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
A calibration standard is, in general, calibrated to a better quality standard at a higher laboratory with better comparison equipment. However, at some point, there has to be a top laboratory with a reference standard which is the end of the chain.
Historically, this was with special specimens kept in very careful conditions, which were carefully built. For example, for many years, a laboratory in Paris kept a stick with two marks 1 meter apart engraved on it, and this was the reference meter. Another laboratory might get a stick and put two marks on it – but it would then have to be shipped to Paris, and measured against the reference meter stick. The lab would then key a record of the exact length.
These days, measures have been redefined to something fundamental which you can measure with a scientific experiment. The official meter is no longer the length of a stick in Paris, but there is an equation for the length of a meter as compared to the result of a scientific experiment. For example, top calibration labs don’t have use sticks as their top reference any more. Instead, they have a scientific apparatus which can perform a laser spectroscopy experiment which allows the time it takes for light to travel a certain distance to be measured. The lab can put a stick in the apparatus, and it will be able to give the exact length based on the equation and the result of the experiment.
Similarly, the second used to be defined as a fraction of the length of the day. A calibration laboratory would do an experiment to measure the height of the sun, and they could compare a clock to when the sun reached it’s highest point in the day marking noon. These days, the second is now defined as a multiple of the frequency of a specific transition of a cesium atom. This transition frequency can be measured by microwave spectroscopy, and you can compare a clock to the transition, and you can adjust the clock as needed. In fact, you can go out and buy an atomic clock, which is just a good quality clock, packaged with a spectroscopy apparatus and an auto-adjust system which checks the clock against the spectroscopy apparatus hundreds of times per second and adjusts the clock as needed.
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
A calibration standard is, in general, calibrated to a better quality standard at a higher laboratory with better comparison equipment. However, at some point, there has to be a top laboratory with a reference standard which is the end of the chain.
Historically, this was with special specimens kept in very careful conditions, which were carefully built. For example, for many years, a laboratory in Paris kept a stick with two marks 1 meter apart engraved on it, and this was the reference meter. Another laboratory might get a stick and put two marks on it – but it would then have to be shipped to Paris, and measured against the reference meter stick. The lab would then key a record of the exact length.
These days, measures have been redefined to something fundamental which you can measure with a scientific experiment. The official meter is no longer the length of a stick in Paris, but there is an equation for the length of a meter as compared to the result of a scientific experiment. For example, top calibration labs don’t have use sticks as their top reference any more. Instead, they have a scientific apparatus which can perform a laser spectroscopy experiment which allows the time it takes for light to travel a certain distance to be measured. The lab can put a stick in the apparatus, and it will be able to give the exact length based on the equation and the result of the experiment.
Similarly, the second used to be defined as a fraction of the length of the day. A calibration laboratory would do an experiment to measure the height of the sun, and they could compare a clock to when the sun reached it’s highest point in the day marking noon. These days, the second is now defined as a multiple of the frequency of a specific transition of a cesium atom. This transition frequency can be measured by microwave spectroscopy, and you can compare a clock to the transition, and you can adjust the clock as needed. In fact, you can go out and buy an atomic clock, which is just a good quality clock, packaged with a spectroscopy apparatus and an auto-adjust system which checks the clock against the spectroscopy apparatus hundreds of times per second and adjusts the clock as needed.
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