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.
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.
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.
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.
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.
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.
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
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)
Level Instrumentation
