Sec. 1-5] 



MECHANICAL IXPUT T RA XSDUCERS 



107 



will acquire a net charge, the glass being negatively, the water mole- 

 cules positively, charged. A movement of the glass particles through 

 the water constitutes a current (streaming current) which can be 

 measured. Conversely, a movement of the liquid through a station- 

 ary fritted glass disk gives rise to a potential difference (streaming 

 potential) between both sides of the fritted disk. 



This phenomenon forms the basis of the electrokinetic pressure 

 meter. The system is illustrated in Fig. (1-5)4 and consists of a 

 porous porcelain disk and a con- 

 tainer filled with a polar liquid 

 (aceto-nitrile) on either side. If 

 the liquid is pressed through the 

 disk, a potential difference arises 

 between both sides of the porce- 

 lain disk which can be picked up 

 by means of two wire-mesh elec- 

 trodes. 



To measure static pressure, the 

 liquid would have to be pressed 

 continuously through the disk. If 

 alternating pressure is applied, the 

 liquid will be pressed back and 

 forth, and an alternating voltage 

 will arise between the electrodes. 

 A commercial form of this pressure 

 transducer operates over a range 

 from 10~ 4 to 10 2 psi. The output 

 is a linear function of the applied 

 pressure difference. The sensitivity is 350 mV/psi under no-load 

 condition and about 3 /jA/psi under short-circuit condition. The 

 output impedance is about 10 5 ohms, shunted by a capacitance of 

 40 fifxF ±10 jujuF, but can be made to have values up to several 

 hundred megohms. 



The output follows the applied pressure without delay. The dy- 

 namic response characteristic is flat within 1 dB, from 4 to 15,000 cps 

 and within 2 dB from 3 to 25,000 cps. The output varies slightly 

 with temperature; the output voltage changes by about 6 per 

 cent if the temperature is changed from 70 to — 10°F or to 

 + 140°F. 



For references see M. Williams, Rev. Sci. Instr., 19, 640 (1948); also C.E.C. 

 Recordings, 10(6), 6 (1956), and Bulletin 1573, both of the Consolidated 

 Electrodynamics Corporation, Pasadena, Calif. 



Impermeable 

 porceloin ring 



Porous porceloin 

 disk 



Thin diaphragms 



Polar liquid 

 [acetonitrile) 



Aluminum ribbons 

 ( output ) 



Fig. (1-5)4. Electrokinetic pressure 

 transducer [from C.E.C. Recording, 10, 

 6 (1956), Consoliiinteil Electrodynamics 

 Corp., Pasadena, Calif . ; by permission]. 



