Articles you may be interested inA simple ac bridge for detecting small resistance change Rev. Sci. Instrum. 80, 086104 (2009); 10.1063/1.3202284 Bridge for measurement of low resistance Rev. Sci. Instrum. 50, 671 (1979);

Real-time observation of the fluctuations in the temperature of a flowing gas may be obtained with a probe containing two geometrically similar thermocouples differing only in size, and an electronic analog circuit using commercially available integrated circuits. The thermocouple lag time constants may vary as gas flow rate varies, and need not be known. Only the thermocouple time-constant ratio, a substantially invariant function of geometry, needs to be known. Under favorable circumstances, even this ratio need not be known in advance and can be determined in situ. The improvement in frequency response depends on the magnitude of the temperature fluctuation and is limited by the quality of the electronics. The system is useful when the thermocouple time constants vary appreciably during the course of the temperature fluctuations because of correlative fluctuations in gas flow rate. A thermocouple simulator is used to adjust the analog circuit and to test its efficacy.

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Total pressure i n a c a l i b r a t i o n volume i s determined by measuring %he f o r c e on a t h i n c i r c u l a r d i s c , of accurately-known area, t h a t i s f r e e l y suspended i n a hole i n t h e container wall, so t h a t t h e d i s c i s s u b s t a n t i a l l y f l u s h with t h e wall. The d i s c almost f i l l s t h e hole, so t h a t t h e r e i s a narrow annular gap. A continuous flow of c a l i b r a t i n g gas, i n j e c t e d i n t o t h e container i n order t o maintain a desired pressure, passes through t h e annular gap t o a d i f f u s i o n pump. The r a t i o of pressures on t h e two faces of t h e d i s c i s on t h e order o f 100:1, so t h a t downstream pressure need be known only nominally i n order t o deduce t h e upstream pressure. Force on t h e d i s c i s measured by a balance t h a t i s c a l i b r a t e d i n s i t u with dead weights, In one arrangement, pressures i n t h e range o f 10-500 pTorr were measured with an estimated probable e r r o r o f ( 1 pTorr + I$).

A system for vacuum gauge calibration using the comparison technique