Most states have weights and measures departments that certify all of the tools used in commerce (scales, gas pumps, taxi meters, etc). These tools are very highly regulated -- often the internals are inspected by state officials, the devices are sealed, and operators are required to bring them back in for periodic re-testing. This may seem obstructionist, but it's based on a long history of dishonest merchants tampering with this type of equipment to defraud customers.