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How would one determine the precise volume of a sample of highly purified water at its minimum density, and then copy its mass precisely to be used as a calibration weight?
The platinum-iridium standard is a bit flakey. http://www.npr.org/templates/story/story.php?storyId=112003322
"Air will not contribute to the weight of a liter of water (unless you are meaning dissolved gases)."Would you care to explain that to these people?http://www.npl.co.uk/upload/pdf/buoycornote.pdf
Atomic S I just bet you meant to say maximum density; the minimum is practically zero.
Normally water used as a masspiece is used in multi ton lots, mostly for testing cranes, weighbridges and other applications where you need a masspiece in the range of 20 to 200 tons. It is easier to move water in a tanker and pump it up to where the big bag is than it is to have multiple lowbed trailers and trucks to move large masses of iron there to do the work. It is easy to get a water meter and calibrate it to within 1% at a certain flow rate and pressure, and this enables you to fill a big bag with accuracy of around 1% or better, which is what is needed to do these tests and certifications.
Why do you want to do this anyway?
but rather one that is based on the number of atoms in a block of Silicon ( as you can get that in multi kilogram lots with impurity levels approaching a single atom per kilogram).
atomic clocks, where they are now small enough to almost literally fit inside a wristwatch,
Anyway, for your task, find a volumetric flask that is calibrated for 1L of water at 25°C (condensation will be a problem at 4°C). Tare your balance with the flask, and fill with water at 25°C, and you'll be all set.
I was surprised to learn that the boiling point of pure water at 1 ATM isn't considered to be 100°C now
For example (not that I'm saying this is the case, and I know it's silly), what if the fluctuations in weight between the standard and its copies were a consequence of preferential variations in the weights of certain elements? Are we sure the compositions of the copies are "exactly" the same as the original? How do we even know what the exact chemical composition of the original is?
How would one determine the precise volume of a sample of highly purified water at its minimum density, and then copy its mass precisely to be used as a calibration weight?Very carefully.
put it in a sealed plastic bag, weigh it on a digital scale ...
Take a one cubic decimeter chunk of glass or metal. A sphere? Of course calibrating your volume as necessary.Weigh it.Drop it into water (with adequate controls).Weigh again.The difference is exactly 1 kilo (assuming one uses the definition 1dm3 of water at maximum density = 1kg).
Thanks Cliff - I reread your earlier post in the light of the above explanation and they both made sense now. Of course the above still suffers from the potential inaccuracies of mass of displaced air and the density of your water again
Also, if you are going to use water, could you let me know what water you plant to use?Most water contains a little heavy water. If you do the experiment with pure H2O (I.e. no DHO) you will get an answer that makes all previous determinations wrong.On the other hand, how much DHO should you use?Your best bet would probably be to go with thishttp://en.wikipedia.org/wiki/Vienna_Standard_Mean_Ocean_Water
Incidentally, the cube shaped box full of water wouldn't work. The vapour pressure of the water would lift the lid on the box as soon as it was under vacuum.You could clamp the lid, but the clamping force would distort the box and the distortion would be different from that of the empty box (because water is practically incompressible).
You could use another interferometric set of measurements to find the size of the clamped box full of water (and surrounded by vacuum) but to do that you would need to know the refractive index of the water (at some poorly defined pressure) to 9 or 10 significant figures.Have fun measuring that
Perhaps this experiment is best carried out in space.