American Magnetics 187 Self-Compensating Liquid Level Controller - Manual (Page 12)

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1 Introduction
1.1 Model 187 Features
The American Magnetics, Inc. (AMI) Model 187 Liquid Level Controller
system (patent-pending) is an advanced, microprocessor-based solution
designed to provide compensated, capacitance-based level monitoring and
control of virtually any cryogenic liquid over a wide range of operating
pressures and temperatures.
1.1.1 Compensated capacitance-based level sensing
The system consists of a Model 187 Liquid Level Controller, sensor,
connecting coaxial cables pairs, and an optional solenoid-operated fill
valve. The primary instrument sensing element is typically a 3/8 inch (9.5
mm) OD cylindrical capacitor constructed of stainless steel which allows a
non-conducting fluid to become the dielectric between the concentric
plates.
In addition to the primary cylindrical sensing element, circular-shaped
parallel plate capacitors are mounted on the sensing element in order to
provide “reference” data to compensate the primary capacitance
measurement. The instrument measures the primary sensing element
capacitance, compensates for any dielectric variations with respect to the
calibration conditions using the “reference” plates, and displays a level
reading. The sensors are normally constructed in one-piece lengths of up to
20 feet (6.1 m).
1.1.2 Factory calibration
The Model 187 is designed to be factory calibrated by AMI for a matched
sensor and is ready for immediate use upon delivery. The capacitance-
based primary and reference sensors provide feedback to the instrument to
measure liquid level for any non-conducting liquid and compensate for a
wide range of operating conditions — therefore, no user calibration
procedure is required, but the Model 187 provides the option of a user
calibration in addition to the factory settings.
All calibration data is password protected to prevent unauthorized
modification and is stored in non-volatile memory. The instrument also
provides a function to select between the user calibration and the AMI
factory calibration.