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.
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, 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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