American Magnetics 150A Portable Liquid Helium Level Meter (CE-Marked) User Manual

Page 4

March 6, 2001

V. MAINTENANCE

The instrument was shipped with a full charge on the batteries. Operating under a
maximum power consumption of measuring a 60" sensor in a near empty dewar, the unit
is capable of making approximately 1,000 level measurements before battery recharging
is required.

The failure of the front panel LED to light when a measurement is being made indicates
low battery power. If the front panel LED fails to stay lit during a measurement, the
batteries should be recharged. The supplied AC-DC adapter plugs into a 90VAC to
132VAC 50/60Hz (or optional 180VAC to 264VAC 50/60Hz for the European model)
source and into the jack provided on the side of the instrument. The batteries should be
charged for a minimum of 1 hour before taking more readings; subsequently, as soon as
it is feasible, batteries should be charged for at least 72 hours to fully charge the battery.

NOTE: Best battery performance will be realized by charging
the Model 150A continuously whenever it is not being used.

VI. CALIBRATION

The instrument is calibrated for a specific sensor length before shipping and should
never need adjustment. However, the calibration procedure is included for your
information. All AMI sensors have a resistance of 4.56 ohms/cm (active length) when in
the normal state and maintained just above the transition temperature of 10K. Thus, if
you are using a 50 cm sensor, you will need a 50 x 4.56 = 228.0 ohm resistor for
calibration purposes (ensure that the resistor power rating is sufficient and that its
temperature coefficient is adequately low such that self-heating due to the 75 mA
excitation current does not cause excessive shift in the resistor’s resistance).

NOTE: The active length of a sensor is usually 1" less than its
overall length.

NOTE: Disconnect the helium level sensor from the instrument.

CHANGING SENSOR ACTIVE LENGTH RANGE

NOTE: PERFORM THIS STEP ONLY IF NEEDED. There are
two sensor active length ranges — 1”-7.99” and 8”-60”. If the
instrument is currently calibrated for a sensor in a particular
range and the new sensor is in the same range, then this step
does not need to be taken—the range-setting jumpers are
already in the correct position. For example, if the instrument is
currently calibrated to an 18” sensor active length, and is being
re-calibrated to a 12” sensor active length, then the jumpers are
already both in the 8”-60” position; there is no need to open up
the unit. All needed adjustments can be made through the side
access holes in the lid.