That doesn't sound right, since in the system we still use in America a pint is 16 oz and a fluid ounce of water still weighs (very close to) one ounce.
That’s because American customary units predate the British Imperial units reform of 1825 - Americans still use the Queen Anne wine gallon for liquid volume and the Winchester bushel for dry volume. (The Imperial gallon more closely matches the earlier ale gallon.)
It’s common for people to refer to all British units as “Imperial” but that name doesn’t apply to the pre-1825 units used in the US. Some of the confusion is that the Imperial reform only made significant changes to volume measures: the yard and avoirdupois weights remained basically the same.
My parent was claiming that the 1825 reform is what made the Imperial fluid ounce weigh one ounce, though, but the pre-1825 units as used in the US also have that property.
No, an ounce avoirdupois is 454g / 16 == 28.4g but a Queen Anne wine fluid ounce is 231 x 2.54^3 / (16 x 8) == 29.6g which is a difference of about 4%. The imperial fluid ounce is exact, because the weight and volume units were designed to work the same way as the metric system. The older units had no such connection.
There is a mnemonic rhyme for the imperial system: “a pint of water weighs a pound and a quarter”. This is exactly precise, due to the way the imperial gallon was defined.
There is a mnemonic rhyme for US customary units: “a pint is a pound the world around”. This is wrong because the first part has a measurement error of 4%, and the second part is wrong because the error is 25% outside the USA.
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 ;)
Yes, 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 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).”
As 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.
That doesn't sound right, since in the system we still use in America a pint is 16 oz and a fluid ounce of water still weighs (very close to) one ounce.