That was the repeatability (and constancy) thing that Jon mentioned a few posts back. The new definition is just more practically reliable when we’re working at the very highest levels of precision.
VB
Just playing devil’s advocate here, but when did the properties of water change? Is it not repeatable or constant enough, in theory, at least?
While I’m at it,
Consider replying to more people
You’ve already replied 3 times to @Valvebloke in this particular topic.
Have you considered replying to other people in the discussion, too? A great discussion involves many voices and perspectives.
If you’d like to continue your conversation with this particular user at length, [send them a personal message
No, fuck off!
7 Likes
Someone thought the VTA could be improved.
3 Likes
I think part of it is a matter of tolerances. Getting a litre of completely pure water, no variation in isotopes etc. isn’t. I wonder also if there are issues with surface tension to factor in, because the water doesn’t ever sit perfectly flat in a container.
I don’t know what the worst thing about water as a reference standard is. But I might guess:
Purity - Its density will be different if it’s got anything dissolved in it. Furthermore the fraction of the water which is deuterium oxide (heavy water) would need to be specified and controlled.
The ‘standard temperature and pressure’ condition is important but it ties the mass standard to two other standards which also have to be rigorously controlled and cross-checked each time you want to determine the kilogramme.
It’s easy to say that all we need is one litre of water. But how do we actually know that that’s what we’ve got ? We’d have to make a (say) cubical vessel all of whose dimensions are controlled to the asccuracy of the metre standard and then fill it with water. Which presumably means having a filling hole. How can we be sure that the water meniscus at the filling hole isn’t depressing or raising the water surface there just a bit ?
In practical terms it’s easy to see why they swapped over to a lump of metal so very quickly.
VB
Ok, I surrender (I knew that was inevitable from the start of this discussion) and genuinely bow to your (vastly) superior knowledge 
I think that there’s no reason why a litre of water might not happen to have the mass of a kg, but best to have that as the effect of its definition, not the cause, as Graeme neatly explains.
But we do enjoy prompting the talking encyclopedia, it keeps the tubes working. 
Will a kilo of onions be more or less expensive ? The public needs to know 
2 Likes
They will be unavailable after Brexit. Only the French will have onions.
2 Likes
It’s just stuff I’ve read in books/on websites. I spent a long time learning it then a longer time working with it including some time teaching it. It was pretty much bound to stick, in the same way that accountants can read balance sheets, electricians understand the pros and cons of different earthing schemes and JB knows about hinges 
.
VB
5 Likes
Ha! I knew that. Read it in one of those book things years ago.
Science, piece of cake.
It’s the long term retention, or rather the lack of it, that kills me.
1 Like
I’ve forgotten almost all the calculus I once knew. A few times per decade it would be useful to be able to pull it back. But each time I need it I don’t have time to re-learn it. I’m reduced to getting a ‘good enough’ answer by building a model in MSExcel. Only a couple of weeks ago I was involved in a discussion with a schoolboy about how to optimise a small-scale coilgun he wanted to build as a physics/engineering project. Calculus might have given quick qualitative insights into which changes would have made the thing work better and which would have made it worse. But I was no longer sufficiently fluent 
.
VB
2 Likes
Outside of a classroom I’ve never used it, although that’s probably a reflection on the limitations of my knowledge of the subject rather than it being unnecessary !
MSExcel, as you mentioned, does/did a good enough job in most cases.
And a nautical mile is the length of the arc of one minute of angle on the earth’s surface from the centre of the earth to the equator.
Roughly 2025 yards.
A cable is a 10th of a nautical mile, 200 yds (No I don’t know what happened to the other 2.5 yds)
An acre can be defined as the area of a rectangle which is one furlong x one chain
Why I remember this stuff I do not know
1 Like
Meh…It’ll all be easier once we go back to Imperial after March. 
btw…a gallon of water weighs 10lb 
3 Likes
I sure as fuck can’t ever remember how many yards in a mile. Utterly unhelpful number.
When I was studying surveying in the 1970’s, we had to use real ‘chains’ on outdoor practical forays. They are remarkably good for measuring odd shaped fields.
1 Like
I got landed last minute by a consultant with setting up and measuring gait speed and distance on a quantified walkway, but in an Aberdeen hospital. He’d no idea of the issues involved.
The best I could do overnight was mark up an old climbing rope with metre divisions by tape, to lay along the corridor.