American Magnetics AMI Liquid Helium Level Sensors - Manual (Page 6)

Rev. 1 6
AMI
EXCELLENCE IN MAGNETICS AND CRYOGENICS
If only 10 inches (25.4 cm) of the sensor is above the liquid helium level then:
Q
h
= 0.028 watts • t
Q
v
= 0.431 joules
Q
s
= 0.143 watts • t
The third term, Q
s
, is again reduced to zero (due to sample-and-hold functionality) and
the sample time is approximately 1.27 seconds, thus the total heat input for a sample
is Q = 0.45 joules
The helium loss is 0.45 joules / (21 joules / gm) = 0.021 grams or approximately
0.166 ml.
The velocity of propagation is the most uncertain term in the calculation. These
calculations are intended to give you an idea of what the worst case helium losses are
and to demonstrate helium loss variability.
V. TROUBLESHOOTING
A. No level reading:
1. Ensure level meter is plugged in.
2. Ensure the leads are connected to the proper instrument terminals.
3. Ensure all lead wires are secure and are not broken.
4. Ensure the vessel is cold and capable of collecting helium.
B. Erratic or erroneous level reading:
1. Ensure there is no ice formations around sensor.
2. Ensure sensor is not installed in a restricted area.
NOTE: Anomalous behavior of the sensor may be seen, under some
conditions, at the lambda point of helium.
If the cause of the problem cannot be located please call an AMI Technical Support
Representative at (865) 482-1056.