Comments on: A new definition of the kilogram?

by: Markus Kuhn

Sat, 23 Jun 2007 16:34:43 +0000

> better than 1 nm (which is approximately two wavelengths of light)

Since the wavelength of visible light is 400–700 nm, two wavelengths of light are about 1 µm, not 1 nm.

by: Martin Vlietstra

Mon, 18 Jun 2007 11:18:56 +0000

Following on George’s comments - when the kilogram was first defined, people’s understanding of thermodynamics was significantly less than it is now – in particular I do not believe that the behaviour of saturated vapour pressure was fully understood in 1799 – Clapeyron who pioneered that work published his papers in the 1830’s.

If we were to use one litre of water at its maximum density as the definition of a kilogram, we have problem – it we weigh the water in vacuum, we will not have a vacuum for long – some water will evaporate, so we could weigh the water in air and take the air’s buoyancy into account. Again, either the water would evaporate or if we used fully saturated air, we would get condensation where we do not want it.

We could try putting the water into a sealed container, but making the container exactly one litre to within one part in 10^8 presents engineering problems that are probably insurmountable. We would furthermore need to ensure that the water in the sealed container was at a specified pressure – water is slightly compressible.

All told, I think that it can be seen that using water instead of a metallic solid presents many problems. The technique used for updating standards is to use the best possible technique of the day and as part of the change-over to calibrate the new standard so that it matches the old as closely as possible.

by: Phil Hall

Sun, 17 Jun 2007 19:48:32 +0000

In answer to the points made by George Carty:

It is important that we don’t change the value of the kilogram.

Although it was originally tied to the properties of water, advancements in measurement technology and physics generally showed that the density of water was subject to too many variables. It was a bit like the metre being bound to a quadrant of the Earth which subsequently turned out not to be the uniform geometric shape envisaged at the time.

The problem facing 21st century metrology is to establish a stable, reproducible reference for mass that does not invalidate existing data (i.e. mass measurements made with reference to the current prototype) along the same lines as the metre and its original prototype.

by: George Carty

Sun, 17 Jun 2007 12:28:06 +0000

Fixing the value of the kilogram?

Would it be possible to change the value of the kilogram so that it was exactly equal to the mass of 1 litre (1 cubic decimetre) of water at 3.98 deg. C (maximum density) and atmospheric pressure?

The standard kilogram was meant to have this value of mass, but an error in its manufacture meant it actually had the mass of 1.000028 cubic decimetres of water.