Page "government" Paragraph 272
from
Brown Corpus
The reproducibilities of helium vapor-pressure thermometers have been investigated in conjunction with a `` constant temperature '' liquid helium bath from 4.2 to 1.8 Af.
Surface temperature gradients have been found to exist in liquid helium baths contained in 15- and 25-liter metallic storage dewars.
The gradient was about one half of a millidegree at 4.2 Af but increased to several millidegrees for bath temperatures slightly greater than the **yl point.
A hydrostatic head correction has been neither necessary nor applicable in the determination of vapor pressures or temperatures for the bulk liquid helium.
However, the surface temperature gradient can produce erroneous vapor-pressure measurements for the bulk liquid helium unless precautions are taken to isolate the tube ( which passes through the surface to the vapor pressure bulb ) from the liquid helium surface.
It has also been observed, in helium 2, that large discrepancies can exist between surface vapor pressures and those pressures measured by a vapor pressure thermometer.
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