4 ms·

Can anyone tell this poor ignoramus whether the kg is getting heavier or lighter - and how much by (approximately, I know it's not going to be an exact figure g

Can anyone tell this poor ignoramus whether the kg is getting heavier or lighter - and how much by (approximately, I know it's not going to be an exact figure given the reason they are changing it)?

artimaeis 8y ago
The mass of the kilogram is remaining the same. These changes don't affect the "size" of any of the units. > "[...] the size of these units will not change (a kilogram will still be a kilogram) [...]"
- tzs 8y ago
  > These changes don't affect the "size" of any of the units. The size of electrical units is changing slightly. According to this BIPM document [1]: "The transition from the 1990 convention to the revised SI will result in small changes to all disseminated electrical units. For the vast majority of measurement users, no action need be taken as the volt will change by about 0.1 parts per million and the ohm will change by even less. Practitioners working at the highest level of accuracy may need to adjust the values of their standards and review their measurement uncertainty budgets". [1] https://www.bipm.org/utils/common/pdf/SI-statement.pdf https://www.bipm.org/utils/common/pdf/SI-statement.pdf
shittyadmin 8y ago
This doesn't impact the actual weight of a kg. What this impacts is how the kg is defined, no longer does it rely on a physical kg. It used to literally be defined as the mass of "IPK" - the international prototype kilogram. https://en.wikipedia.org/wiki/Kilogram#International_prototype_kilogram https://en.wikipedia.org/wiki/Kilogram#International_prototy... This switch now defines it in terms of the force equivalent of the energy exerted by a single photon - if my understanding is correct. Theoretically a more scientifically sound way of keeping 1 kg a constant permanently.
- cco 8y ago
  That doesn't make sense to me, the definition has changed. The IPK was 1 kg and the copies of it were all slightly heavier or lighter, for argument let's say IPK1 was 1.1 kg and IPK2 was 0.9 kg. Now that we have a constant for the mass of 1 kg, all of those IPK's will now have a new mass relative to the new definition. Unless the new definition was based on the current IPK and set its mass as of today as the standard, and that standard now will be unchanging?
  - ekimekim 8y ago
    > Unless the new definition was based on the current IPK and set its mass as of today as the standard, and that standard now will be unchanging? Correct. The current IPK will still be 1kg to within margin of error at the time the new definition comes into effect, but any further changes to it will make it, for the first time, drift away from that value.
    - cco 8y ago
      Ah I see, that makes sense, thanks!
    - Digit-Al 8y ago
      Thanks. That does make sense.
ska 8y ago
It's probably better to think about the other way around. Despite heroic efforts, the "old" kilogram was getting (a tiny big) lighter every time it was handled. The "new" kilogram will finally be a constant. To a reasonable approximation it hasn't changed at all at this moment time.
Digit-Al 8y ago
Thanks for all the comments. Very helpful.

Can anyone tell this poor ignoramus whether the kg is getting heavier or lighter - and how much by (approximately, I know it's not going to be an exact figure given the reason they are changing it)?

artimaeis 8y ago

The mass of the kilogram is remaining the same. These changes don't affect the "size" of any of the units. > "[...] the size of these units will not change (a kilogram will still be a kilogram) [...]"

tzs 8y ago
> These changes don't affect the "size" of any of the units. The size of electrical units is changing slightly. According to this BIPM document [1]: "The transition from the 1990 convention to the revised SI will result in small changes to all disseminated electrical units. For the vast majority of measurement users, no action need be taken as the volt will change by about 0.1 parts per million and the ohm will change by even less. Practitioners working at the highest level of accuracy may need to adjust the values of their standards and review their measurement uncertainty budgets". [1] https://www.bipm.org/utils/common/pdf/SI-statement.pdf https://www.bipm.org/utils/common/pdf/SI-statement.pdf

shittyadmin 8y ago

This doesn't impact the actual weight of a kg. What this impacts is how the kg is defined, no longer does it rely on a physical kg. It used to literally be defined as the mass of "IPK" - the international prototype kilogram. https://en.wikipedia.org/wiki/Kilogram#International_prototype_kilogram https://en.wikipedia.org/wiki/Kilogram#International_prototy... This switch now defines it in terms of the force equivalent of the energy exerted by a single photon - if my understanding is correct. Theoretically a more scientifically sound way of keeping 1 kg a constant permanently.

cco 8y ago
That doesn't make sense to me, the definition has changed. The IPK was 1 kg and the copies of it were all slightly heavier or lighter, for argument let's say IPK1 was 1.1 kg and IPK2 was 0.9 kg. Now that we have a constant for the mass of 1 kg, all of those IPK's will now have a new mass relative to the new definition. Unless the new definition was based on the current IPK and set its mass as of today as the standard, and that standard now will be unchanging?
- ekimekim 8y ago
  > Unless the new definition was based on the current IPK and set its mass as of today as the standard, and that standard now will be unchanging? Correct. The current IPK will still be 1kg to within margin of error at the time the new definition comes into effect, but any further changes to it will make it, for the first time, drift away from that value.
  - cco 8y ago
    Ah I see, that makes sense, thanks!
  - Digit-Al 8y ago
    Thanks. That does make sense.

ska 8y ago

It's probably better to think about the other way around. Despite heroic efforts, the "old" kilogram was getting (a tiny big) lighter every time it was handled. The "new" kilogram will finally be a constant. To a reasonable approximation it hasn't changed at all at this moment time.

Digit-Al 8y ago

Thanks for all the comments. Very helpful.