TM NO. 377 

 In 1961 Marine Advisors, Inc. of La Jolla, California, advertised a so= 

 called "ducted meter" for measuring current flow. This instrument consists 

 of a right circular cylinder within which is mounted an impeller having an 

 axis of rotation coincident with that of the cylinder,, Fluid flow through 

 the cjrlinder causes spinning of the impeller,, This spinning is detected as 

 an induced emf which is generated by iron slugs or magnets (fixed in the 

 impeller blades) as they pass an induction coil. Figure 11=2 is a schematic 

 diagram of the ducted meter. A detailed discussion of the system is giver- 

 later in this chapter. However , there are some important and possibly unique 

 dynamic characteristics of this device that should be examined here. 



The ducted meter is inherently much more responsive to current fluctuation 

 than the unshrouded or open impeller devices. Since the impeller intersects 

 with a straight line path through the cylinder, all flow through the cylinder 

 must produce an impeller rotation. The result, in effect, is that of an inverse 

 pump. Any pressure perturbation occurring at the entrance of the cylinder is 

 hydrostatic ally and almost instantaneously transmitted to the impeller blades. 



In this discussion of instrumentation, the term "response time" is used 

 as a measure of the degree to which an impeller sensor can respond (i.e., 

 commence to spin) to high frequency fluctuations of the fluid flow. Classi- 

 cally, response time is defined as that interval required for the impeller to 

 attain a rotation equivalent to a flow velocity which is (l » i), or about 

 63 percent of an instantaneously applied constant velocity. T§e response 

 time T R is derived from the equation of motion in appendix A s The time 

 response is given by 



__ I 

 'F? = 7 ( > (11-13) 



o 2 "1 \ 



where I - moment of inertia of the impeller (gm car)- s and K, (gm em sec ) is 



a function of the fluid viscosity and density. Obviously, there is a threshold 

 of dynamic pressure below which the combination of fluid pressure and shear on 

 the blades is too small to overcome the inertia and bearing friction of the 

 blade mounting, When this occurs the fluid would just flow around the blades 

 producing no spinning,, However, this threshold of flow response can be mini- 

 mized; (l) by making the impeller as lightweight as possible; (2) by using 

 very low friction pivot bearings upon which the impeller would be axially 

 balanced; and (3) by minimizing the magnetic force coupling between the impeller 

 blade ferromagnets and the pickup coils. 



This dynamic loading cannot occur with an impeller mounted semi -openly, as 

 with the Von Arx and Roberts type instruments, because the fluid pressing upon 

 the blades is completely free to diverge racially from the impeller. This 

 radial divergence reduces the efficiency of the impeller as a flow detector. 

 The inherent differences between a ducted and a semi. -open or open system are 

 analagous to the efficiency gained by using a wind tunnel to provide a pre- 

 scribed air flow at a given point, as opposed to using the same fan in the 

 open air c 



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