The American Magnetics, Inc. (AMI) Model 286 Liquid Level Controller (cryogen monitor) is a microprocessor-based instrument designed to provide multiple sensor capacitance-based monitoring and control of liquid levels. The Model 286 can be used for a wide range of auto-fill applications, with dewar sizes in industrial and laboratory applications down to the very small 0.25 liter dewars used in nitrogen IR detectors.

A typical control system consists of a Model 286 Liquid Level Controller, up to four AMI capacitance-based liquid level sensors, and connecting coaxial cable(s). The primary instrument sensing element is typically a 3/8 inch (9.5 mm) OD concentric-tube cylindrical capacitor constructed of stainless steel which allows the fluid to become the dielectric between the concentric cylinders. The instrument measures the sensor capacitance, which is directly related to the percentage of the sensor immersed in the liquid.

Multiple Sensor Monitoring and Control

The Model 286 is capable of monitoring four independent, capacitance level sensors or controlling liquid level in up to two vessels by directly controlling the actuation of two solenoid-operated flow control valves.

Factory calibrated

The Model 286 is typically calibrated by AMI to mated sensors in liquid nitrogen, and is ready for immediate use upon delivery. If the working fluid is something other than nitrogen, an approximate calibration can be performed or the customer may perform a calibration after installation when the sensor can be immersed in the target fluid.

Multiple calibrations per sensor

Each of the four sensor inputs to the Model 286 can have up to four independent calibrations, which are user-selectable. The user simply selects the desired calibration using the menu-type interface. All calibration data is passcode protected and stored in nonvolatile memory.

Convenient Display and Direct Keypad Entry

The instrument is equipped with a 16-character x 2-line backlit character-mode LCD display which provides liquid level and setpoint indication in inches, centimeters, or percent as selected by the user. 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 length, 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.

HI/LO Alarms

Two user-defined setpoints, HI and LO, are provided for all four channels with front panel LED indications for Channels 1 and 2. The HI and LO setpoints may also operate two pairs of low-current, rear panel relay contacts and a built-in audible alarm. The HI/LO relay contact pairs may be assigned to Channels 1 and 2, or to Channels 3 and 4.

Controller Modes

The Model 286 provides three controller modes: normal, auto-changeover, and pre-cool. The desired mode can be selected via the front panel keypad or remote interface. The level control sequence varies based on the selected mode. See the discussion provided in the Model 286 Manual at AMI's Technical Support page or the Model 286 Brochure for more information about the controller modes.

AMI Model 286 Auto-Changeover System with Vacuum Jacketed Manifold
Switching between LN2 supply tanks to keep critical system cold

Remote Operation

The Model 286 is capable of providing up to two analog 0-10 volt DC signals, which corresponds to 0-100% liquid level, accessible from the rear panel of the instrument for use with a voltage recorder. This is a factory-installed option and should be specified at the time the instrument is purchased. Optionally, up to two instrument-powered 4-20 mA current loop outputs can be ordered.

Digital interface options, including RS-232 or RS-422, are available. The serial remote interface provides a comprehensive command set for remote monitoring and configuration of all functions of the instrument.

View the Model 286 Manual (Technical Support page)

Model 286 Specifications at 25 °C

Level Measurements
0.1%, 0.1 cm, or 0.1 in
± 0.1 %
Analog Output
Integral Non-linearity:
± 0.012 %
16 bits
Total Error:
± 0.75 % for 4-20 mA output
± 1 % for 0-10 VDC output
Current Drift (4-20 mA):
75 ppm  / °C
Voltage Drift (0-10 V):
100 ppm  / °C
Power Requirements
90-132 VAC or 180-264 VAC 50-60 Hz, 
4.2 A max (25 VA plus sum of controller output currents)
Dimensions (Standard):
97 mm H x 213 mm W x 290 mm D
(3.8" H x 8.4" W x 11.4" D)
Weight (Standard):
1.9 kg (4.2 lbs)
Dimensions (Rack Mount):
89 mm H x 483 mm W x 290 mm D
(3.5" H x 19" W x 11.4" D)
Weight (Rack Mount):
2.2 kg (4.9 lbs)
RG-59U Extension Cable Length:
500 ft. maximum
Ambient Temperature:
Operating: 0 °C to 40 °C (32 °F to 104 °F)
Non-operating: -20 °C to 60 °C (-4 °F to 140 °F)
Relative Humidity:
0% to 95%; non-condensing
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