Lake Shore 370, Руководство пользователя

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Lake Shore Model 370 AC Resistance Bridge User’s Manual
APPENDIX C
HANDLING LIQUID HELIUM AND NITROGEN
C1.0 GENERAL
Use of liquid helium (LHe) and liquid nitrogen (LN
2
) is often associated with the Model 370 AC Resistance Bridge.
Although not explosive, there are a number of safety considerations to keep in mind in the handling of LHe and LN
2
.
C2.0 PROPERTIES
LHe and LN
2
are colorless, odorless, and tasteless gases. Gaseous nitrogen 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 properly cooled. A quick comparison between LHe and LN
2
is
provided in Table C-1.
Table C-1. Comparison of Liquid Helium and Liquid Nitrogen
PROPERTY LIQUID HELIUM LIQUID NITROGEN
Boiling Point @1 atm, in K
Thermal Conductivity (Gas), w/cm–K
Latent Heat of Vaporization, Btu/liter
Liquid Density, pounds/liter 4.2
0.083
2.4
0.275 77
0.013
152
0.78
C3.0 HANDLING CRYOGENIC STORAGE DEWARS
Cryogenic containers (Dewars) must be operated in accordance with manufacturer instructions. Safety instructions will
also be posted on the side of each Dewar. Cryogenic Dewars must be kept in a well-ventilated place where they are
protected from the weather and away from any sources of heat. A typical cryogenic Dewar is shown in Figure C-1.
NON-
MAGNETIC
KEEP
UPRIGHT
LIQUID LIQUID
HELIUM HELIUM
NON-
FLAMMABLE
Dewar.eps
Figure C-1. Typical Cryogenic Storage Dewar
Handling LHe and LN
2
C-1