Lake Shore Cryotronics 121, User Manual

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Appendix A: Handling Liquid Helium
and Nitrogen
A.1 General
Use of liquid helium (LHe) and liquid nitrogen (LN
2
) is often associated
with the Model 121 current source. Although not explosive, there are a
number of safety considerations to keep in mind in the handling of LHe
and LN
2
.
A.2 Properties
LHe and LN
2
are colorless, odorless, and tasteless gases. Gaseous nitro-
gen makes up about 78 percent of the Earth's atmosphere, while helium
comprises only about 5 ppm. Most helium is recovered from natural gas
deposits. Once collected and isolated, the gases will liquefy when prop-
erly cooled. A quick comparison between LHe and LN
2
is provided in
TABLE A-1.
Property Liquid Helium Liquid Nitrogen
Boiling Point at 1 atm 4.2 K 77 K
Thermal Conductivity (Gas), w/cm-K 0.083 0.013
Latent Heat of Vaporization, Btu/L 2.4 152
Liquid Density, lb/L 0.275 0.78
TABLE A-1 Comparison of liquid helium and liquid nitrogen