Definitions
A Btu is defined as amount of heat required to raise the temperature of one 1 pound (0.454 kg) of liquid water by 1 °F (0.556 °C) at a constant pressure of one atmosphere. As is the case with the calorie, several different definitions of the Btu exist, which are based on different water temperatures and therefore vary by up to 0.5%: A Btu can be approximated as the heat produced by burning a single wooden match or as the amount of energy it would take to lift a one-pound weight to a height of 778 feet (237 m).
| Nominal temperature | Btu equivalent in joules | Notes |
|---|---|---|
| 39 °F (3.9 °C) | ≈ 1059.67 | Uses the calorie value of water at its maximum density (4 °C or 39.2 °F) |
| Mean | ≈ 1055.87 | Uses a calorie averaged over water temperatures 0 to 100 °C (32 to 212 °F) |
| IT | ≡ 1055.05585262 | The most widespread Btu, uses the International Table (IT) calorie, which was defined by the Fifth International Conference on the Properties of Steam (London, July 1956) to be exactly 4.1868 J |
| ISO | ≡ 1055.056 | International standard ISO 31-4 on Quantities and units—Part 4: Heat, Appendix A. This value uses the IT calorie and is rounded to a realistic accuracy |
| 59 °F (15.0 °C) | ≡ 1054.804 | Chiefly American. Uses the 15 °C calorie, itself now defined as exactly 4.1855 J (Comité international 1950; PV, 1950, 22, 79–80) |
| 60 °F (15.6 °C) | ≈ 1054.68 | Chiefly Canadian |
| 63 °F (17.2 °C) | ≈ 1054.6 | |
| Thermochemical | ≡ 1054.35026444 | Uses the "thermochemical calorie" of exactly 4.184 J |
Read more about this topic: British Thermal Unit
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