Model 187 Self-Compensating Liquid Level Controller

Model 187 Features

The American Magnetics, Inc. (AMI) Model 187 Liquid Level Controller system (US Patent # 6,016,697) 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.

The Model 187 system is the culmination of AMI's experience with capacitance-based level measurements for cryogenic liquids. The errors introduced from dielectric shifts across varying operating conditions, and the complicated calibration procedures required for closed systems (such as pressurized liquid hydrogen or oxygen tanks) have been eliminated.

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). Longer multi-section lengths are available. Example sensor configurations are shown at right.

Factory calibration (ready for use on delivery)

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.

Convenient display and direct keypad entry

The instrument is equipped with a 16 x 2 backlit character-mode LCD display which provides liquid level and setpoint indication in inches, centimeters, or percent as selected by the user. A front panel keypad interface allows the user to easily view and modify the instrument settings. The default display indicates liquid level and fill/loss rate. The current liquid level is updated and visible in many of the display modes used in configuring various settings of the instrument.

The 4 x 4 keypad is provided for direct entry of values for setpoints, sensor lengths, and other functions. The keypad also provides a convenient menu-type interface for configuring various instrument settings that require selection from a list of options.

Automatic level control

The Model 187 provides four level setpoints, two of which (A & B) are control setpoints used to control liquid level through use of a solenoid-operated fill valve. The remaining setpoints (HI & LO) are alarm setpoints and can be set for other control or alarm functions.

The HI and LO setpoints operate relay contacts in addition to the front panel LED indications. LED indication is also provided for the A setpoint, B setpoint, and automatic controller (FILL) function of the instrument. All setpoints are accessible via the keypad. The automatic controller function can also be manually overridden or disabled using the keypad.

A timeout feature is also provided with the automatic controller function. The timeout specifies the maximum amount of time necessary for the Model 187 to fill the target vessel to the A setpoint. If fill is not achieved within the timeout period, the Model 187 ends the fill operation (de-energizes the rear panel controller output) and indicates expiration of the timeout feature by blinking the FILL LED and setting the appropriate flags accessible through the serial or GPIB remote interfaces.

Remote computer monitoring or controlled operation

The Model 187 can be configured to provide an optional analog 0-10 volt DC signal accessible from the rear panel of the instrument for use with a recorder. A 4-20 mA current loop option is available in lieu of the voltage signal.
RS-232 is provided as a standard digital interface for the Model 187. Additional digital interface options, including RS-422 or GPIB (IEEE 488.2 compliant), are available. The serial and GPIB remote interfaces provide a comprehensive command set for monitoring and configuring all functions of the instrument.

Model 187 Specifications at 25 °C

Model 187 Manual in Adobe Acrobat 3.0 format (709 KB)

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