The document describes the monitoring instrumentation used for a helium liquefier system at the University of North Florida Physics Department. It discusses a hygrometer and purity meter that detect impurities like water moisture in stored helium gas and indicate when maintenance is needed. It also describes setting up and collaborating on a LabVIEW program to ensure proper operation of these instruments for efficient liquefaction of helium gas.

1. Monitoring Instrumentation for Helium Liquefier System
By Thomas Olejniczak with help from Abigail Coker
Department of Physics, University of North Florida, Jacksonville, FL, 32224
Liquid helium is in use in several research laboratories in the
University of North Florida Physics Department. The liquid
helium allows professors to study properties of materials at
temperatures near 4.2 K, the boiling point of helium. The
helium liquefier system allows recovery of the boil-off of the
liquid into gaseous helium and re-liquefaction of any stored
helium gas. For efficient operation of the system, constant
monitoring for the introduction of contaminants such as water
moisture and other gases becomes necessary. A hygrometer and
purity meter detect these impurities of the stored gas and
indicate when needed maintenance is required on the liquefying
system. The purpose of this lab was to ensure the proper
operation of the hygrometer and purity meter, which required
some research, installation, and collaboration with a group
editing an operational LabVIEW program.
FIG. 2. The Quantum
Design Advanced
Technology Liquefier
160 liquefies up to 22
– 27 liters of helium
per day and stores up
to 160 liters of liquid.1
FIG. 1. The Quantum Design Advanced
Technology Purifier purifies helium gas to
99.9995% at a rate of 30 liters of gas per
minute.1
ABSTRACT
Fig. 4. The GOW-MAC 20
Series Single Pass Thermal
Conductivity Analyzer tests
the purity of the gas prior to
entering helium. A standard
output of 0 – 5 millivolts is
monitored by LabVIEW. 3
Fig. 3. The Cermet II
hygrometer measures the
change in thermal
conductivity of a gas due to
the humidity present.. The
Cermet II has a standard
output of 4 – 20
milliamps.2
THE KAHN CERMET II HYGROMETER
Hygrometers are instruments used to measure the absolute
moisture content in a gas. The Kahn Cermet II hygrometer is
a ceramic dew point thermal hygrometer that measures
absolute humidity of a gas. A porous layer allows
transmission of water vapor into the sensor and the change in
thermal conductivity due to humidity is measured. 2
The GOW-MAC 20 Series Single Pass Thermal
Conductivity Analyzer compares the difference in the
thermal conductivity between a known purity sample of
Helium and the gaseous Helium to be tested. Four filaments
act as resistors in the purity meter to form a Wheatstone
bridge circuit to measure the difference between the two
gases. Changes in the rate of heat loss due to the thermal
conductivity of the gas changes the resistance of the
filaments and an imbalance in the bridge. 3
THE GOW-MAC GAS PURITY ANALYZER
The Helium Liquefier System consists of several important
instruments. To recover the boil-off of liquid helium the gas is
cycled through two Bauer compressor to store the gas in a bank
of cylinders. The gas stored in the cylinders is tested by the
hygrometer and purity meter. For efficient liquefaction of the
gas, the gas is directed into a purifier prior to being liquefied.
HELIUM LIQUEFIER SYSTEM
THERMAL CONDUCTIVITY
Thermal conductivity is the measurement of a material’s
ability to conduct heat. Thermal conductivity can be thought
of as a proportionality constant when dealing with a uniform
rod of cross-sectional area A and the conduction of heat
through the rod in terms of a temperature gradient,
The Wheatstone bridge circuit used in the purity meter can
accurately measure the voltage output of the circuit. The bridge
itself can be considered as two voltage dividers that provide
balance to the circuit. When the resistances change due to the
thermal conductivity the circuit becomes unbalanced and the
voltage difference for the test side of the circuit will provide
the purity level of the gas compared to the sample gas as an
output voltage.
where
𝑑𝑄
𝑑𝑡
is the flow of heat per unit time, 𝜅 is the material’s
thermal conductivity (W/mK), and
dT
dx
is the temperature
gradient (K/m) along the rod. Thermal conductivity for an
ideal gas relates the molecular velocity, 𝑣, mean free path, 𝜆,
and the molar heat capacity, cv of the gas,4
WHEATSTONE BRIDGE
ACKNOWLEGMENTS
Fig. 5. The Wheatstone Bridge circuit for
the GOW-MAC gas analyzer is shown
with the filaments as the resistors in the
circuit.3
1 http://www.qdusa.com/products/helium-liquefiers.html
2 KAHN Instruments, Inc. CERMET II HYGROMETER
Installation, operation and maintenance
manual.Wethersfielt, CT. May 2003.
3 GOW-MAC Instrument CO. Operating Manual: 20 Series
single pass binary gas analyzer. Lehigh Valley, PA. June
2012.
4
𝜅 =
𝑛 𝑣 𝜆𝑐v
3𝑁 𝐴
.
(2)
𝑑𝑄
𝑑𝑡
= −𝜅𝐴
𝑑𝑇
𝑑𝑥
, (1)