Measurement Technology - The Electronic Nose Artificial Olfaction Technology - page 23

Ammeter and voltmeters, induction wattmeter, electrodynamometer, and AF
power output meter are used for power measurement.

1.9.11 Inductance Measurement

Inductance is the characteristic of an electrical circuit that opposes the initial,
prevent, or a change in value of current [ 8 , 9 ]. The main device that has an output
in the form of a change in inductance is the inductive displacement sensor.
Inductance is measured in henry (H). Inductance is the link between electric
circuits and magnetic fields.
An inductor is a coiled conductor. It is a device to accumulate energy in a
magnetic field. An inductor consists of a wire wound around a core material. Air is
the simplest inductor core material because it is constant, however for physical
efficiency, magnetic materials such as iron and ferrites are commonly used. The
inductor's core material, its length, and number of turns directly affect the
inductor's ability to carry current.
Inductor measurements can be made in series or parallel mode. In the case of a
large inductance, reactance at a given frequency is relatively large, so parallel
reactance becomes more significant than series reactance. A parallel equivalent
circuit should be employed to evaluate a large inductance.
Inductance is a basic electrical property of any coil. The inductance of a coil
depends on the number of turns, diameter of the coils, the length of the coil, and
the nature of the core. To get an accurate inductance measurement, the inductor
must be tested under actual conditions with current flowing through the coil.

1.9.12 Capacitance Measurement

A capacitor is basically consists of two conductors separated by a dielectric medium.
The variable to be measured will cause an effect either by increasing the distance
between two plates or by changing the dielectric constant [ 8 , 9 ]. Capacitance of parallel
plate capacitor (Fig. 1.8 ) whose plates are displaced by a distance d is given as:

C ¼ e 0 e r A = d
where A is the area of cross-section of the plates, e 0 e r are absolute and relative
dielectric constants of the medium.
Any change in A and d will change the capacitance C. However, a change in
separation d between the plates is simple and is widely used to sense the
displacement.
Devices having an output in the form of a change in capacitance include the
capacitive level gauge, the capacitive displacement sensor, the capacitive