You are comparing volume and mass. Water has nothing to do with this.
You cannot compare volume and mass by themselves - you need a reference density.
It is a historical curiosity that the systems used the density of water. Convenient, but arbitrary. As noted, when something changed - like the manner in which length was standardised for one system, the relationship was broken. Even then, the manner in which the mass measure was created required an exact temperature, and won’t work otherwise.
So, the US defined the volume - the fluid ounce - defined with dimensions in inches. They then worked out a mass from this. They go on to create a system with reference masses, crafted ever so carefully in metal. These are the actual definitions of mass measure. Then someone comes along and creates a more carefully definition of the inch, which is accepted world-wide, and - oops - the fluid once is ever so sightly different in size. So, what to do? The answer is nothing. Accept that the inch has changed, and thus the volume it defines has changed too. So the fluid once is no longer exactly tied to the mass once via the density of water at a specific temperature. No big deal.
The metric system didn’t see such large scale changes in the definition of the fundamental units that anything mattered. It is worth noting that the metric system no longer ties the definition of mass to a volume of water, and in principle, one litre of water probably does not exactly equal one kilogram anymore. But the disparity is infinitesimal.
Also, something which is completely irrelevant to your question, is that grams measure mass, whereas ounces measure weight. A subtle difference that makes no difference if you always do your measurements in a gravity field of 9.8 m/s^2.
So if you mass 70 kilos you’ll still mass 70 kilos on the Moon. But if you weigh 150 pounds on Earth you’ll only weigh 25 pounds on the Moon.
I don’t think I’ve ever needed a more accurate weight but I never picked up on that difference. I don’t even remember where I learned that. Oh well, life would be pretty dull if there was nothing left to learn. But on that basis my life should be much more exciting than it is.
The American system of units is a royal mess. Sometimes, the pound is treated as a unit of force, with the corresponding unit of mass being the slug. Sometimes, the pound is treated as a unit of mass, with the corresponding unit of force being the poundal. And sometimes, it’s treated as being a unit of both force and mass, which can’t possibly be consistent but which engineers insist upon doing anyway, which does terrible violence to basically all of the formulas in physics.
Well force is best measured in Newtons, but because we use scales to measure mass on earth, we might use them to measure force due to gravity on the moon…
Isn’t the British mnemonic something like “a pint of water is a pound and a quarter” or something to that effect? There’s an equivalent saying, but I’m not going to bother to look it up.
In one conversion, you’re using Imperial fluid ounces; in the other you’re using US fluid ounces. The two are not the same (they’re about 4% different).
So your two conversions give you answers that are 4% different.
It has nothing to do with volume vs. mass; it’s simply that you’re using two units (Imperial Fluid Ounces and US Fluid Ounces) that sound similar and are close to the same size, but actually slightly different. So you’re of course getting two slightly different answers.
Now, volume vs. mass could be an issue, depending on what you’re measuring, the temperature, and how accurate you need to be, etc. For instance, a cubic centimeter of water at room temperature is going to be about 0.998 grams. But that’s not what’s going on in your calc; you’re just using two different units.
Some of you have brushed by the fact that the old rhyme is nearly correct about the weight of water, but different for other things measured in fluid ounces. Until I checked, I didn’t know just how different.
While a pint of water weighs a little more than a pound, and a US gallon of water a little more than 8 pounds, a US gallon of gasoline (petrol) weighs only a little more than 6 pounds.
Fluid ounces are a measure of volume. They’re in the same family as cups, pints, quarts, and gallons.
Ounces are a measure of weight. They’re in the family of pounds and tons. Think of it this way: comparing fluid ounces to weight ounces is generally like comparing oranges to suspension bridges.
In a sane system, there would be different names for them. Like in the metric system.
You mean like when they called a unit the ton(ne) and said “screw it, 2000 is close enough to 2200”? Or when they said a calorie is the same thing as 1000 calories?
IIRC, the sensible system (the real system) was imperial measure. An imperial gallon of water was 10 pounds. (in the rough and ready days, they did not worry about thermal expansion, but obviously at or around room temperature…) 4 quarts to a gallon, two pints to the quart, 2.5 cups to the pint, 8 (fluid) ounces to the cup…
(IIRC a trivia I never verified - a cubic foot of water is 6 gallons, weighs 60 imperial pounds. if it was Perrier, it cost 60 pounds. )
8 x 2.5 x 2 x 4 = 160 - tada! An ounce of water (imperial) was an ounce weight.
Two and a half cups to the pint… why do you think the Hobbitses were excited that beer came in pints?
