Page 40 - Instrumentation Reference Book 3E

Fluid flow in closed pipes 25

(b)
Figure 4.30 (a) Principle of operation-electromagnetic flowmeter. (b) Electromagnetic flowmeter detector head: exploded
view.

Non-sinusoidal excitation Here it is possible to polarity reversal every cycle. Figure 1.3 l(a) shows
arrange that rate of change of flux density the ideal current waveform for pulsed d.c. excita-
dBldt = 0 for part of the excitation cycle and tion but, because of the inductance of the coils,
therefore there is no transformer action during this waveform cannot be entirely achieved. The
this period. The flow signal is sanipled during solution as shown in Figure 1.31(b) is to power
these periods and is effectively free from induced the field coils from a constant-current source giv-
error voltages. ing a near square-wave excitation. The signal
Square-wave, pulsed, and trapezoidal excita- produced at the measuring electrodes is shown
tions have all been employed initially at frequen- in Figure 1,31(c). The signai is sampled at five
cies around 50 Hz, but most manufacturers have points during each measurement cycle as shown,
now opted for low-frequency systems (2-7 Hz) microprocessor techniques being utilized to evalu-
offering the benefits of minimum power con- ate and separate the true flow signal from the
sumption (Le., only 20 per cent of the power used combined flow and zero signals as shown in the
by a comparative 50 Hz system), automatic com- equation in Figure 1.31(c).
pensatioii for interfering voltages, automatic zero
adjustmeit, and tolerance of light build-up of
material on electrode surfaces. Area of application Electromagnetic flowmeters
An exarr-ple of this type of technique is illus- are suitable for measuring a wide variety of
trated in Figure 1.31, where square-wave excita- liquids such as dirty liquids, pastes, acids, slur-
tion is used. The d.c. supply tQ the coils is ries, and alkalis; accuracy is largely unaffected
switched on and off at approximately 2.6 Hz with by changes in temperature, pressure, viscosity,