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 good news is that, along this chain there are definitely “accuracy multipliers” and forms of natural calibration that are often “good enough” to meet accuracy requirements.

For example: I could measure one foot-pound on a torque wrench with a balanced two-foot bar and a one-pound weight. Both the weight and distance have to be pretty accurate.

But suppose it’s a ten-pound weight and a 20-foot bar, and a mechanism (1:100 gearing) to reduce that torque by a factor of 100. Still one foot-pound, but any inaccuracy on the weight and distance is divided by 100!

Some calibration (when extreme accuracy isn’t needed) is easy. Ice water is always 0C; boiling water is 100C, so there’s probably the most common reference for thermometer calibration.

Answer 2

The good news is that, along this chain there are definitely “accuracy multipliers” and forms of natural calibration that are often “good enough” to meet accuracy requirements.

For example: I could measure one foot-pound on a torque wrench with a balanced two-foot bar and a one-pound weight. Both the weight and distance have to be pretty accurate.

But suppose it’s a ten-pound weight and a 20-foot bar, and a mechanism (1:100 gearing) to reduce that torque by a factor of 100. Still one foot-pound, but any inaccuracy on the weight and distance is divided by 100!

Some calibration (when extreme accuracy isn’t needed) is easy. Ice water is always 0C; boiling water is 100C, so there’s probably the most common reference for thermometer calibration.