Understanding Calibration Part 2
INDUSTRY
PART II: Understanding Calibration
Traceability, Standards and Practical Strategies for Shops
b y
W a l t e r
S h a w l e e
2
Editor’s Note: This is Part II of a two-part series exploring calibration of equipment. Part I in the September issue of Avionics News
focused on the necessity of calibration and repairs for accurate operations. Part II deals with calibration standards and strategies.
C
alibration always traces backward to reference items of
known performance held at
sites of increasing accuracy and stability. “Traceability,” or what standard was
used to establish the value used to set
another unit, is the key here.
In the United States, this trace generally leads back to NIST (National Institute
of Standards and Technology), which
can be found at www.nist.gov. To see the
policy on traceability, visit http://ts.nist.
gov/traceability.
As defined by NIST, “Traceability
requires the establishment of an unbroken chain of comparisons to stated references each with a stated uncertainty.”
In Canada, this standards function
is performed by its National Research
Council’s Institute for National
Measurement Standards, which can be
found at http://inms-ienm.nrc-cnrc.gc.ca/
main_e.html. To see its statements on
traceability and measurement uncertainty, visit http://inms-ienm.nrc-cnrc.gc.ca/
calserv/calibration_services_e.html#Cali
brationReports.
Virtually all calibrations are required
to show the standards used in the process
66
avionics news
•
october
2008
Looking For a Calibration Lab?
Several calibration labs are listed in the Marketplace section
of Avionics News magazine and many are AEA members.
For more information, visit the AEA’s online Member Directory
at www.aea.net/MemberDirectory.
and their traceability. In the U.S., it is
typically back to NIST, and in Canada
to NRC; however, either is acceptable in
either country. Your external calibration
certificate on file, showing the NIST/
NRC standards used, is your in-house
standard’s traceability back to a primary
source.
Standards
The cost of acquiring and maintaining a wide range of primary reference
standards is staggering, along with the
ongoing burden of qualified staff and
secure, environmentally controlled facilities, plus the need for external calibration
and validation of all those standards.
Even a modest calibration facility
can have annual certification costs of
$25,000 to $50,000, and capital/facility
costs of $250,000 or more. Multi-million-dollar lab costs are not uncommon.
In addition, accreditation audits to various standards add annual fees of $5,000
to $25,000, and sometimes much higher
in larger facilities.
Fees for certification at government
agencies and primary cal labs can be
quite high, and delays (if not scheduled
well in advance) can be long. Most
importantly, many will only record data;
they will not do adjustments to bring
a unit into calibration or carry out any
required repairs. They expect these steps
already have been completed prior to
arrival, which is a bit illogical in some
instances, but can be policy.
To do calibration at your facility, you
will need traceable standards linked back
to some known primary references. To
do any in-house calibration, they also
need to still be within their valid calibration cycle. Equally important is a valid
calibration procedure for the items you
want to do (taken from manufacturer’s
data and manuals, or from government
master records).
In addition, a temperature-stable environment (typically +23ºC +/-5ºC for
electronic equipment and +20C +/-2ºC
for dimensional and pressure) is required
for the measurement transfer, along with
high-quality interconnects free of thermal EMF artifacts (un-plated oxygen
free copper wire is ideal). Incredibly,
plain copper telephone wire often is the
ideal interconnect for DC measurements,
with the fewest thermal EMF errors.
Practical Strategies
Primary calibration labs with extensive
certification usually provide the traceable link needed to the master standards
held by governments; they, in turn, serve
secondary facilities (with varying levels
of certification) with practical transfer
measurements for their standards. These
facilities, in turn, serve you as the customer needing equipment calibration.
Many navigation system generators
return to either the manufacturer or their
designated repair and calibration depots
for annual calibration and repair when
needed.
Make certain any item of this type you
send out can, in fact, actually be done at
the facility you intend to use.
It is difficult to do accurate RF level
measurements below -90dBm, and only
costly, specialized equipment, such as
a measuring receiver, can work below
these levels. Most avionics equipment
testing is done below these levels, which
makes accurate RF level certification
especially critical and significantly
reduces the number of cal facilities that
can provide useful support. In addition,
correct navigation modulation signals
must be measurable with high precision,
an uncommon general industry requirement.
In your shop, you should arrange your
test equipment assets so you have the
best time coverage (continuous availability) and greatest ability to crosscheck
your measurements. If your best equipment has at least a 4:1 accuracy ratio
to your lesser items, you might want to
consider developing internal calibration
for simpler items.
