Temperature can be thought of as a description of the level of heat and also may be referred to as heat intensity. Heat level and heat intensity should not be confused with the amount of heat, or heat content. As a substance receives more heat, its molecular motion, and therefore its temperature, increases.
Most people know that the freezing point of water is 32 degrees Fahrenheit (32°F) and that the boiling point is 212 degrees Fahrenheit (212°F), figure 3. These points are commonly indicated on a thermometer, which is an instrument that measures temperature.
We must clarify the statements that water boils at 212°F and freezes at 32°F. These temperatures are accurate when standard atmospheric conditions exist. For example, in Denver, Colorado, which is about 5,600 miles above sea level, water boils at about 203°F. At the top of Mt. Everest, which is about 31,000 feet above sea level, water boils at about 167°F. Standard conditions occur at sea level with the barometer reading 29.92 in. Hg (14.696 psia); this topic is covered in detail later in this unit as part of the discussion of pressure.
Heat theory states that the lowest attainable temperature is 2460°F. This is the temperature at which all molecular motion stops and the temperature at which there is no heat present. This temperature is referred to as absolute zero. This is a theoretical temperature because molecular motion has never been totally stopped; scientists have actually come very close, dropping the temperature in a laboratory setting to within one-millionth of a degree of absolute zero.
The Fahrenheit temperature scale is part of the English measurement system used by the United States. This measurement system is also known as the I-P, or inch-pound, system. The Celsius temperature scale is part of the International System of Units (SI) or metric system used by most other countries. Some important Fahrenheit and Celsius equivalent temperatures are provided in figure 4.
If we have a Celsius temperature that we want to convert to a Fahrenheit temperature, we can use the following formula:
°F = (1.8 x °C) + 32°
For example, if we had a Celsius temperature of 20°C, we can determine the equivalent Fahrenheit temperature by plugging the Celsius value into the formula to get:
°F = (1.8 x °C) + 32°
°F = (1.8 x 20°C) + 32°
°F = 36° +1 32°
°F = 68°
So, 20°C = 68°F
If we have a Fahrenheit temperature that we want to convert to a Celsius temperature, we can use the following formula:
°C = (°F - 32°) / 1.8
For example, if we had a Fahrenheit temperature of 50°F, we can determine the equivalent Celsius temperature by plugging the Fahrenheit value into the formula to get:
°C 5 (°F - 32°) / 1.8
°C = (50°F - 32°) / 1.8
°C = 18° / 1.8
°C = 10°
So, 50°F = 10°C
Up to this point, temperature has been expressed in everyday terms. It is equally important in the HVAC/R industry to refer to temperature in engineering and scientific terms. Performance ratings of equipment are established using absolute temperatures. Performance ratings allow for easy comparison among equipment produced by different manufacturers. The Fahrenheit absolute scale is called the Rankine scale (named for its inventor, W. J. M. Rankine), and the Celsius absolute scale is known as the Kelvin scale (named for the scientist Lord Kelvin). Absolute temperature scales begin where molecular motion starts; and use 0 as the starting point. For instance, 0 on the Fahrenheit absolute scale is called absolute zero or 0° Rankine (0°R). Similarly, 0 on the Celsius absolute scale is called absolute zero or 0 Kelvin (0°K), figure 5. The Fahrenheit/Celsius and the Rankine/Kelvin scales are used interchangeably to describe equipment and fundamentals of this industry.
