Handbook
November 2011
/
59-UMC0071
/
Issue
01
Mercury
iTC
Page 136
LEVEL-METER DAUGHTER BOARD
Original Instructions
16.1.1.2
Operation of the board with a nitrogen-level probe
The nitrogen-probe consists of two concentric steel-tubes. The tubes act as the plates of
a capacitor, while the annular space between them acts as a dielectric. Liquid-nitrogen
has a relative permittivity of 1.45, so the capacitance of the section of probe under the
liquid surface increases by approximately 45%.
The probe-head includes an oscillator whose frequency depends on the capacitance of
the probe. Thus the oscillator frequency also depends on the level of liquid.
The probe-head requires no adjustment. the working range of the oscillator (5 kHz to
65 kHz) accommodates probes up to 2 metres long.
16.1.1.3
Explanation why different methods are used for the two liquids
Liquid helium has a relative permittivity of 1.055, which is similar to the relative
permittivity of helium gas at 4.2 K. The capacitance probe is thus not sensitive to the
liquid level when used with liquid helium.
It is possible to make a liquid-nitrogen probe using a wire made of a high-temperature
superconductor. However this probe would be much less robust than a capacitive probe.
16.2
Installing the level-meter board
16.2.1
Fitting the board
The level-meter board can be fitted in any expansion slot. Only one level meter board
can be fitted in a
Mercury
iTC, because of power consumption limitations.
1
Turn off electrical power to the iTC.
2
Remove the electrical power connector from the back of the unit.
3
Remove the screws that secure the lid of the iTC. There are 6 screws on either side
panel of the iTC and 3 screws on the rear panel.
4
Lift off the lid.
5
Remove the two screws that secure the board-clamp. Remove the board clamp.
Figure 16-1 Board-clamp
