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A dekatherm (dth) is a unit of energy used primarily to measure natural gas. It is a combination of the prefix for 10 (deca, often with the US spelling "deka") and the energy unit therm. One dekatherm is equal to 10 therms or one million British thermal units (Btu) or 1.055 GJ. The energy content of 1,000 cubic feet (28 m3) natural gas measured at standard conditions is approximately equal to one dekatherm.
Natural gas is a mixture of gases containing approximately 80% methane (CH4) and its heating value varies from about or 10.1 to 11.4 kilowatt-hours per cubic metre (975 to 1,100 Btu/cu ft), depending on the mix of different gases in the gas stream. The volume of natural gas with heating value of one dekatherm is about 910 to 1,026 cubic feet (25.8 to 29.1 m3). Noncombustible carbon dioxide (CO2) lowers the heating value of natural gas. Heavier hydrocarbons such as ethane (C2H6), propane (C3H8), and butane (C4H10) increase its heating value. Since customers who buy natural gas are actually buying heat, gas distribution companies who bill by volume routinely adjust their rates to compensate for this.
The company Texas Eastern Transmission Corporation, a natural gas pipeline company, started to use the unit dekatherm in about 1972. To simplify billing, Texas Eastern staff members coined the term dekatherm and proposed using calorimeters to measure and bill gas delivered to customers in dekatherms. This would eliminate the constant calculation of rate adjustments to dollar per 1000 cubic feet rates in order to assure that all customers received the same amount of heat per dollar. A settlement agreement reflecting the new billing procedure and settlement rates was filed in 1973. The Federal Power Commission issued an order approving the settlement agreement and the new tariff using dekatherms later that year, Other gas distribution companies also began to use this process.
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