Large companies can cost-justify hiring specialists in gage calibration and keeping equipment to perform calibration in-house. But for most shops, the economical approach is to hire a calibration service.
ISO 9002, which applies to all manufacturing operations, requires suppliers to calibrate “all inspection, measuring and test equipment and devices that can affect product quality at prescribed intervals, or prior to use, against certified equipment having a known valid relationship to nationally recognized standards—where no such standards exist, the basis used for calibration shall be documented.” (ISO9002.4.10.b)
“Prescribed intervals” usually translates to a minimum of once a year. Where annual calibration is inadequate to ensure accuracy, a shorter interval must be established. “Certified equipment having a known, valid relationship” means that the calibration house must have its own equipment (standards) certified. In the United States, “nationally recognized standards” implies the National Institute of Standards and Testing (NIST), although other standards, such as DIN, may be used to satisfy overseas customers. “Where no such standards exist,” usually refers to highly specific industries or products in which manufacturers must develop their own standards and test methods. Calibration providers issue a certificate of calibration for every gage tested. These certificates are essential for documenting calibration programs.
Some providers automatically remind clients which gages need to be calibrated and when. Most gages can be boxed and shipped to the calibration provider. With large, elaborate gages, the calibration service will come prepared with NIST-traceable gage blocks, precision balls, a thermometer and any other standards needed to perform the job.
How can a machine shop without expertise in calibration select a provider? Naturally, cost and turnaround time are important, but don’t sacrifice quality for convenience. Above all, ISO 9000 requires that consistent procedures be applied, and any professional calibration house should be able to document its methods in a procedures manual. Ask to see it; if it’s unavailable, look elsewhere.
In the past, selection of a calibration lab often came down to the supplier’s reputation. Today, there are national organizations with international agreements that certify a laboratory’s ability to make precision measurements. Organizations such as A2LA and NVLAP provide auditing to ensure that a calibration provider has a procedure that meets the standards defined for the measurements. They ensure that procedure is followed and assess the provider’s ability to perform the calibration process in various areas. This allows the lab to say it is certified.
Calibration houses are required to state their capabilities. This allows customers to make informed decisions as to which provider poses the lowest risk. For automotive companies that have ISO/TS 16949 certification (which was QS-9000), calibration must be performed by an accredited laboratory. In addition, aerospace, medical and nuclear companies are seeking calibration laboratories that have been accredited so they can document that the tightest “measurement of uncertainty” is passed on to customers.
When choosing a provider, ask questions. What are its areas of expertise? How are technicians trained? What equipment is used, and to what standards can methods be certified? What quality control methods are used? What are the uncertainty numbers for the measurements the provider is accredited to make? What are the control tolerances on temperature and humidity? How is equipment protected from outside vibration? A visit to the provider may be in order.