Sec. 1-6] 



MECHANICAL INPUT TRAXSDl VKRS 



12!) 



Fig. (1-6)11. Hot-wire probe for flow- 

 velocity measurements in gases. 



Goodyear 1 has used thermistors for flow-velocity measurements. 

 He claims that velocities as low as 10 -3 cc/hr can be measured and 

 that an operating range of 1 to 10 5 can be obtained. 



A modification of this method for the measurement of very high 

 and rather localized fluid flow velocities has been described by 

 Cady. 2 The method, called "electronics transit-time anemometer 

 (ETTA)," operates in the following way: A fine wire 0.004 in. in 

 diameter and -| in. long is stretched, transverse to the Mow direction; 

 a current pulse of 40 msec dura- 

 tion is applied that heats the 

 wire and the passing air. Farther 

 downstream is a similar wire 

 acting as a thermal transducer; 

 its resistance variation caused 

 by the passing cloud of hot air 



is measured oscillographically. The transit time should be between 

 50 /<sec and 6 msec. The method is applicable to flow velocities 

 from 100 to 5,000 ft/sec. The accuracy is of the order ±2 per cent. 



A similar method, described by Walker and Westenberger, 3 per- 

 mits the measurement of gas velocity in the range of 40 to 400 cm/sec 

 with an error of less than 2 per cent. 



b. Hot-wire Probe. The basic arrangement is shown in Fig. (1-6) 1 1 

 and consists of a fine wire W of a material with a large temperature 

 coefficient of resistivity, such as nickel or platinum The wire is 

 heated by an electric current and cooled by the gas stream. The heat 

 loss causes a decrease of the wire temperature and thus of the wire 

 resistance. The rate of heat loss depends also upon the physical 

 characteristics of the medium and upon the geometry and the surface 

 structure of the wire. 



The system can be used in two ways, either the heater voltage or 

 the heater current is kept constant, and the resistance variations of 

 the wire are used as a measure for the flow velocity of the gas in the 

 vicinity of the probe. The transfer function, resistance variation 

 versus gas flow velocity, increases first steeply and, at higher veloc- 

 ities, with gradually decreasing slope. The method is suitable for 

 the measurement of small velocities. Alternatively, the resistance 

 of the wire, i.e., its temperature, is kept constant by varying the 



1 R. S. Goodyear, Elec. Mfg., October, 1956, p. 90. 



2 W. M. Cady, Physical Measurements in Gas Dynamics and Combustion, in 

 R. W. Ladenburg (ed.), "High Speed Aerodynamics and Jet Propulsion," vol. 

 9, sec. C, 3, part I, Princeton University Press, Princeton, N.J., 1954. 



3 R. E. Walker and A. A. Westenberger, Rev. Set. Instr., 27, 844 (1956). 



