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hwylow t1_j9zfgvi wrote

Reply to comment by r3dl3g in ELI5: Why isn't the Litre (L) considered a "non-SI" unit? by enby-millennial-613

> and is commonly done in Imperial/American standard units in some fields (e.g. kPSI or MPSI for large units of pressure)

Fields, as in scientific fields? Are there any that still commonly use imperial units at all?

> Liter isn't an SI unit simply because it's not the base unit of volume. Volume is inherently just built on distance measurements, and the SI system already has the meter, ergo the base unit of volume is the cubic meter.

Most SI units are not base units, for example the microsecond and the newton. SI just defines a list of units whose use is recommended, and they happened to decide that litre is not one of them. Other than the prefixes, they try and have just one unit for each kind of quantity. However, they include the litre in a list of non-SI units whose use is acceptable, along with the likes of minutes, degrees, hectares, and electronvolts.

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r3dl3g t1_j9zifdk wrote

>Fields, as in scientific fields? Are there any that still commonly use imperial units at all?

More industrial fields, but it's still pretty common in engineering R&D in fields where the academic, scientific, and industrial lines get blurred.

Automotive and Aerospace engineering, for example.

>Are there any that still commonly use imperial units at all?

Aerospace is still overwhelmingly Imperial, at least until you get to space. Altitudes are measured in feet, speed in knots, thrust and payload in pounds, power in horsepower (particularly for piston-cylinder engined aircraft).

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