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
Short answer: certified [gage blocks](https://www.mcmaster.com/2210A85/)
They are traceable, certified accurate, and expensive. But with a full set, you can replicate any measurement required, and create/check a custom gage for that dimension that you can use on your workpiece. You can also calibrate your in-house measuring tools (calipers, micrometers, and the like) using your certified gage blocks.
They come certified to be accurate within a set tolerance. They periodically need to be checked to make sure that they still are within that range, and renew the certification. This is generally done by specialized calibration companies.
The other part is [Handbook 44](https://www.nist.gov/system/files/documents/2017/04/28/hb44-15-web-final.pdf), which gives acceptable tolerance standards for measuring devices. As long as the product falls within the tolerance range, it’s good to go. Linear measures are in Section 5.52, and a one foot ruler has a tolerance of 1/32”.
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