American Magnetics AMI Liquid Helium Level Sensors - Manual (Page 3)

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AMI
EXCELLENCE IN MAGNETICS AND CRYOGENICS
IV. OPERATION
A. The liquid helium level sensor is designed to work with all AMI liquid helium level
instruments. The level meter will be calibrated for a specific length level sensor
(calibrated length will be marked on the calibration label of the level instrument).
NOTE: All sensors have a nominal one-half inch non-active portion at
the top and bottom of the sensor.
Further information on the helium level instrument is contained in the Installation,
Operations and Maintenance Instructions for the particular model instrument you
have purchased.
Helium consumption is a function of the power input to the sensor and will vary with
the current, temperature (resistance) and the length of the sensor. AMI has, under
ideal laboratory conditions, measured the helium consumption for a typical sensor to
be as low as 20 milliliters per hour. This was measured in an open dewar when the hot
gas did not contact the dewar walls. However, in typical installations the helium
consumption will be somewhat higher. The maximum helium consumption (at 70 milli-
amperes and 4.5 ohms/cm) would be 30 ml/hr/cm of active length. To minimize helium
consumption it is recommended the sensor be installed in accordance with the
installation instructions and the power to the sensor turned off at the level instrument
between measurements.
Liquid helium losses due to superconductive helium level sensors can be quite
variable. These losses, due to current in the sensor, are generally a function of physics
and not the manufacturer.
The sensor element is a very small diameter NbTi wire held in a vertical position. The
top of the wire has a small heater attached to initiate a resistive zone. If the current is
adjusted properly, the resistive zone will propagate from the heater area down to the
liquid helium level and will stop without penetrating below the liquid. It takes a rather
large amount of heat to maintain the filament in the resistive state in opposition to the
cooling effects of the surrounding helium gas. In the best case, the heated gas leaves
the system without transferring heat to the liquid helium. In the worst case such as in
a completely closed dewar, all of the heat from the sensor eventually finds its way to
the liquid and causes evaporation.
If the current in a sensor is left on continuously, large losses can occur. It is usually
only necessary to turn the electronics on when it is desired to know the level and then
turn it off. This procedure will minimize the helium losses. For those who want this
process automated, AMI has developed patented "Sample and Hold" instruments.
These instruments combine analog and digital electronics to measure the level on a
periodic basis. The measurement is made by turning on the sensor current and
monitoring the progress of the resistive zone. The instant the resistive zone is
determined to have reached the liquid helium level, the current is turned off and the