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 Run
a “Tight Ship": Calibrate
Competition, regulations, and quality standards
demand the rigorous and documented calibration
of process instrumentation. What is calibration?
What gets calibrated? How is it done? What
tools are used? How often is calibration
performed?
Elements of a discipline
calibration program
Compare to standards
Calibration is the comparison of the output
of an instrument to a known standard. Calibration
involves adjusting the instrument to conform
to a standard. The instrument to be calibrated
is one component in a process loop.
Environment alters performance
The performance of process instrumentation
changes over time due to exposure of transmitter
and/or primary sensing element to temperature,
humidity, abuse, pollutants, or vibration.
Schedule performance tests with a reliable
reference standard will permit you to keep
these shifts under control and will deliver
additional benefits: Costs can be better
controlled as maintenance becomes more proactive
and unplanned shut—downs are avoided.
Safety is enhanced, since a well-calibrated
plant is generally a well-inspected and
well-monitored plant.
Legal requirements such as weights, measures,
custody transfer and chain of custody are
addressed. The calibration requirements
of quality programs (e.g., ISO 17025) and
environmental, occupational safety, or consumer
protection regulations are satisfied.
Calibrate by the book
A complete calibration begins with an “as
found” test to verify how an instrument
is performing. If necessary, an adjustment
is made to bring the instrument back into
conformance with standards. Finally, an
“as left” test is performed
to complete the calibration.
Consider the calibration of a mass flow
meter—comparing mass to a weigh scale.
Consider the transmitter is 0-1000lb/min
to a 4-20ma output signal. The signal is
used to control flow in the process loop.
Full span is 1000lb/min, while full scale
current is 16ma (20ma minus 4ma). The maximum
allowable error or tolerance is .25% of
span.
After meter is installed in flow
loop proceed:
First - close the block valves
up and downstream the flow meter/sensor.
Zero the meter. The zero should read 4ma.
Second - open the valves and start
the actual calibration test.
Third - when test is completed,
compare calibration test error to allowable
error or tolerance. If calibration is within
allowable tolerance the meter passes the
calibration test. If the test shows that
the meter is not within the allowable tolerances,
this will be the documented “as found”
calibration. Note: Calculate the new calibration
factor and input this factor into the transmitter
and proceed.
Adjustments within tolerance
An adjustment is required. Starting again
at step 1; repeat the calibration process.
When test the test is complete and the
acceptable tolerances are satisfied, this
completes the calibration process.
Don’t forget to create a file a calibration
certificate, with date, technician ID, instrument
serial number, test results and other pertinent
information.
Document a traceable chain
An Important element of any calibration
is traceability. A traceable calibration
is supported by an unbroken chain of documented
measurements, made by standards of increasing
accuracy, that ultimately references your
instrument to nationally recognized standards.
This means each piece of calibration equipment
you use must be periodically verified and
certified with equipment which, in turn,
is periodically compared to even higher
standards. Ultimately, the documentation
stretches to the National Institute of Standards
and Technology. (NIST)
The accuracy of a field device or instrument
is the degree of conformity of an indicated
value to a recognized standard value. Accuracy
may be stated in engineering units, in percent
of scale, or, as is often done in process
industries, in percent span.
Next Article “Unbroken Chain
of Custody”
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