Measurement Technology - The Electronic Nose Artificial Olfaction Technology - page 5

Table 1.1 (continued)

S.
no.

Input variable to transducer Output

variable of
transducer

Principle of operation Type of device

(sensor)

13 Force Displacement The application of force against a spring changes its length in proportion to the

applied force

Spring balance

14 Force/torque Resistance

change

The resistance of metallic wire or semiconductor element is changed by
elongation or compression due to externally applied stress

Resistance strain gage

15 Force Voltage An e m f is generated when external force is applied on certain crystalline

materials such as quartz

Piezo-electric device

16 Liquid level/thickness Capacitance

change

Variation of the capacitance due to the changes in effective dielectric constant Dielectric gage

17 Speech/Music/noise Capacitance

change

Sound pressure varies the capacitance between a fixed plate and a movable
diaphragm

Condenser microphone

18 Light Voltage A voltage is generated in a semiconductor junction when radiant energy

stimulates the photoelectric cell

Light meter/solar cell

19 Light radiations Current Secondary electron emission due to incident radiations on the photosensitive

cathode causes an electronic current

Photomultiplier tube

20 Humidity Resistance

change

Resistance of a conductive strip changes with the moisture content Resistance hygrometer

21 Blood flow/any other gas or

liquid or two phase flow

Frequency
shift

The difference in the frequency of the incident and reflected beams of
ultrasound known as Doppler's frequency shift is proportional to the flow
velocity of the fluid

Doppler frequency
shift
Ultrasonic flow meter