EEVblog Electronics Community Forum
General => General Chat => Topic started by: Homer J Simpson on May 22, 2019, 02:04:26 am

https://www.space.com/newkilogram.html (https://www.space.com/newkilogram.html)

https://www.eevblog.com/forum/metrology/changingtothe2019siredefinitionsonmay20/ (https://www.eevblog.com/forum/metrology/changingtothe2019siredefinitionsonmay20/)
https://www.eevblog.com/forum/chat/themethodusedtoredefinethekilogramisjustinsane/ (https://www.eevblog.com/forum/chat/themethodusedtoredefinethekilogramisjustinsane/)

Thanks to science, now my new 1 kg rolls of PLA for my 3D printer will be more precise!

So, how do they use the new standard to verify another weight? With a physical standard, you can at least put them both on a balance.

So, how do they use the new standard to verify another weight? With a physical standard, you can at least put them both on a balance.
As far as a pound it's just a matter of dividing a kg by a number 
"Various definitions have been used; the most common today is the international avoirdupois pound, which is legally defined as exactly 0.45359237 kilograms, and which is divided into 16 avoirdupois ounces."
https://en.wikipedia.org/wiki/Pound_(mass)

So, how do they use the new standard to verify another weight? With a physical standard, you can at least put them both on a balance.
Physical artifacts will still be the way to transport the kg internationally. The difference is that while before, those artifacts were compared against a very carefully maintained hunk of platinum (the "International Prototype Kilogram") before, now they are measured (perhaps indirectly) against the Watt balance. So we could have every single physical artifact on the planet destroyed, but could bring back exactly the same kilogram as before as long as we remember the value for Planck's constant that we decided on: 6.62607015×10−34. In exactly the same way that the length of a metre used to be the length of a particular hunk of metal, but a while back we decided the speed of light to be, by definition, 299792458 m/s, so now instead we can just shine a laser and see how far it travels in 1/299792458 of a second (only conceptually, of course, it's not quite that simple!).
The fact that I can define the second, metre and kilogram (not to mention volts and amps and watts etc) by simply giving you a piece of paper and a laboratory, instead of handing you hunks of metal that I hope haven't deteriorated or changed, is a great accomplishment.

The big problem with said hunks, is that they lose mass (e.g., abrasion, desorption..), AND they gain mass (e.g., dust and fingerprints, adsorption). This happens on the order of micrograms per year, IIRC. All the international standards have been drifting apart from each other independently, including the official standard kg itself (of course, since it was defined as the standard, we wouldn't say that it was varying, but, y'know).
Tim