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A standard cubic foot (scf) is a unit used both in the
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...
industry to represent an amount of natural gas and in other industries where other gases are used. It is the unit commonly used when following the customary system, a collection of standards set by the
National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical s ...
. Another unit used for the same purpose is the standard cubic metre (Sm3), customary when using SI units.


Definitions

A standard cubic foot defines an amount of gas contained in a volume of one cubic foot at standard temperature and pressure. This standard unit of molecular quantity for gases can be used with the
ideal gas law The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas. It is a good approximation of the behavior of many gases under many conditions, although it has several limitations. It was first s ...
to compute the quantity per unit of volume for other pressures and temperatures. In spite of the label "standard", there is a variety of definitions, mainly depending on the type of gas. Since, for a given volume, the quantity is proportional to the
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
and
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
, each definition fixes base values for pressure and temperature.


Natural gas

Since
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...
is an imprecise mix of various molecular species, chiefly
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
but with varying proportions of other gases, a standard cubic foot of natural gas does not represent a precise unit of mass, but a molecular quantity, expressed in moles. For petroleum gases, the standard cubic foot (scf) is defined as one cubic foot of gas at and at normal sea level air pressure. The pressure definition differs between sources, but are all close to normal sea level air pressure. * A pressure of . This is the same pressure as the SI system standard pressure. Gives 1.1953
moles Moles can refer to: * Moles de Xert, a mountain range in the Baix Maestrat comarca, Valencian Community, Spain *The Moles (Australian band) *The Moles, alter ego of Scottish band Simon Dupree and the Big Sound People * Abraham Moles, French engin ...
per scf. * A pressure of . Gives 1.1956 moles per scf. * A pressure of . This value is very close to 30 inches of mercury. Gives 1.1981 moles per scf or 0.002641 pound moles per scf. The standard cubic meter of gas (scm) is used in the context of the SI system. It similarly defined as the quantity of gas contained in a cubic meter at a temperature of and a pressure of . Converting volume units between the standard cubic foot and the standard cubic meter is not exact, as the base temperature and pressure used are different, but for most practical situations the difference can be ignored. Comparing the same volume between the and 101.325 kPa standard cubic meter versus the and standard cubic foot gives an error of 0.04%. A standard cubic foot in the US Customary System is approximately equivalent to 0.02833 standard cubic meters in the SI system. In the natural gas industry, where quantities are often expressed in standard cubic feet, large multiples of standard cubic feet are generally not expressed with
metric prefix A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The pr ...
es, but rather with prefixes based on roman numerals, where the ''s'' for "standard" is often omitted. Common units of gas volumes include ccf (hundred standard cubic feet), Mcf (thousand standard cubic feet), and MMcf (million standard cubic feet). The ''M'' refers to the Roman numeral for thousand, while two ''M's'' represent one thousand thousands, or one million. Bcf (billion standard cubic feet), Tcf (trillion standard cubic feet), Qcf (quadrillion standard cubic feet), etc., are also used.


Compressed or liquefied gases in refillable cylinders

The National Conference on Weights and Measures, a US-based non-profit organization working in cooperation with the US National Institute of Standards and Technology, has defined a set of standards in a regulation entitled the "Uniform Regulation for the Method of Sale of Commodities". This regulation defines a standard cubic foot, for compressed or liquefied gases in refillable cylinders ''other than LPG'' by, "A standard cubic foot of gas is defined as a cubic foot at a temperature of 21 °C (70 °F) and a pressure of 101.325 kilopascals Pa(14.696 psia)".


Industrial gases

Yet other definitions are in use for industrial gas, where, in the US, a standard cubic foot for industrial gas use is defined at and 14.696 psia (101.325 kPa), while in Canada, a standard cubic meter for industrial gas use is defined at and 101.325 kPa (14.696 psia).


Converting actual volumes to standard volumes

An actual volume can be converted to a standard volume using the following equation: :''V''s = ''V''a × ''F''p × ''F''t × (''F''pv)2 Where, :''V''s: standard volume :''V''a: actual volume (sometimes shown as ''V''r for registered volume) :''F''p: pressure factor (sometimes shown as ''P''m for pressure multiplier) ::''F''p: absolute pressure / standard pressure = (line gauge pressure + atmospheric pressure)/base pressure :''F''t: temperature factor (sometimes shown as ''T''m for temperature multiplier) ::''F''t: absolute standard temperature / absolute line temperature = 73.15 + standard temperature (°C)/ 73.15 + line temperature (°C)or 59.67 + standard temperature (°F)/ 59.67 + line temperature (°F):''F''pv: super compressibility factor (often omitted or shown as equaling 1) Example: How many standard cubic feet are in 1 cubic foot of gas at 80 °F and gauge pressure 50 psi? (assuming that there is 13.6 psi atmospheric pressure and ignoring super compressibility) :''V''s = 1 cu ft × 13.6 psi + 50 psi) / 14.73 psi× 60 °F + 459.67 °F) / (80 °F + 459.67 °F):''V''s = 4.16 scf


See also

*
Standard conditions for temperature and pressure Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union ...


References

{{DEFAULTSORT:Standard Cubic Foot Units of amount of substance