TEMPERATURE

The temperature of an object is a measure of the heat level of that object. This level can be measured with a thermometer.

The temperature scales employed to measure temperature are the Fahrenheit (F) scale and the Celsius (C) scale. In engineering and for practically all purposes in the Navy, the Fahrenheit scale is used. You may, however, have to convert Celsius readings to the Fahrenheit scale, so both scales are explained here.

The Fahrenheit scale has two main reference points-the boiling point of pure water at 212°F and the freezing point of pure water at 32°F. The measure of a degree of Fahrenheit is 1/180 of the total temperature change from 32°F to 212°F. The scale can be extended in either direction-to higher temperatures without any limits and to lower temperatures (by minus degrees) down to the lowest temperature theoretically possible, absolute zero. This temperature is - 460°F, or 492°F below the freezing point of water.

In the Celsius scale, the freezing point of pure water is 0°C and the boiling point of pure water is 100°C. Therefore, 0°C and 100°C are equivalent to 32°F and 212°F, respectively. Each degree of Celsius is larger than a degree of Fahrenheit. Only 100° Celsius are between the freezing and boiling points of water, while this same temperature change requires 180° on the Fahrenheit scale. Therefore, the degree of Celsius is 180/100 or 1.8° Fahrenheit. In the Celsius scale, absolute zero is - 273°C. To convert from one temperature scale to another, use the following algebraic equations:

From Fahrenheit to Celsius

°C = 5/9 x (°F - 32)

From Celsius to Fahrenheit

°F=(9/5x°C)+32

shows the two temperature scales

in comparison. It also introduces the simplest of the temperature measuring instruments, the liquid-in-glass thermometer. The two thermom­eters shown are exactly alike in size and shape. The only difference is the outside markings or scales on them. Each thermometer is a hollow glass tube that is sealed at the top and has a mercury-filled bulb at the bottom. Mercury, like any liquid, expands when heated and will rise in the hollow tube. View A o figure 2-10 shows the Fahrenheit thermometer with its bulb standing in melting ice (32°F), and view B shows the Celsius thermometer with its bulb standing in boiling water (100°C).

The main point to remember is that the level of the mercury in a thermometer depends only on the temperature to which the bulb is exposed. If you were to exchange the thermometers, the mercury in the Celsius thermometer would drop to the level that the mercury now stands in the Fahrenheit thermometer. Likewise, the mercury in the Fahrenheit thermometer would rise to the level that the mercury now stands in the

Figure 2-10.-Temperature scales. A. Fahrenheit. B. Celsius.

Celsius thermometer. The temperatures would be 0°C for the ice water and 212°F for the boiling water.

If you place both thermometers in water containing ice, the Fahrenheit thermometer will read 32°F and the Celsius thermometer will read 0°C. Heat the water slowly. The temperature will not change until the ice in the water has completely melted (a great deal of heat is required just to melt the ice). Then both mercury columns will begin to rise. When the mercury level is at the +10° mark on the Celsius thermometer, it will be at the +50° mark on the Fahrenheit thermometer. The two columns will rise together at the same speed and, when the water finally boils, they will stand at 100°C and 212°F, respectively. The same temperature change-that is, the same amount of heat transferred to the water-has raised the temperature 100° Celsius and 180° Fahrenheit, but the actual change in heat energy is exactly the same.

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