Know What Your Gages Are Measuring

May 19, 2003
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Calibration is the comparison of a standard of a known uncertainty to another measurement tool of unknown certainty to detect, correlate, report or eliminate by adjustment any deviation from required performance specifications.

Today, many assume that because a piece of test equipment is working correctly, it must be calibrated and accurate. Only if the equipment has traceable calibration records can this be the case. But how can one be certain that equipment does indeed have traceable records when hundreds, if not thousands, of gages must be calibrated on a regular basis?

The frequency of the calibration depends on tolerance requirements of the job, amount of use and conditions under which the gages are used. But do hard and fast rules apply?

"Though ISO 17025, General Requirements for the Competence of Testing and Calibration Laboratories, clearly states that, 'a calibration certificate shall not contain any recommendation on the recalibration interval,' the best source for establishing calibration intervals is still the equipment manufacturer. Through the use of data collection within their calibration labs, manufacturers are best equipped to adapt experience data from similar applications," said Kevin Shannon, quality manager at DeFelsko Corp. (Ogdensburg, NY).

Richard Pirret, marketing manager of Fluke Process Tools (Everett, WA), agrees that the best place to go when looking for calibration interval recommendations is the manufacturer. But, not all manufacturers will give recommendations. SomeArial the most practical way to determine intervals is from experience. "A pressure transmitter that's critical to the performance of the plant, might be looked at every 90 days or half year. As [a company] develops more confidence in that measurement tool or family of tools, they can relax that. They might move calibration out to 1 year -- they might move it out to 2 years. They can adjust the calibration intervals so it more appropriately matches their experience," said Pirret.

Linda Bogaski, vice president of JBL Systems (Ma-comb, MI), said if a measurement tool's calibration interval is 60 days, but the tool has been sitting on the shelf the entire 60 days or has only been used 5 days, it could be "a waste of money" to calibrate the tool.

One must also consider how variation in the measurement result could affect the product or processes it is intended to measure, said Shannon. "Critical applications should always safeguard their processes by minimizing the calibration interval," he said.

Often it's the management instrumentation packages, or software packages, that look at a measurement tool's history and record how it has behaved over time, whether it's in or out of tolerance and if it's out of tolerance, by how much.

"If measurement tools were found quite within tolerance, that's an indication that you can stretch the calibration interval of that device or that class of device. Software allows you to look at behaviors of classes of measurement tools over time and adjust your approach to them accordingly," said Pirret.

A software solution
Although software is not necessary to keep track of gage history, the pencil and paper method is only as good as its limitations and makes it difficult to retrieve information. Papers might be misplaced, remain unchanged or someone may walk off with the files containing the calibration records. It would be difficult with a manual system to track all the measurement tools used by manufacturers.

A software-based system allows information to be recorded, tracked and retrieved in an effective and efficient manner. Software facilitates searches, particularly those relating to historical data, such as purchase date, past passes or failures, and last calibration.

"If an individual has the time to establish a manual tracking and filing system, it may be sufficient for a small company. However, when considering the demands of today's industry and the availability of custom designed software applications it is hard to imagine a cost-effective manual system," said Shannon. Many gage tracking software packages are available for less than $500.

The software program a company chooses depends on its particular needs, but a list of features for a software package includes:

  • Identify the measurement tool by make, model and serial number.
  • Identify the measurement tool's place in the process.
  • Determine the measurement tool's physical location at a given moment.
  • List the measurement tool's appropriate ranges.
  • List the measurement tool's specifications, including accuracies and certainties, or acceptable tolerances.
  • Identify a correct calibration procedure.
  • Track the date and time that the measurement tools were checked and record any adjustments.
  • Identify the technician who checked the measurement tool.
  • List the types of errors that might be found, a list of parameters having to do with the performance of that particular measurement tool.
  • Gage repeatability and reproducibility.
  • List the standards to which the measurement tool is traceable.
  • Show a picture of the measurement tool so someone new to the process will know to calibrate the correct gage.

Today, many programs generate an e-mail reminder that flags the equipment due for calibration. Reports can be generated by department, equipment or date. Records can be maintained and updated on a continual basis.

