The National Physical Laboratory (NPL) is the UK’s National Measurement Institute and has been managed on behalf of the Government by a company called Serco for 16 years. The NPL was formed in 1900 and is one of the top three such institutes in the world. A prime function for the NPL is to ensure that measurements made in the UK are equivalent to those made in the rest of the world. Every developed country has an equivalent organisation.

This manifests itself in many ways, from facilitating trade through ensuring something weighed or measured has the correct value associated with it (ie weighing a bag of sugar or trading commodities), through to things like the measurement of the temperature of the earth through satellite observation; making sure that the temperature measured by one satellite systems is equivalent to that measured by another.

Measurement is easy?

Measurement is easy, is it not? We all have a ruler, we all have scales we weigh ourselves on in the morning. If you were to go back 2,000 years, the Romans could do measurement – they were very good at it – it facilitated engineering achievements that remain impressive even by modern standards. An example of this is the water system built to take water to Nimes in France. Over a 50 km distance the height of the waterway changes by less than 17 metres, including the construction of aquaducts such as the Pont du Gard. To get the surveying right, to actually get the water to flow in the right direction over the whole course of it, remains impressive.

Indeed even earlier than the Romans, the Egyptians were using measurement to great effect, for example in the construction of the pyramids. However, to underestimate the difficulty of measurement is to ask for problems. In the 20th century the Laufenburg Bridge was constructed between Germany and Switzerland, both extremely impressive, leading engineering countries. Unfortunately when the construction teams reached the middle there was a 54 cm difference in height between the two halves of the bridge. In the UK, about five years ago, a survey of blood pressure measurement was carried out in southeast London: about 30% of all the measurements were actually incorrect, and not incorrect by a subtle amount. They were incorrect by an amount that meant people were getting incorrect medication. So, it is important that people get measurement right, even in something where it looks routine. It is very easy to become complacent about whether a measurement is correct or not.

It is very easy to become complacent

The Measurement System in the UK

The National Measurement Office, on behalf of the Department for Business, Innovation and Skills, funds four different organisations to deliver the core of the UK Measurement system, shown in Figure 1.

Mass measurement lies within the scope of physical measurement and the UK’s mass scale is derived from NPL, where the national mass standard (kilogram 18) is held. Once NPL has realised the mass scale it is disseminated throughout the UK either directly to industrial users or via accredited laboratories. The United Kingdom Accreditation Service(UKAS) is responsible for ensuring that these accredited laboratories are competent to perform measurements in an appropriate manner.

There has been a lot of investment over many years into this accreditation system, and it is, by definition, the way that all mass measurements should be traceable. Mass is also interesting as it is a base unit meaning that other units of measurement are derived from it, such as pressure, humidity and force. So, we also calibrate items to facilitate that.

The exception to the traceability route outlined above is weights used for trading purposes (legal metrology). In this case the dissemination happens from NPL via the National Measurement Office.

Users of Measurement

Who uses measurement? Industry: for example, a company like Rolls Royce, building jet engines, needs to know that the parts fit together and align correctly, and that the turbine blades are not going to melt in the engine. In medicine NPL provides traceable measurements throughout all the radiography departments in the UK, where cancer is being treated with very precise radiation doses. A further example is in Health and Safety where it is vital to know if a cable used to tether an oil rig can withstand the force that will be exerted on it.

In Europe, it is estimated that between 2% and 7% of gross domestic product is directly related to measurement. It is absolutely a key parameter, and it is essential that everybody gets it right.

Mass Measurement

Today, the situation for dissemination of mass measurements in the UK is virtually identical to that of all the other industrialised countries in the world.

The kilogram

Figure 2 shows the International Prototype of the kilogram (a weight that is defined to weigh exactly 1 kilogram and from which the mass of all other weights in the world are derived). It is a cylinder of platinum-iridium that is kept in a vault just outside Paris under two bell jars, to protect it from atmospheric contamination. Around the world, there are about 90 copies of this kilogram. They were made somewhere between 1880 and 2000, in three or four different batches.

This definition of the kilogram dates back to the late 19th century when scientists where seeking practical ways to define the basic measurement units. Considerable thought was given to the choice of artefact in order to make it as stable as possible:

Shape – The international prototype of the kilogram is a cylinder of height equal to its diameter (about 39 mm). With the exception of a sphere, which would be difficult to handle in a safe manner, this shape represents the minimum surface area for a given volume of material. This minimises the chance of contamination on the weight.

Material – Platinum iridium was chosen as it is chemically unreactive and it is dense – so minimising the volume and hence surface area of the weight. It should also be noted that an alloy of platinum-iridium is used as this is much harder than pure platinum. This is important if the weight is to avoid being damaged in use. The material for most of these platinum- iridium kilograms was manufactured at Johnson Matthey in London.

This definition has served us well for over 100 years, but it is not as robust as the definitions of other base measurement units (such as the metre or the second) which are based on fundamental scientific constants (eg the speed of light). The definition means that any change in the mass of the international prototype of the kilogram would change the value of all of the mass scales in the world. Figure 2 shows how the mass of other copies of the kilogram have varied relative to the international prototype. There has been a 50 microgram, or 50 parts in a billion relative change. Given that these copies were made at the same time as the international prototype using the same production methods, there is no sound reason why there should be significant changes in their relative values. If you are weighing pharmaceuticals or if you are measuring very large forces, such as the sort that tether oilrigs, these uncertainties and errors extrapolate and grow. So these anomalies are important.

