The US Department of the Navy has an enlightening short film about why calibration matters. In the film, cannonballs are made outside of specifications: too large or too small. When it comes time to fire them, they are either too big to fit inside the cannon barrels or too small and don’t fire strongly enough to damage enemy ships. The manufacturers, storage depots, naval yards and ships stores are all continuously checking the cannonballs for quality control. However, each of these points is using a calibration instrument that has drifted off target: the cannonballs move like pinballs back and forth through the system, rejected by one station and accepted by another, consuming time and money and massively reducing the Navy’s fighting effectiveness.

Clearly illustrated here are two fundamentals concerning calibration:

1. Without a calibration system, both output and measurement tools will drift off target over time
2. The results of failing to calibrate correctly and consistently are large losses of money, time and effectiveness.

Assuming you’re not in the Navy, what consequences of ignoring calibration of your equipment can you expect? Products being tested could be accepted when they are bad, and rejected when they are good. Inaccurate readings can lead you in a false direction in research, product development or troubleshooting. A contractual agreement may state that your output has to be within specific tolerances, and ignoring calibration may result in the customer breaking the contract, issuing fines, revoking your operating license or other punishment. Consumer and employee safety may be threatened. The environment could be damaged by emissions. Nielsen research and ATS (in 2006 and 2008) averaged the cost of manufacturers’ calibration errors to be over $1.7 billion each year.

Calibration requires a known standard of accuracy. The performance of an instrument is compared to this known standard, and adjusted back to conformance. You do this after the power goes out at your home. When electric service returns, you find a clock that you know is displaying the correct time, and then go around resetting all the flashing clocks in the house. The reading provided by all of the clocks is then known to be accurate – or, accurate enough for your tolerances.

However, testing one instrument against another is not calibration. This is called field testing. Both instruments could be out of calibration by the same amount and direction, and you would learn nothing. Also, you would not know which instrument read correctly. You must have a standard which you know is accurate, and the most proper calibration procedure has a chain of information that links it back to a maser standard at the National Institute of Standards and Technology (NIST). A minimum accuracy of ten times the accuracy of the equipment being tested is required for a calibration standard. This way, calibration within overlapping tolerances is not possible and the equipment will assuredly be calibrated correctly.

What causes things to shift out of calibration tolerances? Most components degrade over time – some very quickly, some over long periods. A calibration schedule is usually what discovers and corrects this drifting. Many items can go out of calibration because of shock: from being dropped, for example. Some can be overloaded by electricity which knocks out protective devices, causing damage to reading meters.

Calibration testing must therefore be completed:

  • When the manufacturer recommends
  • When the work to be done specifies
  • Before and after an important testing or measuring task
  • After a physical shock or electrical event
  • At regularly scheduled intervals (annually, quarterly, monthly)

Calibration often requires sending the equipment to a service depot or back to the manufacturer. They will calibrate it using NIST-certified standards. Keeping employees and consumers safe, controlling emissions, considerably reducing production and rework costs, saving time, ensuring quality measurements and output, and improving the effectiveness of results are all important reasons to calibrate your equipment and instruments.

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Jason Kanigan is a technical writer for Global Test Supply, a distributor of test and measurement equipment.