Sec. 1-4] 



MECHANICAL INPUT TRANSDUCERS 



95 





1-4. Liquid-level Transducers 



A direct conversion of a liquid-level position into an electric signal 

 is rarely used. In most cases the level position is detected indirectly, 

 either by the mechanical movement of a float which is connected to 

 a displacement transducer, by hydrostatic or pneumatic-pressure 

 measurements, by optical or acoustic means, or by gamma-ray 

 absorption (1-1 5) } 



1-41. Resistive Methods 



a. Contact Systems. The simplest direct-acting liquid-level trans- 

 ducer consists of one or more contacts at appropriate levels of the 

 liquid; an electric circuit is closed through the liquid when it reaches 

 the contact. If the resistivity of the liquid is too 

 high for convenient current measurement or if 

 the resistivity is variable, the level variation of 

 the liquid may be made to change the length of 

 a mercury column, so that the opening and 

 closing of the circuit take place between the 

 mercury and the contact points. A system of this 

 type with 10 contacts (in 10 U tubes) has been 

 described by Norman 2 and is schematically 

 shown in Fig. (1-4)1. 



The contacts can be connected to a circuit in 

 such a way that the variation of the mercury 

 column causes a stepwise variation of a resist- 

 ance. The resistance variations can be made to be a linear 

 function of the length of the mercury column or any other desired 

 function. A function in which the resistance variation is proportional 

 to the root of the mercury column displacement has sometimes been 

 used for orifice-type flow meters; in these meters the flow velocity 

 is proportional to the root of the pressure difference. Contact 

 manometer systems with electrical temperature and pressure 

 compensation (incorporating a second manometer and a resistance 

 thermometer) are described by Lohmann and von Grundherr, 3 and 

 by Geyger. 4 The contact systems have the disadvantage that the 



1 For a synopsis of such mostly mechanical systems, see Bulletin 1161, sec. 4, 

 pp. 8 and 9, Minneapolis-Honeywell Regulator Company, Philadelphia, Pa. 



2 E. E. Norman, Elec. World, 120, 635 (1943). 



3 H. Lohmann and F. von Grundherr, Arch. tech. Messen, V 1245-2, May, 

 1934. 



4 W. Geyger, Arch. Elektrotech., 28, 57 (1934). 



Fig. (1-4)1. Liquid- 

 level transducer with 

 multiple contacts. 



