Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

11.2 Measurement of Temperature and Its Units 311

Absolute Zero Temperature

Because both the Celsius and the Fahrenheit scales are arbitrarily defined, as we have explained,

scientists recognized a need for a better temperature scale. This need led to the definition of an

absolute scale, the Kelvin and Rankine scales, which are based on the behavior of an ideal gas.

You have observed what happens to the pressure inside your car’s tires during a cold winter day,

or what happens to the air pressure inside a basketball if it is left outside during a cold night.

At a given pressure, as the temperature of an ideal gas is decreased, its volume will also decrease.

Well, for gases under certain conditions, there is a relationship between the pressure of the gas,

its volume, and its temperature as given by what is commonly called the ideal gas law. The ideal

gas law is given by

(11.3)

where

Pabsolute pressure of the gas (Pa or lb/ft

2

)

Vvolume of the gas ( m

3

or ft

3

)

mmass (kg or lbm)

Rgas constant

Tabsolute temperature (Kelvin or Rankine, which we will explain soon)

Consider the following experiment. Imagine that we have filled a rigid container (capsule)

with a gas. The container is connected to a pressure gauge that reads the absolute pressure of

a

J

kg#K

or

ft#lb

lbm#R

b

PVmRT

■Figure 11.5

Examples of temperature-

measuring devices.

Copper

Constantan

T-type Thermocouple

RTD

Thermistor

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