American Magnetics 135 & 136 Liquid Helium Level Instruments (CE-Marked) - Manual (Page 13)

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1 Introduction
1.1 Model 135/136 Features
The American Magnetics, Inc. (AMI) Model 135/136 Liquid Helium Level
Instrument is an advanced, microprocessor-based instrument utilizing
AMI’s patented sample-and-hold principle with automatic helium sensor
vacuum burnout protection. The instruments are designed for unattended
operation in systems where it is important to monitor and/or control the
liquid helium levels and minimize the liquid helium losses.
The Model 135/136 is designed to be used in conjunction with an AMI
liquid helium level sensor. The liquid helium level sensor consists of a
small diameter NbTi filament in a hollow tube. A constant current is
passed through this filament causing the portion of the filament in helium
gas to become resistive, while the portion in the liquid helium remains
superconducting. The resulting voltage across the resistive portion of the
filament is read by the instrument, converted to a liquid level, and
displayed on the front panel LED display.
1.1.1 Minimal liquid helium losses
In order to minimize liquid helium loss, the Model 135/136 automatically
energizes the liquid helium level sensor at predetermined time intervals
and monitors the normal (resistive) zone as it progresses from the top of
the sensor toward the surface of the liquid helium. As soon as the normal
zone reaches the liquid surface the level reading is saved and the sensor
current is turned off until the next sample interval occurs. The SENSOR
CURRENT LED is illuminated during each sample. Sample intervals are
user programmable from the front panel and can be set between 0.0 (no
delay between samples) to 600.0 minutes or hours. A sampling mode toggle
switch provided on the front panel can be toggled for continuous readings
during a helium transfer period or for just a quick level reading update.
1.1.2 Automatic Level Control
The Model 136 provides automatic level control capabilities. Two
independent setpoints, A and B, are provided for determining a control
band to activate/deactivate a power receptacle on the rear panel. The
Model 136 is designed for unattended operation in systems where
automated refills are required.
1.1.3 Burnout protection
The Model 135/136 provides automatic helium sensor vacuum burnout
protection. A sensor which is energized in a vacuum environment without
contact with liquid helium will self-heat to the point of burnout. AMI’s
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