"Weirdly, it’s not even known if the IPK is getting lighter, or if the national prototypes are getting heavier — but either way, something is causing these kilos to change weight, by around 50 micrograms every 100 years."
Well they COULD always do that thing with the water where they heat it to 4 degrees celcius and measure its volume and weight. Then they'd know which it is.
Well, they're using the static, known density at 4 degrees Celsius to produce water with a known mass, and then test what the measurement of that mass comes to. Basically they're using a known mass of water as a calibration aid. But I don't know that this level of precision could compete with the efforts underway.
Exactly. They'd COMPARE it to the existing physical object used as the reference for the kg. Assuming water allover the world hasnt changed its properties over the last few centuries they'd know whether it became heavier or lighter.
Well they COULD always do that thing with the water where they heat it to 4 degrees celcius and measure its volume and weight. Then they'd know which it is.
That said, there's a nice video on this: http://youtube.com/watch?v=ZMByI4s-D-Y