ELI5, How are precision calibration tools, themselves calibrated?

Question

Feels like a chicken and egg senario. Let’s say I get my torque wrench from work sent off to be calibrated, and that’s calibrated with something itself needs to be calibrated, and so on and so fourth. How’s that figured out?

In: 430

Answer 1

The SI system is what defines meters, seconds, kilograms, Newtons etc. Your wrench may use foot-pounds-force, but feet and pounds-force are nowadays defined as certain numbers of meters and Newtons.

The SI system is defined in such a way that scientists can carry out experiments to get the length of a meter etc precisely.

For example, a particular atom in a particular state will give off radio waves with a specified number of waves per second. By counting waves, you can measure a second. And this is not an approximation, the second is defined as the time it takes for a certain number of waves.

Another example: Light and radio waves travel through vacuum at a fixed speed, which is specified in the SI standard. Using an accurate clock (see previous paragraph), you can measure how far light goes in a certain fraction of a second, and that is a meter.

Of course, all the above is completely impractical for day to day use.

So there are a small number of labs worldwide, typically one per country, which specialise in measurement. They will have a number of standards, such as 1kg lumps of metal or metal sticks with two marks precisely 1m apart. Those standards will have been checked by the experiments above, or against standards that were calibrated against those experiments. (E.g. There are only a handful of labs that have done the kilogram experiment).

In turn, those standards will be used to calibrate other standards or measuring devices, which will be used to calibrate other standards or measuring devices, and this repeats many times until one of those calibrated devices is used to calibrate your wrench.

Each time you calibrate something you end up with less accuracy than you started with. But your wrench probably doesn’t need to be accurate to one part per million, even one part per thousand is probably overkill.

Answer 2

The SI system is what defines meters, seconds, kilograms, Newtons etc. Your wrench may use foot-pounds-force, but feet and pounds-force are nowadays defined as certain numbers of meters and Newtons.

The SI system is defined in such a way that scientists can carry out experiments to get the length of a meter etc precisely.

For example, a particular atom in a particular state will give off radio waves with a specified number of waves per second. By counting waves, you can measure a second. And this is not an approximation, the second is defined as the time it takes for a certain number of waves.

Another example: Light and radio waves travel through vacuum at a fixed speed, which is specified in the SI standard. Using an accurate clock (see previous paragraph), you can measure how far light goes in a certain fraction of a second, and that is a meter.

Of course, all the above is completely impractical for day to day use.

So there are a small number of labs worldwide, typically one per country, which specialise in measurement. They will have a number of standards, such as 1kg lumps of metal or metal sticks with two marks precisely 1m apart. Those standards will have been checked by the experiments above, or against standards that were calibrated against those experiments. (E.g. There are only a handful of labs that have done the kilogram experiment).

In turn, those standards will be used to calibrate other standards or measuring devices, which will be used to calibrate other standards or measuring devices, and this repeats many times until one of those calibrated devices is used to calibrate your wrench.

Each time you calibrate something you end up with less accuracy than you started with. But your wrench probably doesn’t need to be accurate to one part per million, even one part per thousand is probably overkill.