Sec. 1-6] MECHANICAL INPUT TRANSDUCERS 117 



leakage problems. Redhead 1 has succeeded in developing a gauge 

 (inverted magnetron) which is capable of measuring pressure in a 

 range from 10~ 3 to 10~ 12 mm Hg. 



The gauge is generally operated with direct current of 1,000 to 

 3,000 volts. It can be driven with alternating current since the 

 different geometry of the electrodes causes rectification, but the a-c 

 discharge extinguishes at a pressure below 2 x 10~ 5 mmHg. Start- 

 ing difficulties are encountered, even at d-c operation if the gauge is 

 turned on at low pressures. According to Mcllwraith, 2 such diffi- 

 culties can be overcome by momentarily flashing a filament that is 

 sealed in a side tube close to the gauge while the high voltage is 

 turned on. 



The magnetic fields used by different authors vary from 300 to 

 8,000 oersteds. The accuracy of a calibrated gauge is in general of 

 the order of 3 per cent. 



1-6. Transducers Responding to Flow Velocity of Liquids 



The only direct-acting flow-velocity transducer, in the strict sense 

 of the word "direct," is the inductive system described in 1-62. 

 All others can be considered as indirect (mechanoelectric) systems, 

 such as the resistive systems (1-61), the sonic system (1-64), or the 

 radioactive methods (1-65). The thermoelectric system (1-63) is also, 

 in principle, an indirect-acting system. 



A considerable number of methods for flow-velocity measure- 

 ments have been described which are combinations of mechanical 

 systems with electrical-displacement transducers. For instance, the 

 flowing medium can be made to cause a pressure difference in a pitot 

 tube or a restricted orifice and the pressure is measured with electric 

 transducers, or the flowing medium causes a displacement of the float 

 in a rotameter which is transformed in an electric signal. 3 Other 

 methods use rotating impellers driven by the liquid medium; either 

 the torque caused by the flow or the rotation velocity of the impeller 

 is picked up with electric transducers. These combination systems 

 are omitted from the description. A survey (mostly mechanical) of 

 basic solutions for flow measurement can be found in Ziebolz. 4 



Linearity between the flow velocity and the transducer output 



1 P. A. Rtdhead, paper read at the Physical Electronics Conference, M.I.T., 

 March, 1957. 



2 C. G. Mcllwraith, Rev. Set. Instr., 18, 683 (1947). 



3 E. C. Crittendon and R. E. Shipley, Rev. Sci. Instr., 15, 343 (1944). 



4 H. Ziebolz, Rev. Sci. Instr., 15, 80 (1944). 



