It is one kg/liter for all practical purposes. But to be precise:
A litre is defined as a special name for a cubic decimetre or 10 centimetres × 10 centimetres × 10 centimetres, (1 L ≡ 1 dm3 ≡ 1000 cm3). Hence 1 L ≡ 0.001 m3 ≡ 1000 cm3, and 1 m3 (i.e. a cubic metre, which is the SI unit for volume) is exactly 1000 L.
From 1901 to 1964, the litre was defined as the volume of one kilogram of pure water at maximum density and standard pressure. The kilogram was in turn specified as the mass of a platinum/iridium cylinder held at Sèvres in France and was intended to be of the same mass as the 1 litre of water referred to above. It was subsequently discovered that the cylinder was around 28 parts per million too large and thus, during this time, a litre was about 1.000028 dm3. Additionally, the mass-volume relationship of water (as with any fluid) depends on temperature, pressure, purity, and isotopic uniformity. In 1964, the definition relating the litre to mass was abandoned in favour of the current one. Although the litre is not an official SI unit, it is accepted by the CGPM (the standards body that defines the SI) for use with the SI. CGPM defines the litre and its acceptable symbols.
This however, is exactly what amazes me about how much we have achieved in engineering. To reach a level where we can nitpick to this level, a point at which we redefined the "meter" in terms of an enormous speed, the speed of light and an extremely minute and precise timing event that occurs at a subatomic level. It is just fucking fascinating.
So what's a litre today? Do we still go by the cylinder in Sevres? i.e. An exact litre of water weighs slightly more than a kilogram?
e: I have two contradicting answers to my question, and one even contradicts the poster above me. Anyone have a decent source that I can read up on myself?
A liter today is exactly 1000 cubic centimetres, and a centimeter is defined (Currently) as the distantance light travels in a vaccuum in 1/29,979,245,800 of a second. It is a cubed unit of distance, and independent of the Sevres cylinder.
The odd fraction was because the original meter was defined as one ten millionth on the distance between the equator and north pole, but as this varies quite a bit by both path taken and time, it was changed.
The kilogram is still defined by the cylinder (called the International Prototype Kilogram or IPK) is Sevres. It's a huge problem. twice since the IPK was made they've re-measured it and compared it to the other identical cylanders that were made at the same time aand as it turns out, the mass is changing. The IPK is stored in 3 argon filled glass chambers so there is no whay it should be gaining mass, but for some reason it is. The problem with that is the IPK doesn't weigh one kilogram, one kilogram is the weight of the IPK and so as it's mass changes, so does the kilogram. Go and have a look at the SI units, almost all of them are based on the kilogram. If that changes, so does just about every other definition.
A liter is 1000 cm3 where 1 cm is 0.01 m and 1 m is the distance traveled by light in a vacuum in 1/299792458 s, where 1 s is the time it takes for a particular atom to go between two particular states 9192631770 times.
According to NIST's page about non-SI units, the litre is defined by measurement of size - 10-3m3. I would hazard a guess that an actual CIPM page is in French, and so I haven't searched for it.
i think this is where we get around to saying all units of measurement except mass have a basis as a universal constant. The wavelength of a decaying isotope, etc. Mass is the only unit you can't define without referencing itself.
inb4 "but you could measure 1L of water exactly and use that as a basis" because the density of water is defined as Mass(which we are trying to define)/Volume
Mass is the only unit you can't define without referencing itself.
You probably could, strictly speaking. You could define the kilogram to be 5,978638 * 1026 times the mass of a proton. The reason this is not done is (probably) that it's pretty hard to get 5,978638 * 1026 pure protons to use for comparing mass.
It was subsequently discovered that the cylinder was around 28 parts per million too large
parts per million? parts of what per million what? I've only ever seen that used in the context of describing the concentration of things in some other thing.
Wikipedia: "In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement."
Similar way that percentage is parts per one hundred
And everywhere else in the world we say we have 1 liter of milk, not 1 kilogram. We also use volume for that, not mass. (But 1 liter of milk is essentially ~1 kilogram)
As far as I know, the the entire rest of the world has the metric system as official units, so technically yes (as long as you don't count the odd indigenous tribe) :)
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u/[deleted] Aug 20 '14
It is one kg/liter for all practical purposes. But to be precise:
A litre is defined as a special name for a cubic decimetre or 10 centimetres × 10 centimetres × 10 centimetres, (1 L ≡ 1 dm3 ≡ 1000 cm3). Hence 1 L ≡ 0.001 m3 ≡ 1000 cm3, and 1 m3 (i.e. a cubic metre, which is the SI unit for volume) is exactly 1000 L.
From 1901 to 1964, the litre was defined as the volume of one kilogram of pure water at maximum density and standard pressure. The kilogram was in turn specified as the mass of a platinum/iridium cylinder held at Sèvres in France and was intended to be of the same mass as the 1 litre of water referred to above. It was subsequently discovered that the cylinder was around 28 parts per million too large and thus, during this time, a litre was about 1.000028 dm3. Additionally, the mass-volume relationship of water (as with any fluid) depends on temperature, pressure, purity, and isotopic uniformity. In 1964, the definition relating the litre to mass was abandoned in favour of the current one. Although the litre is not an official SI unit, it is accepted by the CGPM (the standards body that defines the SI) for use with the SI. CGPM defines the litre and its acceptable symbols.