A standard cubic foot (scf) is a unit representing the amount of gas (such as

natural gas Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...

) contained in a volume of one

cubic foot The cubic foot (symbol ft3 or cu ft)
, . is an United States customary units, 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 Outline of p ...

. Another unit used for the same purpose is the standard cubic metre (Sm³), derived from

SI unit The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of units of measurement, system of measurement. It is the only system ...

s, representing the amount of gas contained in a volume of one

cubic meter The cubic metre (in Commonwealth English and international spelling as used by the International Bureau of Weights and Measures) or cubic meter (in American English) is the unit of volume in the International System of Units (SI). Its symbol is m ...

at different reference conditions. The reference conditions depend on the type of gas and differ from other standard temperature and pressure conditions.

Usage

The scf and the scm are units 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 stat ...

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 and eve ...

and

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

, each definition fixes base values for pressure and temperature.

Natural gas

Since

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 abundance of methane on Earth makes ...

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 The cubic foot (symbol ft3 or cu ft)
, . is an This is the same pressure as the SI system standard pressure. Gives 1.1953 Mole (unit), moles per scf. * A pressure of . Gives 1.1956 moles per scf. * A pressure of . This value is very close to 30

inches of mercury Inch of mercury (inHg, ″Hg, or in) is a non- SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States. It is the pressure exerted by a column of mercury in ...

. 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 The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of units of measurement, system of measurement. It is the only system ...

. It is similarly defined as the quantity of gas contained in a cubic

meter The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...

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 a double "M" ("MM") 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 The National Council on Weights and Measures (NCWM) is a not-for-profit standards development organization, dedicated to developing the United States technical standards for weights and measures in commerce. The organization's official mission is ...

, 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 Industrial gases are the gaseous materials that are Manufacturing, manufactured for use in Industrial sector, industry. The principal gases provided are nitrogen, oxygen, carbon dioxide, argon, hydrogen, helium and acetylene, although many other ...

, 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

References