calorie

Various units of energy, specifically defined for heat flows. In principle, 1 calorie is the amount of heat necessary to raise the temperature of 1 gram of water 1° Celsius at a pressure of 1 atmosphere. Sometimes called a gram-calorie. Symbol, cal.

the kilocalorie (what dieters call a calorie)
International Steam Table calorie or international calorie
thermochemical calorie

A problem arises because the amount of heat required differs depending on the initial temperature of the water. Different choices of starting temperature have led to different calories, usually identified by subscripts. The most common calorie, in which the water temperature is raised from 14.5°C to 15.5°C, is sometimes called the 15°C calorie. Abbr, cal₁₅. One cal₁₅ = 4.1855 joules, a value adopted by CIPM in 1950.¹

Other calories have been based on initial temperatures of 0°C, 3.5°C, and 19.5°C. Still another definition made the calorie ¹⁄₁₀₀th of the quantity of heat needed to raise the temperature of a gram of water from 0°C to 100°C. The values produced by the various definitions differ by somewhat less than 1%.

According to the current national standard in the United States², the calorie in any form is not to be used. Joules should be used instead.

Thermochemical calorie

The thermochemical calorie (cal_th), used in some branches of chemistry, was defined by the U.S. Bureau of Standards in 1953 as = exactly 4.1840 joules.

International Steam Table calorie

In 1929, the International Steam Table Conference (London) defined the international calorie as ¹⁄₈₆₀ of the international watt hour, = 4.1860 international joules. Symbol, cal_IT. The adoption of the absolute system of electrical units changed these values to ¹/_859.858 watt hours and approximately 4.18674 joules.

At present, one International Table calorie (cal_IT) = exactly 4.1868 joules (Fifth International Conference on the Properties of Steam, London, 1956). This value was suggested by E. J. Le Fevre because it is evenly divisible by 9, which in pre-computer days facilitated conversions between specific heats described in kilocalories per kilogram-degree Celsius and those in Btu per pound-degree Fahrenheit.³

kilocalorie

The calorie used by physiologists and dieters is based on raising the temperature of a kilogram of water 1°C, and is about a thousand times bigger. It is properly called a “kilogram calorie” or kilocalorie, = 1,000 cal₁₅, and abbreviated “C”, capitalized to distinguish it from the lowercase “c” for the gram calories.

1. Bureau International des Poids et Mesures.
Procès-Verbaux des Séances du Comité International des Poids et Mesures, volume 22, 1950.

Pages 79 & 80.

2. IEEE/ASTM SI 10™-2002.
American National Standard for Use of the International System of Units (SI): The Modern Metric System.
New York: IEEE, 30 December 2002.

See Section 3.3.3.

3. Ernst Schmidt.
International system of units. MKSA system in applied thermodynamics.
in Systems of Units. National and International Aspects.
Carl F. Kayan, editor.
Publication No. 57 of the AAAS.
Washington, D. C.: American Association for the Advancement of Science, 1959.

Page 280.