[cmsmith]

I work at NIST and took a tour of this machine a few months ago. A few bonus facts:

- The spatial distribution of the local gravity field is a significant part of the uncertainty of these measurements. The weights extend into the basement of the lab, and the gravity in the basement is less than that above the surface. They produce gravity maps by dropping things in a vacuum to get a handle on it.

- The drift in the dead weight standard was mostly caused by the individual masses welding themselves together under the immense pressure of the weight stack. The interface has been designed to reduce this effect.

- The same group is also working to count the number of atoms in the kilogram, so that the mass of the dead weight stack will not be 500,000 times the mass of a piece of platinum in Paris, but will be 500,000 times the mass of 6.XX E23 silicon atoms.

[aftbit]

More about that piece of platinum in Paris, the International Prototype Kilogram (IPK): https://en.wikipedia.org/wiki/Kilogram#International_prototy...

Also, something I had never heard of, Vienna Standard Mean Ocean Water: https://en.wikipedia.org/wiki/Vienna_Standard_Mean_Ocean_Wat...

[bambax]

It's not in Paris but in Sèvres, 10 km west of Paris (where I live).

[knodi123]

The most precise reference for a kilogram in the whole world? It's approximately in Paris, more or less- we care about precise weights, not precise locations. /s

[cr1895]

Can you expand on this: "the individual masses welding themselves together under the immense pressure of the weight stack."

It's not really even that heavy (~450mt). Is this effect something that is entirely negligible for things which don't require such immense precision? Like, what kind of drift are we talking about...milligrams, micrograms?

[elbrownos]

Those sound like great reasons to use something sensible like a hydraulic ram to apply 1,000,000 lbf. Force = pressure x area, and neither pressure nor area is difficult to measure precisely. I can't see the upside of using 1,000,000 lbm of stainless steel.

[kabouseng]

It's much simpler to compensate for dead weight to get as exact as possible measurement. With a hydraulic press you need to compensate for the compression and heat buildup of the liquid etc.

[adaml_623]

It's easy to measure area but how do you calibrate your pressure sensor?

I think you'll find that this device is used not as a testing tool but as a calibration tool.

[pif]

> The spatial distribution of the local gravity field is a significant part of the uncertainty of these measurements.

Interesting! Could you please add a few numbers, just to get the order of magnitude?

[joezydeco]

I was curious about the tolerance in a mass of this size. Article didn't mention that. Do you happen to know?