96 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



contact points tend to deteriorate or corrode and that the indication 

 is discontinuous. 



b. Electrolytic Resistance. A continuous method to convert the 

 level variation of a moderately conducting liquid (water, sea water) 

 into electric signals is shown in Fig. (1-4)2. It consists of two wires 

 A and B held at a constant distance a, for instance by means of insu- 

 lating spacers C, and immersed in the liquid. The resistance between 

 the wires varies in inverse proportion to the height h of the liquid 

 level. Therefore, if a constant voltage is applied, the current through 



=-n= 



f 



c 



-B 



f l 



Fig. (1-4)2. Liquid-level 

 transducer for use in con- 

 ducting liquids (electro- 

 lytes). 



Fig. (1-4)3. Equivalent 

 circuit of the electrolytic- 

 level transducer shown in 

 Fig. (1-4)2. R, resistance of 

 the electrolyte; C, capac- 

 itance between the elec- 

 trodes; R F , equivalent loss 

 resistance of the capaci- 

 tance C. 



the liquid is proportional to the level h. The current sensitivity of 

 the method, AI/Ah (variation of the current / caused by a variation 

 of the liquid level h), decreases approximately hyperbolically with 

 the distance a between the wires. One can, therefore, increase the 

 current sensitivity by bringing the two wires close together, but 

 such a procedure will increase the difficulty of keeping the distance 

 between the wires constant, and a small variation in distance will 

 produce a large error. 



Electrolytic polarization makes it necessary, in most cases, to use 

 alternating current for the determination of the resistance variation. 

 The impedance between the terminals of A and B depends upon the 

 resistance as well as the capacitance between both wires and can be 

 represented by the equivalent circuit of Fig. (1-4)3. The magnitude 

 of the capacitance C can be considerable. 1 



Capillary forces between the wires and the liquid tend to cause 

 errors. Under static conditions (no movement of the liquid surface) 

 the liquid rises on the wire by an amount which depends upon the 

 wire diameter and the surface tension of the liquid and which is of 



1 W. Oesterlin, Arch. tech. Messen, V 1123-12, November, 1952. 



