Calibration procedure
Install the sensor in the target vessel. Refer to the particular drawings provided with
the sensor for proper connection between the sensor and the 188CPS instrument.
The instrument should be energized with the sensor(s) connected to the instrument via
the oscillator(s) (see the system diagram on page 5).
1. Cool the sensor to the approximate operating temperature. Drain the target
vessel so that all plate pairs installed on the sensor are fully exposed to gas.
Note
It is important to cool the entire sensor assembly within the target vessel to the
approximate operating temperature. The capacitance of the coaxial cabling
within the sensor shifts as a function of very large temperature changes.
Therefore, calibration at room temperature and subsequent operation at
cryogenic temperatures is not recommended. The sensor should be thoroughly
cooled for best calibration accuracy and point sensing performance.
2. While the plate pairs are exposed, press the MIN calibration button. The
instrument will sound a constant tone for approximately 2 seconds. One short
beep will sound at the end of the MIN calibration indicating successful
calibration and storage to internal memory.