American Magnetics 135 & 136 Liquid Helium Level Instruments (CE-Marked) - Manual (Page 14)

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Introduction
Features
innovative microprocessor-based circuitry detects incipient sensor burnout
and de-energizes the sensor before damage can occur. A 5% increase in
sensor resistance will trigger this protection, causing the current to be
switched off for 6 seconds before attempting to resume normal operation.
1.1.4 Floating supply
Due to safety concerns, the high voltage power supply used for the sensor
is a floating supply. This minimizes the possibility of personal injury in the
inadvertent event of someone who is grounded coming in contact with the
energized sensor electrical wires.
1.1.5 Convenient display
The Model 135/136 is equipped with a 4-digit LED digital display which
provides liquid helium level indication in inches, centimeters or percent as
selected by a front panel switch. A front panel switch allows the user to
adjust the instrument quickly and easily for any length sensor up to 80
inches (203 cm). The sensor active length can be entered in either inches or
centimeters.
1.1.6 Microprocessor-based electronics
Microprocessor-based electronics provide 0.1% readout accuracy.
Nonvolatile memory maintains instrument calibration without battery
backup. Watchdog timer circuitry and low line voltage (brownout) detector
prevent microprocessor lockup and provide fail-safe operation.
1.1.7 Remote computer monitoring or controlled operation
The Model 135/136 is provided with a 0-10 volt recorder output. A 4-20 mA
current loop option is available in lieu of the recorder output. Available
computer interface options include RS-232/422 Serial Port/Data Logger or
IEEE-488.
The Model 135/136 may be optionally configured for a maximum of one
analog output option and one computer interface option.