OTHER FLOW METERS
To measure this flow, the motion of the shaft generates a cone with the point, or apex,
down. The top of the shaft operates a revolution counter, through a crank and set of
gears, which is calibrated to indicate total system flow. A variety of accessories, such
as automatic count resetting devices, can be added to the fundamental mechanism, which
perform functions in addition to measuring the flow.
The hot-wire anemometer, principally used in gas flow measurement, consists of an electrically
heated, fine platinum wire which is immersed into the flow. As the fluid velocity increases, the
rate of heat flow from the heated wire to the flow stream increases. Thus, a cooling effect on
the wire electrode occurs, causing its electrical resistance to change. In a constant-current
anemometer, the fluid velocity is determined from a measurement of the resulting change in wire
resistance. In a constant-resistance anemometer, fluid velocity is determined from the current
needed to maintain a constant wire temperature and, thus, the resistance constant.
The electromagnetic flowmeter is similar in principle to the generator. The rotor of the generator
is replaced by a pipe placed between the poles of a magnet so that the flow of the fluid in the
pipe is normal to the magnetic field. As the fluid flows through this magnetic field, an
electromotive force is induced in it that will be mutually normal (perpendicular) to both the
magnetic field and the motion of the fluid. This electromotive force may be measured with the
aid of electrodes attached to the pipe and connected to a galvanometer or an equivalent. For a
given magnetic field, the induced voltage will be proportional to the average velocity of the fluid.
However, the fluid should have some degree of electrical conductivity.
Ultrasonic Flow Equipment
Devices such as ultrasonic flow equipment use the Doppler frequency shift of ultrasonic signals
reflected from discontinuities in the fluid stream to obtain flow measurements.
discontinuities can be suspended solids, bubbles, or interfaces generated by turbulent eddies in
the flow stream. The sensor is mounted on the outside of the pipe, and an ultrasonic beam from
a piezoelectric crystal is transmitted through the pipe wall into the fluid at an angle to the flow
stream. Signals reflected off flow disturbances are detected by a second piezoelectric crystal
located in the same sensor. Transmitted and reflected signals are compared in an electrical
circuit, and the corresponding frequency shift is proportional to the flow velocity.