Calibration: What You Need to Know
Calibration, certification and accreditation are terms that are sometimes used interchangeably, but they are actually not the same.
Calibration is not just about having stickers on your equipment with dates that say everything is okay. All gaging equipment must be calibrated periodically to ensure that it’s capable of performing the job for which it’s intended, i.e., measuring parts accurately. Whether you’re a small machine shop or a large multi-million dollar manufacturing giant, you undoubtedly use any number of gages or gaging systems to maintain the standards for quality in your production. The accuracy of these dimensional measuring instruments must be periodically checked to ensure that they are working properly.
Maintaining and checking the performance of hundreds or thousands of gages can be a pretty costly proposition, whether you buy the equipment and hire the people to do it internally, or send the gages to an outside calibration service to perform the checking for you. For most manufacturing facilities, the most economical approach is to hire a calibration service to do this verification for you.
How can a small machine shop or even a large high-production facility without expertise in calibration intelligently select a provider? Naturally, cost and turnaround time are important, but you don’t want to sacrifice quality for convenience just to save a few dollars.
Calibration, certification and accreditation are terms that are sometimes used interchangeably but they are actually not the same. When it comes to selecting a provider for these services to ensure that your measuring equipment is measuring parts accurately, you should understand the differences between these terms.
Calibration and Certification
Calibration is a method or a process to determine if the item under test is meeting or not meeting its performance requirements. A gage block is a good example of this. It is brought into an environmentally controlled room and measured on a piece of equipment that compares it to traceable standards. It is measured by a trained operator using a defined measuring process that has been established and had all potential errors relating to the process identified and their potential effect quantified. Once the gage block has been measured by this calibration process, it can be said to be within specification or not. If within specification, a calibration certificate is issued stating the block is within the specification defined and the actual measured value is shown. When a gage block is found to be out of spec, there is usually not much you can do to bring it into calibration specification; therefore it is commonly replaced or may be downgraded to a lower grade block where it would meet the calibration specs.
Many gages are more complicated than this simple gage block example. In these cases, the calibration process is performed and if the gage under test does not meet specification, then there is an opportunity to adjust the performance (or calibrate) the gage so that it does meet specification. This process will work whether the gage is a micrometer, dial indicator, bench amplifier/probe combination, or even a complex surface finish or form measuring system.
When selecting a calibration supplier for the gaging and gaging systems in your facility, accreditation is an important consideration. While calibration is the process of measuring—or measuring and adjusting—an item to ensure that it is within the performance specification, “accreditation” is the process of verifying that the organization doing the calibration is able to do so correctly.
Since the mid-1990s, there has been a revolution in setting standards for calibration laboratories. ANSI/NCSL Z540-1 was introduced in 1994 and was based on two previous standards: ISO/IEC Guide 25 and MIL-STD 54662A. In 1999, the international community first released 17025. ISO/IEC 17025 has been accepted by Mutual Recognition Arrangement as the international standard for accreditation of calibration and testing laboratories.
ISO 17025 accreditation for a lab or a field service organization performing calibrations is really a sign to you that they have been examined and tested by an independent third party to determine if the methods and processes they are using are sound and if the sources of error and uncertainties associated with them are deemed to be reasonable.
You can imagine that getting these accreditations is no inexpensive proposition for the calibration facility. In order to receive these accreditations, they must establish processes and procedures, define all of their sources of errors, test the procedures to provide data to validate them, and demonstrate to the inspectors from the certifying body that all are reasonable and the practices are always followed.
The accreditations and the numbers that back them up are the keys to selecting the appropriate calibration house for your needs.
If your shop only has the basics to control and the tolerances are not too tight, there are many good “economy” calibration houses that can meet your needs. However, should you be manufacturing parts that might be life critical or have extremely small tolerances, you may want to dig deeper into the calibration houses’ accreditations and scope of capabilities to see if their procedures are adequate for what you need.
With today’s computer-controlled measuring systems, software plays an important part in the system’s performance. There are calibration routines and hardware locks to prevent unintended or unwanted calibration adjustment. You have to have the right service software to do these processes, to measure the artifacts, apply correction factors from the error correction files, etc. It’s all done in the software.
The economy calibration service may come to your facility, put a standard on the machine, run a trace and give you a certificate. But the question is: a certificate for what? Is that calibration service accredited with the process and tests to do the calibration and provide you a certificate? If the machine does not pass, do they have the means to adjust the calibration to bring it into specification? Unless they can provide you the accreditation certificate showing they are qualified to certify that piece of equipment, chances are that you may not have a gage performing to specification and there is a good chance your gage auditor may also find this issue. If in a life-critical manufacturing facility, this could mean big recalls of product, or worse.
Therefore, for a system such as this, you want to be sure that the provider you choose is accredited (which also means trained and has access to all the calibration tools) to ensure the system is performing to specification, and you are in possession of an approved “certification of calibration.”
The simplest way to select your calibration supplier is to ask questions. Ask about accreditation: what gages have they been accredited to certify; what are their sources of errors and how big are they. And when in doubt, a visit may be just the thing to convince you that your supplier cares as much about the performance of your gage as you do.