Weight gain at NIST
We like to measure things – and NIST is our source of the best measurement advice. We knew that time has been measured with astonishing accuracy, almost 1 part in 10^15.
This shows the improvement in clock accuracy at NIST from 1950 to 2000.
Given how accurately time is measured, we wondered ‘how is the kilogram calibrated?’
We learned that, according to NIST,
“The magnitude of many of the units comprising the SI system of measurement, including most of those used in the measurement of electricity and light, are highly dependent upon the stability of a 131-year-old, golf ball-size cylinder of metal stored in a vault in France”.
And over the years the International Prototype Kilogram, the IPK, and its copies that are distributed around the globe, have ‘gained weight’. There’s more here at wiki.
Here’s the graph to show it:
Weight change of the standard kilogram and its copies over 100 years.
NIST further explains:
“Mass drift over time of national prototypes K21–K40, plus two of the IPK’s sister copies: K32 and K8(41).[Note 9] All mass changes are relative to the IPK. The initial 1889 starting-value offsets relative to the IPK have been nulled.[12] The above are all relative measurements; no historical mass-measurement data is available to determine which of the prototypes has been most stable relative to an invariant of nature. There is the distinct possibility that all the prototypes gained mass over 100 years and that K21, K35, K40, and the IPK simply gained less than the others”.
Uncertainty, it seems, is here to stay.
However, there’s an improved kilogram standard being worked on, called the Watt Balance, which measures the electrical power used to null the weight of a one kilogram test specimen. To get accurate results, NIST must establish the gravitational force accurately. Here’s a link to a NIST article describing how they do it:
http://nvl.nist.gov/pub/nistpubs/jres/106/4/j64schw.pdf
Which reminds me of a Jim Williams article from Linear Tech, where he describes a VERY accurate electronic weigh scale, capable of measuring your heartbeat – since all the blood pumping and flowing changes your weight, a little bit. He called it a ‘ballisto-cardiogram’. Here’s where you can read more, in AN43 that Jim wrote for Linear Tech, about bridge circuits – including how to make a scale that can resolve 0.01 pound at 300 pounds full scale, or about 33 parts per million. It uses a clever circuit to achieve balance quickly and accurately, check it out: