heat to warm a room. Another example tells us that IO pounds of water at 80F will melt more ice in a given length of time than 1 pound of water at 100^o F. The former has more heat, but the latter has a higher temperature. Temperature is the measurement of heat intensity in degrees Fahrenheit or Celsius. Therefore, temperature measurements can be made by using a glass thermometer calibrated either in degrees Fahrenheit or Celsius. The generally accepted way of stating measurements of temperature in English-speaking countries is in degrees Fahrenheit.

The thermometer measures the degree of sensible heat of different bodies. The thermometer can make a comparison only between the temperature of a body and some definitely known temperature such as the melting point of ice or the boiling point of water. Figure 4-1 shows a comparison of the scales of Fahrenheit and Celsius thermometers. It also shows the marking of the freezing and boiling points of pure water at sea level. The range of the Fahrenheit thermometer between the freezing point and the boiling point is 180 (32 to 212 = 180). On the Celsius thermometer, the range is 100 (0 to 100 = 100) from the freezing point to the boiling point.

Figure 4-1. - Comparison of Fahrenheit and Celsius thermometers.

To convert Fahrenheit readings to Celsius:

(F - 32) 1.8 = C

To convert Celsius readings to Fahrenheit:

(C x 1.8) + 32 = F

The heat that can be measured by a thermometer and sensed or felt is referred to as "sensible heat." An example of sensible heat is presented by placing a small vessel of cold water over a gas flame and putting a thermometer in the water. Upon observation, you note that the thermometer indicates a rise in temperature. Also, if you place your finger in the water several times, you will feel (or sense) the change in temperature that has taken place.

The unit of measurement for a given quantity of heat is the British thermal unit, abbreviated and commonly known as Btu. One Btu is the amount of heat needed to change the temperature of 1 pound of water 1 Fahrenheit at sea level. If one Btu is added to 1 pound at 50F, the temperature of that pound of water will be raised to 51F.

All substances above absolute zero contain heat. There is heat even in ice, and its melting point is fixed at 32F. Because of a fundamental law of nature, when ice at 32F melts into water at 32F, a change of state takes place. The ice (solid) has turned into water (liquid). A certain amount of heat is required during this change of state. This heat is known as latent heat. Latent heat is the amount of heat required to change the state of a substance without a measurable change in temperature.

There are other types of heat that you will encounter in heating. These are as follows:

Specific heat - The ratio between the quantity of heat required to raise 1 pound of any substance 1^o F and the amount of heat required to raise the temperature of 1 pound of water 1^o F.

Superheat - The amount of heat added to a substance above its boiling point.

Total heat - Is the sum of sensible heat plus latent heat.

We previously mentioned absolute zero. But, what is absolute zero? Scientists have determined that when the temperature of a substance has been reduced to -460F that all the heat has been removed from a substance. At this point all the molecules cease to have motion. Absolute zero is the lowest temperature obtainable. Heat is present in all substances when the temperature is above absolute zero.