310 Chapter 11 Temperature and Temperature-Related Parameters
100
80
60
40
20
0
Ice water
100
80
60
40
20
0
Boiling water
■Figure 11.4
Calibration of a mercury
thermometer.water boils is assigned a value of 212. The relationship between the two temperature scales
is given by(11.1)
(11.2)
As with other instruments, thermometers evolved over time into today’s accurate instruments
that can measure temperature to 1/100C increments.
Today we also use other changes in properties of matter, such as electrical resistance or
optical or emf (electromotive force) changes to measure temperature. These property changes
occur within matter when we change its temperature. Thermocouples and thermistors, shown
in Figure 11.5, are examples of temperature-measuring devices that use these properties. A ther-
mocouple consists of two dissimilar metals. A relatively small voltage output is created when a
difference in temperature between the two junctions of a thermocouple exists. The small volt-
age output is proportional to the difference in temperature between the two junctions. Two of
the common combinations of dissimilar metals used in thermocouple wires include iron /
constantan ( J-type) and copper/constantan (T-type). (The J and the T are the American
National Standards Institute (ANSI) symbols used to refer to these thermocouple wires.) A ther-
mistor is a temperature-sensing device composed of a semiconductor material with such prop-
erties that a small temperature change creates large changes in the electrical resistance of the
material. Therefore, the electrical resistance of a thermistor is correlated to a temperature value.T 1 °F 2
9
5
3 T 1 °C 24 32
T 1 °C 2
5
9
3 T 1 °F 2 324
Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