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Home / Calibration / What is calibration ?
CALIBRATION
What is calibration ?
S Bharadwaj Reddy1 Comment
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WHAT IS CALIBRATION?
There are as many definitions of calibration as there are methods. According to ISA’s The
Automation, Systems, and Instrumentation Dictionary, the word calibration is defined as “a
test during which known values of measurand are applied to the transducer and
corresponding output readings are recorded under specified conditions.” The definition
includes the capability to adjust the instrument to zero and to set the desired span. An
interpretation of the definition would say that a calibration is a comparison of measuring
equipment against a standard instrument of higher accuracy to detect, correlate, adjust,
rectify and document the accuracy of the instrument being compared.
The zero value is the lower end of the range. Span is defined as the algebraic difference
between the upper and lower range values. The calibration range may differ from the
instrument range, which refers to the capability of the instrument. For example, an
electronic pressure transmitter may have a nameplate instrument range of 0–750 pounds per
square inch, gauge (psig) and output of 4-to-20 milliamps (mA). However, the engineer has
determined the instrument will be calibrated for 0-to-300 psig = 4-to-20 mA. Therefore, the
calibration range would be specified as
0-to-300 psig = 4-to-20 mA. In this example, the zero input value is 0 psig and zero output
value is 4 mA. The input span is 300 psig and the output span is 16 mA.
Different terms may be used at your facility. Just be careful not to confuse the range the
instrument is capable of with the range for which the instrument has been calibrated.
WHAT ARE THE CHARACTERISTICS OF A CALIBRATION?
Calibration Tolerance: Every calibration should be performed to a specified tolerance. The
terms tolerance and accuracy are often used incorrectly. In ISA’s The Automation,
Systems, and Instrumentation Dictionary, the definitions for each are as follows:
Accuracy: The ratio of the error to the full scale output or the ratio of the error to the
output, expressed in percent span or percent reading, respectively.
As you can see from the definitions, there are subtle differences between the terms. It is
recommended that the tolerance, specified in measurement units, is used for the calibration
requirements performed at your facility. By specifying an actual value, mistakes caused by
calculating percentages of span or reading are eliminated. Also, tolerances should be
specified in the units measured for the calibration.
For example, you are assigned to perform the calibration of the
previously mentioned 0-to-300 psig pressure transmitter with a specified calibration
tolerance of ±2 psig. The output tolerance would be:
2 psig
÷ 300 psig
× 16 mA
—————————-
0.1067 mA
rounding to 0.11 mA would exceed the calculated tolerance. It is recommended that both
±2 psig and ±0.10 mA tolerances appear on the calibration data sheet if the remote
indications and output milliamp signal are recorded.
Note the manufacturer’s specified accuracy for this instrument may be 0.25% full scale
(FS). Calibration tolerances should not be assigned based on the manufacturer’s
specification only. Calibration tolerances should be determined from a combination of
factors. These factors include:
• Requirements of the process
• Capability of available test equipment
• Consistency with similar instruments at your facility
• Manufacturer’s specified tolerance
Example: The process requires ±5°C; available test equipment is capable of ±0.25°C; and
manufacturer’s stated accuracy is ±0.25°C. The specified calibration tolerance must be
between the process requirement and manufacturer’s specified tolerance. Additionally the
test equipment must be capable of the tolerance needed. A calibration tolerance of ±1°C
might be assigned for consistency with similar instruments and to meet the recommended
accuracy ratio of 4:1.
Accuracy Ratio: This term was used in the past to describe the relationship between the
accuracy of the test standard and the accuracy of the instrument under test. The term is still
used by those that do not understand uncertainty calculations (uncertainty is described
below). A good rule of thumb is to ensure an accuracy ratio of 4:1 when performing
calibrations. This means the instrument or standard used should be four times more
accurate than the instrument being checked. Therefore, the test equipment (such as a field
standard) used to calibrate the process instrument should be four times more accurate than
the process instrument, the laboratory standard used to calibrate the field standard should be
four times more accurate than the field standard, and so on.
With today’s technology, an accuracy ratio of 4:1 is becoming more difficult to achieve.
Why is a 4:1 ratio recommended? Ensuring a 4:1 ratio will minimize the effect of the
accuracy of the standard on the overall calibration accuracy. If a higher level standard is
found to be out of tolerance by a factor of two, for example, the calibrations performed
using that standard are less likely to be compromised.
Suppose we use our previous example of the test equipment with a tolerance of ±0.25°C
and it is found to be 0.5°C out of tolerance during a scheduled calibration. Since we took
into consideration an accuracy ratio of 4:1 and assigned a calibration tolerance of ±1°C to
the process instrument, it is less likely that our calibration performed using that standard is
compromised.
