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Since water is not a fixed density you can't have an exact conversion unless you specify a lot of other things. For example, a US fluid ounce of water at 212F w
by jefftk 6y ago
Since water is not a fixed density you can't have an exact conversion unless you specify a lot of other things. For example, a US fluid ounce of water at 212F weighs almost exactly one US ounce ;)
- fanf2 6y agoYes, I did not specify which standard temperature and pressure were required for the correspondence to work, because I thought that was an obvious requirement when defining units in the metric style. In fact stp differs between the metric system and the imperial system. There was no stp for the Queen Anne wine gallon because there was no intention to link units of volume and weight. The stp for the metric system was initially freezing point but was changed to the point of maximum density of water because that was easier to practically implement - https://en.wikipedia.org/wiki/History_of_the_metric_system#The_units_of_weight_and_length https://en.wikipedia.org/wiki/History_of_the_metric_system#T... The Wikipedia article I linked above says, “The 1824 Act defined as the volume of a gallon to be that of 10 pounds (4.54 kg) of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury (102 kPa) at a temperature of 62 °F (17 °C).”
- jefftk 6y agoAs far as I know, that a fluid ounce of water weighs an ounce at the boiling point is a complete coincidence, but it does illustrate that joining weight and volume this way is tricky business.