Measurement Technology - The Electronic Nose Artificial Olfaction Technology - page 22

1.9.9 Resistance Measurement

It is normally seen that methods which involve the measurement of change in
resistance are ideal to those use other principles. This is because alternating as well
as direct current and voltages are suitable for resistance measurement [ 7 ].
The resistance of metal conductor is expressed by an equation that takes up a
few physical quantities. The relationship is R = qL/A; where

R Resistance, X

L Length of conductor, m

Cross-sectional area of conductor, m 2 A and

q Resistivity of conductor material, X-m

Any method of changeable one of the extent involved in the relationship can be
the design basis of resistance measurement sensor. The standard devices and
methods available for measuring change in resistance, which is measured in units
of ohms, include the DC bridge circuit, the voltmeter-ammeter method, the
resistance-substitution method, the digital voltmeter and the ohmmeter. Devices
that convert the measured quantity into a change in resistance include the resis-
tance thermometer, thermistor, the wire-coil pressure gage, and the strain gage.

1.9.10 Power Measurement

Power may be defined as the rate at which energy is transformed or made avail-
able. The power in a circuit at any instant is equal to the product of the current in
the circuit and the voltage across its terminals at that instant [ 8 , 9 ].
In a DC circuit if V is voltage and I is the current then power is given by,

P ¼ V I

If the circuit resistance is known power may be calculated from one of the
equivalent formula,

P ¼ V 2 = R ¼ I 2 R

In an AC circuit, the instantaneous power varies continuous as the current and
voltage go through a cycle of values. Power in an AC circuit is given by,

P ¼ V I cos/

V RMS Value of AC voltage

I RMS value of AC current

/ phase angel between voltage and current, and

cos / power factor to the AC current