Sec. 1-5] 



MKCIIA.XICAL INPUT THASSDUGERS 



111 



Empirical calibration is required. Typical calibration curves are 

 shown in Fig. (1-5)9. Since different gases have different heat con- 

 ductivities, the calibration curve for each will be different. For some 

 applications, notably in the absence of hydrogen, this difference may 

 be neglected. The Piraui gauge indicates the total pressure, i.e., the 

 sum of all partial pressures of any gases and vapors present. 



The ordinary range of the gauge is from about 1 to 10~ 3 mm. 

 Different investigators have extended the range to as high as 15 mm 1 

 and to as low as 10~ 5 mm Hg. 2 Extension to a limit of 5 x 10 -9 mm 



0.25 0.50 0.75 



Pressure, mm Hg 



[a) 



1.00 



10~ 3 10~ 2 10"' 



Dry air, pressure, mmHg 



Fig. (1-5)9. Characteristics of thermal vacuum gauges: (a) output from a commercial 

 thermocouple gauge for different gases; (6) resistance variation of a Pirani gauge. 



has been claimed by Ellet and Zabel, 3 who used a flattened-nickel 

 filament and a liquid-air bath surrounding the gauge, and by Weise, 4 

 who used a film of magnesium titanate 20 /u thick, with a surface of 

 about 10 cm 2 . This gauge has a range from 760 mm Hg to the lowest 

 measurable pressure. At high pressures (above 200 mm Hg) the 

 effect seems to reside in a cooling of the film brought about by a gas 

 convection current created within the vessel. 



Operation of the gauge over the full sensitive range causes, in 

 general, a resistance variation of 10 per cent, e.g., from 15.5 to 17 

 ohms. The time required for a thermal gauge to come to equilibrium 

 varies from several seconds to minutes. 



Pirani gauges show a variation of reading with ambient tempera- 

 ture. In order to compensate for ambient-temperature variation of 

 the gauge, a "dummy gauge," i.e., a gauge with similar physical 

 properties but operated in a closed and evacuated vessel, should be 

 connected in an adjacent bridge arm. A constant-temperature bath 



1 E. S. Rittner, Rev. Sci. Instr., 17, 113 (1946). 



2 A. M. Skellett, J. Opt. Soc Am., 15, 56 (1927). 



3 A. Ellet and R. M. Zabel, Phys. Rev., 37, 1102 (1931). 



4 E. Weise, Z. tech. Physlk, 24, 66 (1947). 



