Page 192 - Mechanical Engineers' Handbook (Volume 2)

9 Flow Rate 181

Figure 29 Swirlmeter (registered trademark, Fischer and Porter Co.). (From Ref. 67, with permission.)

to the flow, which then enters a diverging passage. The rotating flow field develops a stall
(a region of relatively low axial velocity), which rotates in the diverging passage. An ane-
mometer button in the wall senses the low-speed region each time it passes, generating a
pulse train. The frequency of this pulse train is linearly related to the volume flow rate.
Mass flow rate calculation requires knowledge of the volume flow and the density. The
equation is
total counts per minute
ACFM (29)
calibration constant
W ACFM (30)
act

SCFM ACFM act (31)
STD
Vortex-shedding flowmeters use a prismatic bluff body to generate the instability and
one of several means to sense the oscillations of the stagnation streamline, which occur when
the bluff body is shedding a vortex trail. A plan view schematic is shown in Fig. 30.
Again, these meters can operate over a turn-down ratio of 100 1 or better and claim
0.25% accuracy.
The advantages of the instability meters are due to their pulse train signal, which is not
easily obscured by random noise and can be transmitted over long distances.

9.7 Ultrasonic Flowmeters
An ultrasonic flowmeter is a device for estimating the flow rate in a duct based on measuring
the propagation velocity of sound waves along one or more paths through the fluid, assuming
the velocity profile in the duct to be known. There are two general types of ultrasonic
flowmeters: transit time and Doppler. In both types, a transmitter unit injects an ultrasonic