Model 137 Liquid Helium Level Monitor - Discontinued

The Model 137 is an advanced, multi-sensor liquid helium level monitor which incorporates AMI's patented sample-and-hold design with automatic helium level sensor vacuum burnout protection. The Model 137 is ideal for operation in systems where it is important to monitor the liquid helium levels of multiple containers while also minimizing the helium losses.

Multi-sensor capability

The Model 137 Liquid Helium Level Instrument incorporates multiple sensor input capabilities. Up to eight (8) liquid helium level sensors can be monitored on an individual basis by the Model 137. Selection between sensors is performed from the front panel of the instrument or via a remote communications program. All non-selected sensors are electrically disconnected from the instrument.

High and low alarm functions

The Model 137 provides one set of "High" and "Low" alarm setpoints which activate front panel LED warning indicators and rear panel relay outputs in the event of an overfill or liquid loss condition for the currently selected sensor. The "Low" level alarm also energizes an audible warning which can be silenced from the front panel. "High" and "Low" setpoints are programmable from 0 to 100 percent of sensor active length.

Minimal liquid helium losses

In order to minimize liquid helium loss, the Model 137 automatically energizes the currently selected liquid helium level sensor at user programmable time intervals and monitors the normal (resistive) zone as it progresses from the top of the sensor toward the liquid surface. As soon as the normal zone reaches the liquid surface, the level reading is saved and the current in the sensor is turned off until the next sample interval occurs. An LED sensor current indicator is illuminated during each sample. Sample intervals are user programmable from the front panel and can be set between "0.0" (continuous reading) to "600.0" minutes or hours. A manual update switch provided on the front panel can also be set to obtain continuous readings during a helium transfer period or to manually force an immediate measurement.

Sensor burnout protection

The Model 137 provides automatic helium level sensor vacuum burnout protection. A liquid helium sensor energized in a vacuum environment will self-heat to the point of burnout within seconds. AMI’s 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.

Convenient display

The Model 137 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. Front panel sensor calibration allows the user to calibrate the instrument quickly and easily for any length sensor. Calibration can be performed in either inches or centimeters of sensor active length with automatic reading crossover between inches, centimeters, and percent.

Remote computer monitoring or controlled operation

The Model 137 can be provided with an optional 0-1 volt DC signal on 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. Computer interface options, including RS-232 Serial Port/Data Logger or IEEE-488, are also available.

Purchase of an RS-232 or IEEE-488 control option is highly recommended with the Model 137. The Model 137 internally maintains only one set of individual calibration and alarm settings. However, the use of an RS-232 or IEEE-488 option allows the user to develop a remote control program to select a sensor and save/restore the individual sensor calibration and alarm data. The LabVIEW drivers supplied by AMI support remote sensor selection and individual sensor calibration settings and can be used as is or modified for a particular application.

The Model 137 provides as standard a IEC-320 power inlet module with built in input voltage selector, dual line fusing and integral RFI line filter. The RFI line filter prevents external radio frequencies (RF) from disrupting the function of the Model 137 while also preventing any internal RF from escaping the instrument. The RFI line filter is provided for NMR environments.

Microprocessor-based electronics provide 0.1% readout accuracy. Nonvolatile, read only memory maintains instrument calibration without battery backup. Watchdog timer circuitry and low line voltage (brownout) detector prevent microprocessor lock-up and provide fail-safe operation.

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