Page 396 - Mechatronics with Experiments

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Mercury in glass
thermometer

100 Fixed point:
boiling point (1 atm)

50 interpolated point
L
L
2
0 Fixed point:
freezing point (1 atm) FIGURE 6.52: Mercury-in-glass thermometer for
manual temperature measurement. It uses the
transduction principle that the volume of the mercury
expands as a linear function of temperature.

6.9.2 Temperature Sensors Based on Resistance
RTD Temperature Sensors A RTD (resistance temperature detector) temperature
sensor operates on the transduction principle that the resistance of the RTD material changes
with the temperature. Then the resistance change can be converted to a proportional voltage
using a Wheatstone bridge circuit.
A good approximation to the resistance and temperature relationship for most RTD
materials is
R = R (1 + (T − T )) (6.174)
o
o
where is the sensitivity of the material resistance to temperature variation. The construc-
tion of a typical RTD sensor is shown in Figure 6.53. The sensitivity constant for various
materials is shown in Table 6.1.

Evacuated
space

Resistive
1
coil
Pyrex tube
( 3 in.O.D.)
8
Mica cross form
FIGURE 6.53: Resistance temperature detector (RTD) sensor for temperature measurement.