Sec. 1-4] 



MECII.WWAL INPUT TRANSDUCE US 



97 



the order of a millimeter if the wires are clean and immersed in clean 

 water. Contamination of the wires can cause considerable variations 

 of this value. Under dynamic conditions, i.e., if the water rises or 

 falls, the water line around the wires can remain behind the true water 

 level by several millimeters. Where possible, the admixture of a 

 wetting agent to the liquid, to reduce surface tension, is recom- 

 mended. 



Variations of the resistivity of the liquid, brought about by a 

 change of concentration of dissolved matter or a change of tempera- 

 ture, will cause errors. Two methods have been applied to reduce 

 such errors. One method uses auxiliary contact points arranged at 

 fixed (known) levels h v h 2 , etc. As the liquid rises, its surface makes 

 electrical contact with one of these points. This occurrence is re- 

 corded by a mark on the continuous record of the liquid level from 

 the electrodes A and B. and the contin- 

 uous record can thus be calibrated. An- 

 other method uses an auxiliary electrode, 

 as shown in Fig. (1-4)4. Three electrodes 

 A, B, and C are immersed in the liquid. 

 A change of the liquid level causes a vari- 

 ation of the resistance between the elec- 

 trodes A and B but not of that between 

 B and C. The resistance ratio, measured 

 in a bridge or with a ratio meter, is prac- 

 tically independent of the conductivity 

 of the liquid. 



An instrument of this type which 

 measures the water level on a weir, and thus the flow rate, has been 

 described. 1 Ware 2 describes a liquid-level indicator which is based 

 upon the resistance variation between the liquid and a servosystem- 

 controlled contact bar. 



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

 ducer with compensation for 

 conductivity variation of the 

 electrolyte. 



1-42. Inductive Method 



An inductive method for the detection of the level of liquid sodium 

 in stainless-steel tubes has been described by Pulsford. 3 The arrange- 

 ment, Fig. (1-4)5, consists of two coils L x and L 2 of about 1,000 turns 

 wound around a steel tube T containing the liquid sodium. The 

 inductance of each coil is about 250 juH. One coil (search coil) can be 

 set at a predetermined level. The inductance of the search coil 



1 Anon., The Electrician (London), 89, 209 (1922). 



2 L. A. Ware, Electronics, 13, 23 (March, 1940). 



3 E. W. Pulsford, J. Set. Instr., 32, 362 (1955). 