If you have to ship key items out (as
most of us do), make certain you have
rugged, well-padded shipping containers
for them. Many mil-spec transit containers are available on the surplus market to
protect your gear, and it is money well
CALIBRATION Q&A
Q: Do I need to calibrate everything?
A: Yes, at least once initially, such as validating a test harness
as correct and working. You might designate some items to be “Cal
Not Required” or “Cal as Required,” but then you never really know
their status or how they impact your operation, nor can you use
them in any approved process unless you have specifically allowed
this. No reference, active or data display item (meter, power supply,
generator or load) should ever be left outside the calibration loop,
although for some low-use or known low-drift items, you might elect
to lengthen the interval beyond one year.
You also can limit the use of some items marked, such as “Not
to be Used for Equipment Calibration or Certification,” when they
are used for internal non-critical functions and never will be used
to calibrate any external customer items. Marking tags of this kind
should be vivid yellow, orange or red for clarity.
spent. And be sure your insurance coverage correctly protects the item in transit.
You certainly don’t want to discover the
new $18,500 spectrum analyzer was
uninsured when it returns after being
crushed during shipping.
Picking a Cal Lab
When choosing a calibration facility,
pick one with the best quality, accuracy,
service and reporting, not just the lowest
price.
“Paper compliance” based on price
alone can be a dangerous strategy, especially if serious errors creep into your
equipment from substandard or inadequate calibration, and from there to
your customers.
If you are on good terms with your
other local shops, you might want to
compare cal results on your equipment to
build up some history on different cal labs
and crosscheck your own gear. Always
crosscheck any item just received back
from calibration, and contact the cal lab
if you find anything unsatisfactory, especially if it significantly disagrees with
your supplied test data.
Many people can afford a high-accuracy GPS-disciplined frequency standard,
which can serve as an ongoing shop frequency reference and give an additional
level of confidence to measurements.
Your frequency generators and counters
often can accept an external clock input,
driven by this stable source through a
distribution amplifier. This technique can
improve frequency accuracy by up to
three or more orders magnitude over the
internal clock reference.
It is not always well understood that
many high-end manufacturers intended
for their instruments (especially synthesized generators, spectrum analyzers
and counters) to be driven by a master
external standard (typically 10Mhz), and
the basic internal reference might be only
adequate to permit basic operation. You
might want to investigate this strategy
for your equipment, as it also can reduce
your overall calibration costs. RF generators, such as HP 8647A, 865A/B, 8657A
and many HP spectrum analyzers, ben-
Continued on following page
avionics news
•
october
2008
67
CAlibration
Continued from page 67
CALIBRATION Q&A
efit from this external clock technique.
You also should be aware of the diminishing quality of many modern pieces of
equipment, which might be “digital” but
might not offer high stability, high accuracy or high durability. There also have
been big shifts in manufacturers’ product
support, which might mean only three to
five years of support for even brand-new
items. Be certain you know the product
support plan and calibration costs for
any new items, and consider that some
used gear can be of higher quality and at
a lower price.
I am still amazed that a 30-yearold Tektronix 465B oscilloscope can
be fixed or calibrated almost anywhere,
and can still give excellent service, while
many modern digital scopes already are
completely out of support and can be
fixed nowhere, not even at the factory.
Your shop equipment can be a powerful asset to you when well maintained,
so consider what strategies will work
best for you, and try to weed out any
problem items that spend too much time
in repair or cost too much to calibrate.
Use the daily check technique to catch
any problems before they affect customer
equipment, and always protect your gear
in transit. q
If you have comments or questions
about this article, send e-mails to
avionicsnews@aea.net.
68
avionics news
•
october
2008
Q: How often do I need to calibrate something?
A: The generally accepted interval is one year, based on the
equipment specification drift called out by most manufacturers.
Some items exhibit very little drift (particularly higher-end Fluke
meters, such as the model 87) but might have wear and battery
issues triggering failures or errors; therefore, a one-year cycle
makes sense for most things. If you have a regular daily check
procedure, you can justify a three-month extension window in your
quality-assurance plan to allow for local timing or access issues.
Equipment does not suddenly go inaccurate on the 366th day
since calibration — regular daily checks reveal far more problems
than annual checks. In addition to a periodic calibration, all items
must be removed from use and sent for repair as soon as any fault
is detected no matter where they are in the calibration cycle.
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertising