SCFM ⇄ ACFM
Standard-to-actual airflow via the ideal-gas temperature and pressure correction.
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The engineering
SCFM counts molecules — mass flow wearing a volumetric costume at agreed reference conditions — while ACFM is the volume the gas really occupies where it is. Compress air to 700 kPa and 200 SCFM shrinks to about 30 ACFM in the pipe; that ratio is the whole point of the correction.
The card's booby trap is the word 'standard': CAGI uses 14.5 psia/20 °C, many US process specs use 14.696 psia/60 °F, gas utilities have their own. That's why the reference fields are editable — always ask whose standard, and use *absolute* pressures, never gauge.
Where this math comes from
The correction is the ideal gas law doing shift work. Robert Boyle nailed pressure-volume in 1662, Jacques Charles and Joseph Gay-Lussac added temperature (Gay-Lussac published 1802, crediting Charles's unpublished circa-1787 work), and Émile Clapeyron fused them into PV = nRT in 1834.
'Standard conditions' arrived with commerce: once gas was sold by volume through 19th-century town-gas meters, buyer and seller had to agree what a cubic foot *was*. The compressed-air industry inherited the problem — a compressor's SCFM rating is a chemistry statement, and its ACFM at your plant is this card's arithmetic.
- 1662Robert BoyleP·V constant at fixed temperature.
- 1802Gay-Lussac (crediting Charles)Volume–temperature law.
- 1834Émile ClapeyronCombined ideal gas equation PV = nRT.
- 1915Compressed Air Institute eraIndustry standard reference conditions emerge (circa).
See the full timeline of the math behind every calculator →
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