Sec. 1-2] MECHANICAL IX PUT TRANSDUCERS 43 



Probably the most accurate electrolyte transducer has been de- 

 scribed by Pappenheimer. 1 The S3 7 stem is used as a pressure-sensing 

 device; it consists of two cylindrical pressure chambers made from 

 quartz, with a cross section like that shown schematically in Fig. 

 ( 1-2)26. The two chambers are separated by a quartz or glass mem- 

 brane (0.25 mm thick, 12 to 15 mm diameter). The chambers are 

 rilled with isotonic sodium chloride solution (for biological experi- 

 ments) and a trace of detergent is added to lower the surface tension. 

 Silver tubes inserted at four places serve as electrodes and as conduit 

 to the places where the pressure is to be measured and to a reservoir 

 to fill the chambers. From about 70 to OS per cent of the resistance of 

 each cell is located in the gap G. The width of this gap changes when a 

 pressure difference between both chambers occurs. The performance 

 of this gauge is extraordinary ; its lowest useful pressure range is from 

 to ±2 mm Hg, the highest range from to ±1,000 mm Hg. The 

 accuracy is high ; 0.0 1 per cent of the full-scale range can be measured. 

 The liquid volume of the chambers is 0.2 to 3 ml (for different embodi- 

 ments), and the change of volume is in the order of 3 x 10 -6 ml/mm 

 Hg. The output level is 0.12 to 0.4 mV/mm Hg of pressure for 1 volt 

 applied to the bridge, the output impedance on each side, 1,000 to 

 3,000 ohms. The response time is short; to achieve 95 per cent of the 

 output for a sudden change of pressure, 2 to 12 msec are required 

 with critical damping. The influence of temperature variation is 

 negligible if the system is used in a balanced bridge. 



For the use as a displacement transducer, an electrolytic system 

 offers a number of advantages: The transducer can be made very 

 small, the force or energy requirements to cause a variation of the 

 conductive path can be made negligible for many applications, the 

 resistance of the gauge can be selected within wide limits by selecting 

 electrolytes of proper concentration, and the resistance variations 

 caused by a displacement of the electrodes can be large. 



Electrolytic transducers suffer in general from the variation of 

 electrolyte resistivity with temperature. The resistivity changes for 

 most aqueous solutions by about —2 per cent/°C, but can change by 

 as much as 8.9 per cent/°C (42.7 per cent sodium hydroxide solution, 

 see temperature transducers, Table 9). A solution with an extremely 

 small temperature coefficient is Magnanini's solution (121 grams man- 

 nite, 41 grams boric acid, 0.04 grams potassium chloride to 1 liter of 

 solution; the temperature coefficient, at 18°C, is —0.1 per cent/°C; 

 the resistivity is very high). A further difficulty, in particular if the 

 transducer is operated at direct current, arises through polarization 

 1 J. R. Pappenheimer, Rev. Sci. Instr., 25, 912 (1954). 



