Mechanical Engineering Principles

252 MECHANICAL ENGINEERING PRINCIPLES

Scale

Float

Tapered

tube

Direction of

fluid flow

Figure 22.10

be achieved using a Pitot-static tube or, more often,
by using a weighing meter in an instrument repair
workshop.

Advantages of float and tapered-tube flowmeters

(i) They have a very simple design.

(ii) They can be made direct reading.

(iii) They can measure very low flow rates.

Disadvantages of float and tapered-tube flowmeters

(i) They are prone to errors, such as those caused

by temperature fluctuations.

(ii) They can only be installed vertically in a

pipeline.

(iii) They cannot be used with liquids containing
large amounts of solids in suspension.

(iv) They need to be recalibrated for fluids of
different densities.

Practical applications of float and tapered-tube
meters are found in the medical field, in instrument
purging, in mechanical engineering test rigs and in
simple process applications, in particular for very

low flow rates. Many corrosive fluids can be handled

with this device without complications.

22.11 Electromagnetic flowmeter

The flow rate of fluids that conduct electricity, such

as water or molten metal, can be measured using

an electromagnetic flowmeter whose principle of

operation is based on the laws of electromagnetic

induction. When a conductor of lengthLmoves at

right angles to a magnetic field of densityBat a

velocityv, an induced e.m.f.eis generated, given

by:e=BLv.

Velocity,

v

L

S

e= BLv

N

Direction of

fluid flow

Magnetic field

of flux density, B

Non magnetic

tube

Figure 22.11

With the electromagnetic flowmeter arrangement

shown in Figure 22.11, the fluid is the conductor

and the e.m.f. is detected by two electrodes placed

across the diameter of the non-magnetic tube.

Rearranginge=BLvgives:

velocity,v=

e

BL

Thus withBandLknown, wheneis measured, the

velocity of the fluid can be calculated.

Main advantages of electromagnetic flowmeters

(i) Unlike other methods, there is nothing directly

to impede the fluid flow.

(ii) There is a linear relationship between the fluid

flow and the induced e.m.f.

(iii) Flow can be metered in either direction by

using a centre-zero measuring instrument.