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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 pans 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 person-hours needed to screen the parts and verify equipment
accuracy. These lost hours increase your labor 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 schedule that they often calibrate equipment
unnecessarily. 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.
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Setting initial calibration intervals
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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, which is now available in hard
copy, microfiche, floppy disk, and CD-ROM formats. For MIDAS information, call (909)
273-5286 or visit the Government-Industry Data Exchange
Program Web site at http://www.gidep.corona.navy.mil.
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 where 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, such as GAGEtrak from
CyberMetrics, can help you keep better records. As technicians perform calibrations,
they enter the measurements directly into a computer database using computerized
forms you create with the program. Once entered, these data are continually available,
and you can easily use the information to calculate equipment measurement uncertainties,
identify stability problems, and perform gage R&R studies.
Some packages will also allow you to automatically adjust calibration
intervals. GAGEtrak, for example, 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 cal lab first tests an instrument to see
if it is within tolerance. If it is, GAGEtrak lengthens the calibration interval
for the instrument by 10%. If the instrument is not within tolerance, it shortens
the calibration interval by 55%.
GAGEtrak 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 cal lab finds an instrument’s calibration
to be in tolerance. You can also program the software to take into account the test
measurements when setting the calibration interval. If the test readings are quite
close to the nominal readings, for example, you can have the software set a longer
calibration interval than if the readings are further from the nominal.
Another feature is the ability to analyze measurements from a group
of gages when setting calibration intervals. Using this method, you set the calibration
interval for a whole group of gages using measurements made with 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.
One user who has benefited from calibration-management software is
Ron Storteboom, calibration lab manager for Kysor Medallion, Spring Lake, MI. Kysor
Medallion manufactures sensors and electronic modules used in heavy-duty trucks
and farm equipment. He 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 program helps him find the appropriate calibration intervals
of Kysor’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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