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r3dl3g t1_j9z7otx wrote

What you're describing isn't "the metric system," and is commonly done in Imperial/American standard units in some fields (e.g. kPSI or MPSI for large units of pressure). Prefixes in units aren't monopolized by SI.

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.

"Lack of coherence" means it isn't derived explicitly from a core SI unit. The fact that it can be expressed in SI units without rounding doesn't matter.

This also ties to a huge misunderstanding that people have about the metric system. The strength of SI isn't remotely related to the prefix units (i.e. the "moving the decimal point over"). Instead, the core power of the metric system is that more complicated units are all derived directly from other units; so, 1 Pascal is explicitly 1 Newton spread over 1 square meter. 1 Newton is explicitly 1 kg accelerating at a rate of 1 meter per second per second. Building on those basic units is what makes the SI system as powerful as it is, as it means you don't have weird constants that you have to factor into all of your calculations (e.g. the wonderful world of pounds-force vs. pounds-mass).

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kinyutaka t1_j9z90un wrote

To be more specific, the Liter is simply a special name for the cubic decimeter (1 tenth of a meter, cubed)

The Liter is defined by its relation to the Cubic Centimeter, which is equal to the milliliter.

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TheRealStorey t1_ja5s637 wrote

A Liter is the volume of 1kg of water at STP (Standard Temperature and Pressure)

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kinyutaka t1_ja5w157 wrote

That is also true.

A cm^3 of water is one gram.

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TheRealStorey t1_ja5zb7s wrote

It was made to be universally transferable, everyone has water, there is a standard meter stick in France and it was updated to the length traveled by some light so it's universally reproducible. SAme with a second, it's the vibrations of some atom, but the idea is anyone can perform the same experiment anywhere and have the same precise measurement.

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hwylow t1_j9zfgvi wrote

> 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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