Calibrating measurement and test equipment (M&TE) is expensive, but using measurement and test equipment that is out of calibration can be even more costly. Equipment that is out of calibration will produce suspect parts and once you’ve discovered that your M&TE is out of calibration, you will have to screen the suspect parts and repair or scrap the parts that are not in spec. If you have already shipped the parts to a customer, you may have to recall them.
You not only lose the bad parts, which increases your material costs, you also lose the many labor-hours needed to screen the parts and verify equipment accuracy. These lost hours increase your costs and delay your production schedule.
To avoid the problems that inaccurate M&TE can cause, companies must calibrate their equipment regularly. Some companies, however, are so strict about sticking to their calibration schedules that they often unnecessarily calibrate equipment. While this helps them avoid producing bad parts, the unnecessary calibrations also increase costs.
Is it possible to ensure that your M&TE is properly calibrated and keep calibration costs low? Yes. By establishing realistic calibration specifications, keeping good calibration records and analyzing the measurements, you can set up an optimum calibration interval for each piece of equipment that you use. There also is commercially available software that helps take the drudgery out of both the record keeping and data analysis.
Setting Initial Calibration Intervals
The first thing you must do is set an initial calibration interval for a new piece of M&TE, taking into account the manufacturer’s recommendations. This is especially true if you have no prior experience with a particular piece of equipment. If you have similar equipment already in service, you can use your experience with that equipment to set the calibration interval for the new piece of equipment.
You can also take into account the recommendations of industry and government related organizations. The U.S. Navy’s Metrology Information and Document Automation System (MIDAS) program, for example, has information gathered from many years of experience with test and measurement systems. Their calibration requirements are spelled out in the Metrology Requirements List. For MIDAS information, visit the Government-Industry Data Exchange Program Web site at http://www.gidep.org.
Other questions to ask when setting initial calibration intervals include:
- How stable is the equipment?
- How complex and critical are the measurements that will be made?
- Will the M&TE be used frequently or only rarely?
- What are the environmental conditions (such as dust, vibration and temperature) in which the equipment must operate?
- What is the risk of damage or misuse? Are skilled technicians or unskilled technicians using the equipment? Is the measurement automated or manual?
- Is the equipment company-owned or is it employee-owned?
- Are there any customer contract obligations regarding calibration intervals?
- Are there any regulatory agency obligations regarding calibration intervals?
- What are the risks associated with using improperly calibrated equipment?
Once you’ve set the initial calibration intervals, it is important to keep good calibration records. These records should include the equipment’s manufacturer, its model number, identification numbers associated with the instrument and perhaps the location at which the equipment is normally used. It should also contain the procedure number for the procedure used to calibrate the equipment and an indication of how frequently that calibration is to be performed.
The record should also contain data on the calibrations and the measurements performed. This information should include the date of the calibration, the technician who performed it, the equipment characteristic that was calibrated and measurements of that characteristic before and after the calibration.
Using modern calibration management software can help you keep better records. As technicians perform calibrations, they enter the measurements directly into a computer database using computerized forms in the program. Once entered, this data is continually available and you can easily use the information to calculate equipment measurement uncertainties, identify stability problems and perform gage R&R studies.
Some software packages will also allow you to automatically adjust calibration intervals. GAGEtrak, for example, in addition to using Method A3 also uses the widely recognized NCSL guideline RP-1, Method A1, the Simple Response Method, to calculate calibration intervals based on calibration data. Using this method, a calibration lab first tests an instrument to see if it is within tolerance. If it is, GAGEtrak lengthens the calibration interval for the instrument. If the instrument is not within tolerance, it shortens the calibration interval. The amount of adjustment in these scenarios is defined by the user within the software settings. GAGEtrak also allows you modify the rules it uses to adjust the calibration intervals. For example, you may feel more comfortable lengthening the calibration interval after the second or third time the lab finds an instrument’s calibration to be in tolerance.
Another feature is the ability to analyze a group of gages when setting calibration intervals. Using this method, you set the calibration interval for a whole group of gages using the calibration results of all of the gages in a group. This method gives you a more statistically significant sample, which in turn should give you more confidence in your calibration intervals.
Ron Storteboom is calibration lab manager for Medallion Instrumentation Systems, Spring Lake, MI and has greatly benefitted from calibration software. Medallion manufactures sensors and electronic modules used in heavy-duty trucks, farm equipment and recreational vehicles. Ron manages more than 1,300 pieces of equipment, including mechanical and electronic instruments and has been using GAGEtrak for more than two years.
The software helps him find the appropriate calibration intervals of Medallion’s custom electronic test equipment. Since this is custom test equipment, he often has no history to help determine the correct intervals. In this case, he sets a conservative initial calibration interval and then uses the interval-adjustment feature of the software to find the most cost-effective interval.
Another big cost-saver is the software’s ability to set calibration intervals based on usage. For several types of pin gages and gage blocks, Storteboom has programmed the software to call for calibration of the instrument after so many uses instead of after a set period of time. When they switched over to this type of calibration interval, Storteboom discovered that more than half of the gages are rarely used. As a result, he was able to cut back on the number of times he calibrated those gages.
Overall, Storteboom figures that he saves thousands of dollars each year as a result of using calibration management software. Not only does it make it easier to manage his M&TE inventory, it gives him the data he needs to justify lengthening calibration intervals without increasing the risk of producing nonconforming products.
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