Yokogawa ADMAG CA - Manual (Page 89)

IM 1E8B0-01E
11-2
11. OPERATING PRINCIPLE
11.2 Principal and Features of the capacitance Mag-
netic flowmeter
The measuring principle of the capacitance magnetic flowmeter (ADMAG CA) is
basically the same as that of a conventional wetted-electrode magnetic flowmeter. The
capacitance magnetic flowmeter, however, picks up the e.m.f. signal generated in a
liquid through the ceramic tube wall's capacitance by placing both electrodes outside the
metering tube (ceramic tube) making them non-wetted.
The ADMAG CA can offer stable measurement by taking the following measures
against fluid noise ( slurry noise and flow noise) which are problematic in flow measure-
ment by magnetic flowmeters.
(1) Slurry Noise
Slurry noise is the noise generated when solid matter in a fluid collides with the elec-
trodes. Against this noise, stable measurement is realized by employing a capacitance
detection construction in which the electrodes are not wetted.
(2) Flow Noise
Flow noise voltages generated when measuring low-conductivity fluids have the charac-
teristic, the higher the frequency, the smaller the noise voltage. The ADMAG CA
provides the flow measurement of fluids having a very low conductivity by adopting a
high-frequency excitation affected less by flow noise and by taking zero-stability
measures by improving the magnetic circuit and employing capacitance electrodes.
11.3 Capacitance Magnetic flowmeter circuit Configu-
ration
(1) Circuit Configuration
A capacitance magnetic flowmeter differs from a wetted-electrode magnetic flowmeter
in forming area electrodes outside the metering tube. The magnetic flow detector
consists of a ceramic tube, capacitance electrodes formed outside the ceramic tube, a
magnetic circuit including excitation coils and a core, earth rings and shield cases.
Since electrode impedances are extremely high for capacitance electrodes, driven shield
is implemented for capacitance electrodes by providing shield cases in the form of
enclosing the capacitance electrodes to prevent the effect of stray capacitance.
The converter is composed of the exciting circuit for dual square wave frequency
excitation, an amplify circuit to amplify the e. m. f. generated between capacitance
electrodes and digital circuit including an A/D converter and timing circuits.