Absolute Scale (Kelvin)

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Absolute Scale (Kelvin)

Another scale in wide use by scientists in many fields is the absolute scale or Kelvin scale. It was developed by Lord Kelvin of England. On this scale the freezing point of water is 273°K and the boiling point of water is 373°K. The absolute zero value is considered to be a point at which theoretically no molecular activity exists. This places the absolute zero at a minus 273° on the Celsius scale, since the degree divisions are equal in size on both scales. The absolute zero value on the Fahrenheit scale falls at minus 459.6°F.

Scale Conversions

Two scales, Fahrenheit and Celsius, are commonly used. With the Celsius and Fahrenheit scales, it is often necessary to change the temperature value of one scale to that of the other. Generally a temperature conversion table, like table 1-4-1, is used or a temperature computer. If these are not available, you must then use one of the following mathematical methods to convert one scale to another.

Mathematical Methods

It is important to note that there are 100 divisions between the freezing and boiling points of water on the Celsius scale. There are 180 divisions between the same References on the Fahrenheit scale. Therefore, one degree on the Celsius scale equals nine-fifths degree on the Fahrenheit scale. In converting Fahrenheit values to Celsius values the formula is:

In converting Celsius values to Fahrenheit values the formula is:

One way to remember when to use 9/5 and when to use 5/9 is to keep in mind that the Fahrenheit scale has more divisions than the Celsius scale. In going from Celsius to Fahrenheit, multiply by the ratio that is larger; in going from Fahrenheit to Celsius, use the smaller ratio.

Another method of converting temperatures from one scale to another is the decimal method. This method uses the ratio 1°C equals 1.8°F. To find Fahrenheit from Celsius, multiply the Celsius value by 1.8 and add 32. To find Celsius from Fahrenheit, subtract 32 from the Fahrenheit and divide the remainder by 1.8.

Examples:

To change a Celsius reading to an absolute value, add the Celsius reading to 273° algebraically. For example, to find the absolute value of – 35°C, you would add minus 35° to 273°K algebraically. That is, you take 273° and combine –35° so you use the minus ( – ) function to arrive at 238°K.

To change a Fahrenheit reading to an absolute value, first convert the Fahrenheit reading to its equivalent Celsius value, which is then added algebraically to 273°. Consequently, 50°F is equivalent to 283° absolute, arrived at by converting 50°F to 10°C and then adding the Celsius value algebraically to 273°.

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