Modified version of Sievert's apparatus accurately calibrates low pressure measuring instruments. Metal-gas system is composed of hydrogen in two-phase equilibrium with erbium to obtain reproducible hydrogen pressures

Lundin, C. E.

English

NASA Technical Reports Server

.August 1970	 Brief 70-10223 
NASA TECH BRIEF 
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Use of Thermodynamic Properties of Metal-Gas

Systems as Low-Pressure Standards 
rumps 
Schematic Diagram of the Modified Sievert's Apparatus 
A method has been developed for accurately cali-
brating low-pressure measuring instruments, using 
the thermodynamic properties of a metal-gas system 
composed of hydrogen (H) in two-phase equilibrium 
with erbium (Er), to obtain reproducible hydrogen 
pressures.
A schematic diagram of a modified Sievert's appa-
ratus used for making precision measurements is 
shown. For the two-phase region, Er-H solid solution 
plus erbium dihydride, a thermodynamic relationship 
was established. The manner in which the equilibrium 
pressure of hydrogen over erbium varied as a function 
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
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will be free from privately owned.rights.
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of temperature and composition was determined ex-
perimentally for temperatures of 8200 to 1220°K and 
pressures of 4 x 10 1 to 10 torr. 
The data were fitted by the van't Hoff equation: 
–A log 10 P =—+B 
where P is the pressure (in torr) of hydrogen gas in 
equilibrium with the two phases (dihydride and solid-
solution), T is the absolute temperature, and A and B 
are constants. If the logarithm of pressure is plotted 
against the reciprocal of absolute temperature, a 
straight line is obtained. The slope and intercept of the 
straight line are represented by A and B, respectively. 
These constants were very accurately determined for. 
the erbium-hydrogen system. A statistical analysis of 
the measurement errors was performed. 
Data from 12 independent runs representing 252 
data points provided the following relationship: 
log 10 P -11,490 18 + 10.668 ±O.019 
T 
where the uncertainties in the measurements (the plus 
and minus terms of the above equation) are one-stand-
ard-deviation values determined by computer least-
squares analysis. For example, one standard devia-
tion, the uncertainty for a pressure of 10 torr, is 
0.34% of the magnitude of pressure plus the un-
certainty of the temperature measurement. Once the 
equilibrium equation is established, the corresponding 
erbium temperature can be calculated for the desired 
hydrogen pressure.
An erbium-hydrogen system will serve as a reliable 
secondary standard for the calibration of low-pressure 
measuring instruments. Metal-oxygen and metal-
nitrogen systems may also be suitable as secondary 
standards. 
Note: 
The following documentation may be obtained 
from:
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.65) 
Reference: 
NASA-CR- 1271 (N69-16099), The Use 
of Thermodynamic Properties of Metal-
Gas Systems as Reduced-Pressure Stand-
ards 
Patent status: 
No patent action is contemplated by NASA. 
Source: C. E. Lundin of 
Denver Research Institute 
University of Denver 
under contract to 
Langley Research Center 
LAR- 10452
. 
F_ 
Brief 70-10223	 Category 03


Use of thermodynamic properties of metal-gas systems as low-pressure standards

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