Traceability is accomplished by ensuring the test standards we use are routinely calibrated
by “higher level” reference standards. Typically the standards we use from the shop are
sent out periodically to a standards lab which has more accurate test equipment. The
standards from the calibration lab are periodically checked for calibration by “higher level”
standards, and so on until eventually the standards are tested against Primary Standards
maintained by NIST or another internationally recognized standard.
The calibration technician’s role in maintaining traceability is to ensure the test standard is
within its calibration interval and the unique identifier is recorded on the applicable
calibration data sheet when the instrument calibration is performed. Additionally, when test
standards are calibrated, the calibration documentation must be reviewed for accuracy and
to ensure it was performed using NIST traceable equipment.
Uncertainty: Parameter, associated with the result of a measurement that characterizes the
dispersion of the values that could reasonably be attributed to the measurand. Uncertainty analysis
is required for calibration labs conforming to ISO 17025 requirements. Uncertainty analysis is
performed to evaluate and identify factors associated with the calibration equipment and process
instrument that affect the calibration accuracy. Calibration technicians should be aware of basic
uncertainty analysis factors, such as environmental effects and how to combine multiple calibration
equipment accuracies to arrive at a single calibration equipment accuracy. Combining multiple
calibration equipment or process instrument accuracies is done by calculating the square root of the
sum of the squares, illustrated below:
It makes sense that calibration is required for a new instrument. We want to make sure the
instrument is providing accurate indication or output signal when it is installed. But why
can’t we just leave it alone as long as the instrument is operating properly and continues to
provide the indication we expect?
Instrument error can occur due to a variety of factors: drift, environment, electrical supply,
addition of components to the output loop, process changes, etc. Since a calibration is
performed by comparing or applying a known signal to the instrument under test, errors are
detected by performing a calibration. An error is the algebraic difference between the
indication and the actual value of the measured variable. Typical errors that occur include:
Zero and span errors are corrected by performing a calibration. Most instruments are
provided with a means of adjusting the zero and span of the instrument, along with
instructions for performing this adjustment. The zero adjustment is used to produce a
parallel shift of the input-output curve. The span adjustment is used to change the slope of
the input-output curve. Linearization error may be corrected if the instrument has a
linearization adjustment. If the magnitude of the nonlinear error is unacceptable and it
cannot be adjusted, the instrument must be replaced.
To detect and correct instrument error, periodic calibrations are performed. Even if a
periodic calibration reveals the instrument is perfect and no adjustment is required, we
would not have known that unless we performed the calibration. And even if adjustments
are not required for several consecutive calibrations, we will still perform the calibration
check at the next scheduled due date. Periodic calibrations to specified tolerances using
approved procedures are an important element of any quality system.
Honesty and Integrity: A CST must possess honesty and integrity above all else. Most
technicians work independently much of the time. Calibrations must be performed in
accordance with procedures and must be properly documented. Additionally, the calibration
department may be understaffed and production schedules may demand unrealistic
completion requirements. These factors can have a real impact on proper performance and
documentation of calibrations. Remember: Nobody can take away your integrity; only you
can give it away.
Some basic concepts on how calibrations should be performed need to be discussed before
we go on. Some of these may be new concepts not used in your facility, but you should be
familiar with them. Some of these practices are industry dependent. Although calibrations
are generally performed the same, some different practices have developed. These practices
are:
Some basic concepts on how calibrations should be performed need to be discussed before we go
on. Some of these may be new concepts not used in your facility, but you should be familiar with
them. Some of these practices are industry dependent. Although calibrations are generally
performed the same, some different practices have developed. These practices are:
An individual instrument calibration is a calibration performed only on one instrument. The input
and output are disconnected. A known source is applied to the input, and the output is measured
at various data points throughout the calibration range. The instrument is adjusted, if necessary,
and calibration is checked
2. Mistakes on re-connect
3. Less efficient use of time to do one calibration for each loop instrument as opposed to
one calibration for the loop
When observations are not accurate or instrument indicators do not match the output of a
surrogate instrument
An instrument has had a shock, vibration, or exposure to adverse conditions, which can put
thermometers with area of sensitive nature, uncalibrated instruments may cause potential
safety hazards.
Wastage: If the instrument is not perfectly calibrated, it might lead to potential wastage of
resources and time consumed in the operations, resulting in an overall increase in expenses.
faulty or questionable quality of finished goods arises. Calibration helps maintain the
quality in production at different stages, which gets compromised if any discrepancy arises.
Fines or litigations: Customers who have incurred damage may return the product against a
full refund, which is still alright; but if they go for litigation due to damages, you could be
Increased downtime: Poor quality of finished goods is the first indicator of disrepair in your
equipment. Regular calibration programs identify warning signs early, allowing you to take
action before any further damage is caused.
established procedure that every business using machinery or instruments must conduct
periodically as specified in their machinery or instruments requirement