Handbook
November 2011
/
59-UMC0071
/
Issue
01
Mercury
iTC
Page 135
LEVEL-METER DAUGHTER BOARD
Original Instructions
16
LEVEL-METER DAUGHTER BOARD
This chapter describes the level-meter daughter board.
16.1
Description of the level-meter board
The level-meter board monitors the level of cryogen in a reservoir. The board can be
used with liquid helium and liquid nitrogen systems. The level-meter raises an error if
the cryogen level falls below a safe level.
The level-meter board uses a superconducting-wire probe to measure the depth of
liquid helium, or a capacitance probe to detect the level of liquid nitrogen.
There are two 9-way D connectors on the board; one for a nitrogen sensor and one for a
helium sensor.
16.1.1
Principles of operation
16.1.1.1
Operation of the board with a helium-level probe
The probe is a length of superconducting wire extending from the bottom to the top of
the helium reservoir. The probe is normally mounted vertically, although other
geometries are possible.
The section of wire below the liquid surface is cooled more efficiently that the section in
the gas above the liquid. The level-meter controls the current through the wire such that
the temperature of the section in the gas is above the critical temperature (and is thus
resistive), while the section in the liquid is below the critical temperature (and thus
superconducting). The length of the resistive section of the wire thus depends on the
level of liquid in the reservoir.
To minimise heating of the sample, the current in the wire is turned off after a
measurement has been made. Because there is zero current through the probe, the
whole length of wire becomes superconducting between measurements. When the next
measurement is made, it is necessary to introduce heat into the section of wire in the
gas to make it resistive. A small heater-resistor is mounted in thermal contact with the
top of the wire to heat a small section of wire. If the current in the wire is sufficient, the
resistive section propagates down the wire until it extends to the surface of the liquid.
When the measurement has been made, the current in the wire is turned off again. The
measurement is performed at intervals between a few second to a few hours,
depending on the expected rate of change of the liquid level. Do not perform
measurements too often, as each measurement boils off a small quantity of helium. A
fast sampling rate is available for use when the reservoir is being filled.
The maximum probe-length depends on the resistivity of the probe wire:
For probes up to 1.4 m long, a wire resistivity of 166 Ohms/meter is used, with a
current of 130 mA.
For probes up to 2 m long, a wire of resistivity of 61.2 Ohms/meter is used, with a
current of 250 mA.
