Sec. 1-5] MECHANICAL INPUT TRANSDUCERS 101 



The resistances of both the part of the wire in the liquid and that 

 in the vapor atmosphere vary with the applied current, as shown in 

 Fig. (1-4) 10 (generalized characteristic, relative values). In the liquid 

 (curve a), the resistance increases only slowly with increased current 

 (only at a higher critical current value does the resistance increase 

 sharply). In the vapor (curve 6), the resistance rises with gradually 

 increasing slope. The exact location of these curves varies with the 

 medium and with the temperature. 



The sensitivity increases with decreased wire diameter. Platinum 

 wires (Wollaston wire, silver removed) of about 10 ju diameter or 

 platinum ribbons are recommended. The calibration characteristic 

 (current in the diagonal of a Wheatstone bridge versus liquid level) 

 is nearly linear if the vapor is at the same temperature as the liquid. 



A liquid-nitrogen-level indicator based upon this method is de- 

 scribed by Maimoni. 1 The instrument permits the determination of 

 the liquid level to within 0.1 in. and has a working range of 2 in.; 

 changes in the liquid level over this range causes the current in the 

 bridge diagonal to vary by 20 /liA. The power dissipation is 3.4 mW/ 

 in. Wexler and Corak 2 describe an instrument with a very small wire 

 which, when arranged in a horizontal position, permits the deter- 

 mination of the liquid level to better than 0.2 mm. 3 



1-5. Pressure Transducers 



The field is divided into two sections, one for transducers operating 

 at high pressures above atmospheric pressure and one for operation 

 at pressures considerably lower than atmospheric pressure (vacuum 

 gauges). 



Direct-acting transducers for the high-pressure range are based 

 either upon the variation of a resistance (1-51) or of a capacitance 

 ( 1-52) or upon piezoelectric effects (1 -53), electrokinetic effects (1-54), 

 or electric gas discharges (1-55). 



Direct-acting vacuum gauges are based upon either thermal con- 

 duction (Pirani gauge, 1-56) or upon ion currents in gases (Ionization 

 Gauges, 1-57). 



A great number of indirect-acting pressure transducers have been 

 described in which the pressure to be measured causes the mechanical 

 deformation of a membrane or of a Bourdon tube; the deformation 



1 A. Maimoni, Rev. Sci. Instr., 27, 1024 (1956). 



2 A. E. Wexler and W. S. Corak, Rev. Sci. Instr., 22, 941 (1951). 



3 See also E. H. Quinnel and A. H. Futch, Rev. Sci. Instr., 21, 400 (1950), and 

 M. S. Fred and E. G. Rauh, Rev. Sci. Instr., 21, 258 (1950). 



