How do companies that make measuring tools like rulers make sure the product is accurate? Is there a universally ruler that is used to check? How do they make sure the measurements are exact?
In: Engineering
There are primary standards locked away from which secondary standards (copies) are made and certified. Those copies are sent around and used to make calibration standards. All standards are traced everytime they are copied, and become a little less accurate with each copy.
1. Company makes ruler.
2. Company checks ruler against their calibration standard and passes or fails ruler.
3. Company calibration standard is sent to a Metrologist every so often. Metrologist checks company standard against their standard.
4. The metrologist standardis checked against a secondary standard.
5. The secondary standard is traced to the primary.
There could be more copies than listed in my example above but that is the general idea. The unit of length is no longer based on a physical object but rather the speed of light in a vacuum. NIST is responsible for maintaining this system.
Edit: adding a great video that is easy to follow. https://youtu.be/EufnURd1U2s
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