In the UK, we have three official copies and an unofficial copy of the kilogram, which allows us to ensure that our copy of the kilogram is not changing mass relative to the others.

Figure 2

Improving Definitions

There is a requirement to improve how we define mass. Our current method is good enough for industrial practice because everything is consistent. This is the whole purpose of having an organisation like the NPL. It makes sure that the measurements made in the United States are equivalent to those in Germany, the UK, France, Malaysia, wherever. However, it does not mean that it is right; it just means we all get the same answer because we use the same methods.

It is worth noting that we believe the UK’s kilogram has been stable to within 14 micrograms – 14 parts per billion – over the last 100 years. The weight gets dirty even though we keep it in a glass bell jar with a filtered air inlet. It is just an aspect that metal gains weight, as you store it in air, because it gets contaminated. We have systems here we can use to clean this and always have a consistent answer. If you look at most of the weights that have been kept within Europe and North America, they are of a similar performance to that of the UK.

Redefining the kilogram

Within the next five to 10 years, the kilogram will be redefined, and it will be redefined in terms of a constant of nature, probably Planck’s Constant. This will remove the reliance on a single artefact that could change over time. There are two ways that this can be done:

Silicon-based kilogram

One of these methods is to use a known weight such as an atom of silicon. If you have enough atoms, you can make a kilogram. So, if you can measure the volume of a sphere made from pure silicon and know all about its surface and know the spacing between the silicon atoms, you can define a kilogram in those terms. It is actually very difficult to do, and work (through an international consortium) has been in progress on this for 20 or 30 years.

An electrical kilogram

Another possibility, which we at NPL like, because we invented the method, is that you can balance the gravitational force on a weight against an electrical force. The electrical force is analogous to an electric motor. A current flows through a wire wrapped around a magnet that generates a force. We know the electrical unit of current and we can measure magnetic fields – or actually we can cancel them out – very well. So, if you can balance that electrical force with a gravitational force, you can define the kilogram in terms of that. This is likely to be the method adopted to redefine the kilogram within the next five years.

There are around five different experiments around the world looking into this. The reason it cannot be redefined at the moment is that, whilst we can achieve quite low uncertainties, they do not agree. We are pursuing experimentation to make sure that the different experiments come together and establish what the true value is. This will probably happen somewhere around 2015-18.

Concluding Remarks

The measurement procedures for precious metals are fine in terms of where they are now, and they have evolved to where they are now such that the requirements in terms of the uncertainty you can achieve in a measurement, the potential error in that measurement, are very closely linked to the technologies used in the weighing process.

The one warning I would give is that if, all of a sudden, you improve one parameter or one method in this – say, for example, you go and use improved balances (which are available) – there is a real risk that you think you are doing things with lower measurement uncertainties than you actually are. When you compare platinum-iridium with a stainless steel weight, it will depend on the density of the air at the time. In the UK, the density changes by about 10% during the year. So, you will get a different answer in the summer, when the pressure is high, than in the winter, when the air pressure is lower unless, you make appropriate corrections. The same is also true if you compare gold with stainless steel weights. As you move to better weighing technologies, you need to take into account these other effects, or you could create a problem going forward. It is just a question of being aware of that and making corrections.

Questions and Answers

Stewart Murray

In the bullion market, we use beam balances and we also use electronic scales. Will the effect that you mention of different air densities affect both of these equally or one more than the other?

Dr Ian Severn

Both equally. The key thing is the difference in volume or density between the standard with which the electronic balance was calibrated and the density of the item you are weighing, so in your case, gold, which is of course much more dense than steel. There is nothing wrong with the procedures at the moment. I suspect that, as with most procedures, they have evolved to a common well-defined place.

Stewart Murray

What is the difference that you could get between the highest and the lowest air pressure, weighing something like a 400 troy ounce gold bar? Are we in the eighth or ninth significant figure?

Dr Ian Severn

No, it is fewer significant figures than that. It is going to be at the fourth or fifth significant figure, potentially – perhaps fifth or sixth.

The other thing that you have to bear in mind that as you are at different altitudes, the air density changes even more between locations. For example, in the UK, the typical air density is about, ignoring the units, 1.2. In Madrid, it is typically about 1.02. So, there is a huge change: 20% or more.

Stewart Murray

When you have got an electronic scale and you take it to Madrid or Colorado, do the manufacturers correct for the altitude and the different gravity in these different places?

Dr Ian Severn

It actually does not matter in terms of the gravity and the altitude: because if you calibrate the scale using a weight in the same location, it all cancels out. So, electronic balances work well wherever; it is just about being aware of the more subtle effects like buoyancy and the air density that are the key things.

Gareth Owen-Jones (Sartorius UK)

It really becomes an issue if you move the balance from one place to another, because if you weighed something here and then took the balance to Mexico City and did not calibrate it, there would be an enormous difference.


If you weigh something in your own part of the world and then you compare the weight to the LBMA, according to their assaying and their weighing balances, would that have a difference?

Dr Ian Severn

As long as weighing is done correctly, gravity should not make any difference. Whether you are using a two-pound balance or an electronic balance, gravity is compensated for by the weighing process, as long as it is done correctly. The particularly important thing is that the electronic balance should be calibrated in the location where it is going to be used. If that is done, then there should not be a problem.

Ian Severn is head of the Engineering Measurement Division at the National Physical Laboratory, the United Kingdom's national measurement institute. He started his career there 20 years ago as a mass metrology specialist and became Head of the Mass and Density Group. The area he heads includes responsibility for the realisation of the UK measurement scales associated with length, mass, temperature and optical radiation.