128 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



temperature difference t 2 — t x constant. The electric power neces- 

 sary to keep this temperature difference constant is proportional to 

 the flow rate. 



The output of the system is an indication of the mass rate of flow 

 (in kilograms per second) rather than the volume rate of flow (in 

 cubic meters per second). A reduction of the reading to standard 

 temperature and pressure is not required, therefore. 



The method has been applied primarily to the measurement of 

 large flow rates, as high as 75,000 ft 3 /hr; the power requirements for 



Fig. (1-6)9. Flow-velocity transducer 

 system for gases, thermal method. 



Fig. (1-6)10. Thermal flow-velocity 

 transducer for gases, modification of 

 the system illustrated in Fig. (1-6)9. 



such rates are of the order of 1 kW. The restriction of the conduit by 

 the wires is negligible. Turbulence arising on the wires may be con- 

 sidered an advantage because it provides for better mixing. 



Errors are likely to arise from nonuniform heating of the gas, from 

 nonuniform distribution of the flow velocity, and from heat loss to 

 the environment (thermal insulation of the conduit is recommended). 

 Deposit of dust on the heater will tend to make the response slower 

 but will not affect the accuracy. Errors resulting from the variation 

 of the specific heat can become large if the composition of the flowing 

 gas is altered. The change of specific heat of gases with temperature 

 and humidity will generally be small and not cause any serious error. 



Thermoelements can be used instead of the resistance thermom- 

 eters. 1 The heat capacity of the thermal transducers, and thus 

 their inertia, can be kept very low so that the thermal system is 

 capable of following pulsations of the gas or liquid stream. 



1 G. W. Penney and C. F. Fecheimer, J. AIEE, 47, 181 (1928). 



