fluid_mechanics

446 Chapter 8 ■ Viscous Flow in Pipes
Magnetic sensor
IN
OUT
Flow
out
Flow
in
FLOW
Turbine
F I G U R E 8.47
Turbine-type flow meter.
(Courtesy of E G & G Flow
Technology, Inc.)
Another useful pipe flowrate meter is a turbine meter as is shown in Fig. 8.47. A small, freely
rotating propeller or turbine within the turbine meter rotates with an angular velocity that is a function
of 1nearly proportional to2 the average fluid velocity in the pipe. This angular velocity is picked
up magnetically and calibrated to provide a very accurate measure of the flowrate through the meter.
Volume flow meters
measure volume
rather than volume
flowrate.
8.6.2 Volume Flow Meters
In many instances it is necessary to know the amount 1volume or mass2 of fluid that has passed
through a pipe during a given time period, rather than the instantaneous flowrate. For example,
we are interested in how many gallons of gasoline are pumped into the tank in our car rather than
the rate at which it flows into the tank. There are numerous quantity-measuring devices that provide
such information.
The nutating disk meter shown in Fig. 8.48 is widely used to measure the net amount of water
used in domestic and commercial water systems as well as the amount of gasoline delivered to
your gas tank. This meter contains only one essential moving part and is relatively inexpensive and
accurate. Its operating principle is very simple, but it may be difficult to understand its operation
without actually inspecting the device firsthand. The device consists of a metering chamber with
spherical sides and conical top and bottom. A disk passes through a central sphere and divides the
chamber into two portions. The disk is constrained to be at an angle not normal to the axis of symmetry
of the chamber. A radial plate 1diaphragm2 divides the chamber so that the entering fluid
causes the disk to wobble 1nutate2, with fluid flowing alternately above or below the disk. The fluid
exits the chamber after the disk has completed one wobble, which corresponds to a specific volume
of fluid passing through the chamber. During each wobble of the disk, the pin attached to the tip
Calibration gears
V8.14 Water meter
Pin
Metering
chamber
Flow
in
Flow
out
Casing
Diaphragm
Sphere
Disk assembly
F I G U R E 8.48
Nutating disk flow meter.
(Courtesy of Badger Meter,
Inc.)