<P> The British thermal unit (Btu or BTU) is a traditional unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit . It is part of the United States customary units . Its counterpart in the metric system is the calorie, which is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius . Heat is now known to be equivalent to energy, for which the SI unit is the joule; one BTU is about 1055 joules . While units of heat are often supplanted by energy units in scientific work, they are still important in many fields . As examples, in the United States the price of natural gas is quoted in dollars per million BTUs . </P> <P> A BTU was originally defined as the amount of heat required to raise the temperature of 1 avoirdupois pound of liquid water by 1 degree Fahrenheit at a constant pressure of one atmosphere . There are several different definitions of the BTU that are now known to differ slightly . This reflects the fact that the temperature change of a mass of water due to the addition of a specific amount of heat (calculated in energy units, usually joules) depends slightly upon the water's initial temperature . As seen in the table below, definitions of the BTU based on different water temperatures vary by up to 0.5% . In the table, thermochemical and steam table (IT) values, which are now defined in terms of exact values in joules, have been rounded to four decimal places . </P>

What is the meaning of btu in ac
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