Agfdhyk

With the redefinition of kilogram currently underway, I see recent online articles claiming that a precise definition of the kilogram is critical for drug development and nanotechnology. E.g. BBC.com

In a world where accurate measurement is now critical in many areas, such as in drug development, nanotechnology and precision engineering - those responsible for maintaining the international system had no option but to move beyond Le Grand K to a more robust definition.

Or joe.ie

Accurate measurement is critical in many areas of the world today, such as in drug development, nanotechnology and engineering and is among reasons the Le Grand K rule is being changed.

Is this claim justified? Can someone please exemplify by describing such a process where more accurate mass measurements make a difference?

Is redefining the kilogram useful for drug development and nanotechnology?

Probably not, and almost certainly not with regard to drug development.

The redefinition of the kilogram achieved two key goals:

• Getting rid of the very last physical prototype that underlies the International System of Units.


• Breaking past the tenth of a part per million accuracy level with regard to mass.

Neither the pharmaceutical industry nor nanotechnology industry cares about the first goal, which is a rather academic concern. I don't know about the uncertainty or accuracy requirements of the nanotechnology industry, but for the pharmaceutical industry, those requirements are on the part of a part per thousand. To wit, from USP General Chapter 41, Balances:

Repeatability is satisfactory if two times the standard deviation of the weighed value, divided by the desired smallest net weight (i.e., smallest net weight that the users plan to use on that balance), does not exceed 0.10%.

The accuracy of a balance is satisfactory if its weighing value, when tested with a suitable weight(s), is within 0.10% of the test weight value.

Routinely attaining better than a part per thousand uncertainty and accuracy in assessing mass is rather nontrivial. Doing so requires very precise calibrated equipment and very careful attention to environmental details. This is the scale on which the pharmaceutical industry operates.

Attaining a tenth of a part per million uncertainty and accuracy is well beyond nontrivial. This is the scale at which the former prototype-based definition of the kilogram was problematic. That extreme of uncertainty and accuracy is the scale at which the very few laboratories of a national level research institutes operate, along with the level at which the very few academic laboratories funded by those national level research institutes operate. This extreme accuracy level is not the scale at which the pharmaceutical industry operates.

From the NPL (National Physical Laboratory) in the UK, there are three problems with the current system (using Le Grand K):

1. Its weight changes over time


2. These changes are unpredictable


3. The national copies cannot be monitored with the highest level of
   accuracy

Further, the system is expensive and prone to difficulties. The maintenance of Le Grand K is expensive and a logistical nightmare. The national copies exhibit varying drift from LGK - as much as 2 micrograms per year. Rectifying these drifts can leave the nations that depend on the copies without them for as much as 6 months.

Further, the kilogram is the only remaining SI unit still defined by a physical artifact.

According to NIST, even the US and UK pound measurement of mass is defined in relation to the kilogram. This makes the kilogram the standard for the most widely used units of mass in the world. Also from the article:

Moreover, this mass-comparison system is not easily scalable from
large to small. The smaller the scale, the larger the uncertainty in
measurement because a very long sequence of comparisons is necessary
to get from a 1 kg standard down to tiny metal mass standards in the
mg range, and each comparison introduces an added uncertainty.
As a result, although a 1 kg artifact can be measured against a 1 kg
standard to an uncertainty of a few parts in a billion, a milligram
measured against the same 1 kg has relative uncertainties of a few
parts in ten thousand.

However, the question isn't whether it is a good idea to change the definition to be based on a fundamental constant (it undeniably is), but whether medical and nano-technological processes are negatively affected by the small drift witnessed in the baselines.

To start, it's important to realize that the kilogram is a base for a number of other SI measurements based on mass - such as the volt or the ohm. It is likely that the correction of the small amounts of drift in the kilogram will benefit these things much more than straight mass measurements, but that is mere speculation on my part.

Additionally, NIST says this:

That uncertainty is not satisfactory for the ever-more-demanding needs
of modern measurement science, device manufacture, material science,
pharmaceutical research and testing, and environmental monitoring, to
name only a few. Increasingly, those endeavors require accurate
measurements on the order of micrograms (millionths of a gram) and
smaller.

For nanotechnology, the benefit seems plain - at the scale of molecules, a micro-gram is an error magnitudes larger than those in use to measure the components. And electrical measurements are also likely critical in this space. As nanotechnology can involve the manipulation of a single atom, which weighs on the order of zeptograms.

For biochemistry (what drug development mostly boils down to, though the analysis of body processes - required for research/testing - could involve electrical measurements as well), measurements down to the microgram are considered common enough to be given as an example in a discussion of measures. As such, 2 micrograms/year of drift would constitute a significant fraction of the measurement.

So, yes, the accuracy of the definition of a kilogram is critical today for scientific endeavors, and will grow more critical in the future.