100 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



shown schematically in Fig. (1-4)8. It consists of two plates P x and 

 P 2 kept at a fixed distance and electrically insulated from each other. 

 An a-c voltage E n (700 volts, 60 cps) is applied between the electrode 

 P x and the water. The electrode P 2 assumes an a-c potential between 

 zero and that of P x ; this potential varies with the level of the water 

 surface. 



The system can be used in two different ways: With stationary 

 electrodes, the output voltage serves as a measure of the liquid level; 

 the output voltage decreases with increased water level. The transfer 

 characteristic (output voltage versus water level) is nonlinear. An- 

 other method of operation consists in the use of a servosystem which 

 moves the two electrodes P x and P 2 up and down and keeps the 

 distance between P 2 and the water surface constant. The latter 

 method furnishes more accurate results. In the practical embodi- 

 ment, the minimum distance between P 2 and the water surface is 

 2.5 cm. The instrument operates over a total range of water-level 

 variation of 4.5 cm, and the error is in the order of 0.5 mm. 



1-44. Thermal Method 



This method has been applied primarily for the level determination 

 of liquefied gases but is, in principle, applicable to other systems. A 

 schematic diagram is shown in Fig. (1-4)9. 



A fine wire W extends vertically through a container which is 



ftj 



Fig. (1-4)9. Thermal liquid- 

 level transducer. 



Current,/ 



Fig. (1-4)10. Character- 

 istics of the transducer 

 shown in Fig. (1-4)9: (a) 

 wire immersed in liquid; 

 (6) in gas. 



partly filled with a liquid. The wire is heated by a constant current. 

 The heat transfer from the wire to the liquid is greater than that from 

 the wire to the vapor. Thus, that part of the wire immersed in the 

 liquid will be cooler and its resistance lower than the part extending 

 in the vapor atmosphere. As the level of the liquid recedes, the 

 average temperature of the wire, and thus its resistance, will increase. 