But who should have access to these records is in debate. Some will argue that only the metrology manager, or person in charge of calibration, should have access to the information. Others contend that all employees using gages should have at least read-only access. A database should contain the gage's entire history. "For a critical measurement, you wouldn't want to use a gage that's on its last legs," said Bogaski.

Once a gage has been calibrated, a calibration label should be attached. At a minimum, these labels should include an identification number, date of last calibration, identifying marks for the person placing the gage in service and the date of the next calibration. Additional information, if room allows, includes a description of the gage, status of the gage -- repaired, limited use, pass or fail -- and how it's been used.

The rigors of the shop floor can cause labels to fall off or become unreadable. "Any label damaged so that it may affect the readability of its contents must be replaced. It is helpful to maintain a database of all calibration information to facilitate the recreation of label information as required," said Shannon. Laminating labels and using labels with strong adhesives can help reduce label degradation.

Calibration intervals
So why must gages be calibrated at regular intervals? A variety of conditions make gages move out of tolerance. Here are a few:

  • Time. The general drift of any device over time.
  • Frequency of use.
  • Misuse and abuse, including using the wrong gage for a job or dropping it.
  • Environmental conditions, such as temperature, vibration and chemical pollutants.
  • Internal contamination, such as things that coagulate and clog the measurement tool, particularly in pressure instrumentation.

Even though rugged gages address these problems, they tend to be heavier and more awkward to handle than nonrugged gages, resulting in less use by operators.

Lab vs. workstation
Some will contest that keeping the measurement tool in its original environment during calibration will yield more accurate results, but others argue that the readings are more accurate in a lab environment.

"We contend that you want your measurement tools calibrated in situ.

Calibrate them onsite because they're in the temperature environment, and they're in the vibration environment. They're in the environment in which they're actually going to be working," said Pirret. Also, taking that measurement tool out of a process to calibrate in a lab, may not only be a difficult physical process, but it disrupts the workflow, and other accommodations must be made to continue the process, whether it's installing a replacement or shutting the machine down.

Others will argue that the lab is the better place to perform the calibrations. Bogaski said a lab provides better conditions and eliminates interference with the machine's operator. It's difficult to do the calibration if the operator is standing over your shoulder, waiting for the calibration to be done so he can get back to work.

Bottom line
Management commitment and employee training are vital to gage management programs, but it goes beyond these requirements. "An effective calibration program needs to have the ability to correctly capture gage performance history, record it and provide access to that data in the future," said Pirret.

Bogaski said gage control is a recurring problem that weakens metrology programs. "The people in charge of gages are given the responsibility but not the authority to do their jobs." Gages are passed from person to person within the plant, and the measurement tools are not returned to the person in charge to record its usage.

Shannon said, "An effective gage calibration program requires a well-thought out system, with a competent and conscientious individual responsible for its implementation and maintenance. Individuals calibrating and using equipment must be aware of the critical role that correctly calibrated equipment plays in product verification."

Test Reports and Calibration Certificates
Each test report or calibration certificate shall include at least the following information, unless the laboratory has valid reasons for not doing so:

  • Title, i.e., Test Report of Calibration Certificate.
  • Name and address of laboratory and location where the tests and calibrations were carried out, if different from the address of the laboratory.
  • Unique identification of the test report or calibration certificate, such as the serial number, and on each page an identification in order to ensure that the page is recognized as part of the test report or calibration certificate, and a clear identification on the end of the test report or calibration certificate.
  • Name and address of the client.
  • Identification of the calibration method used.
  • Description, condition and unambiguous identification of the items tested or calibrated.
  • Date of receipt of test or calibration items where critical to the validity and application of the results and dates of performance of the test or calibration.
  • Reference to sampling plan and procedures used by the laboratory or other bodies where these are relevant to the validity or application of the results.
  • Test or calibration results with, where appropriate, the units of measurement.
  • The names, functions and signatures, or equivalent identification, of the persons authorizing the test report or calibration certificate.
  • Where relevant, a statement to the effect that the results relate only to the items tested or calibrated.

Source: ISO 17025

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