Calibration
• Definition (ANSI,1994)
The set of operations which establish, under specified conditions, the
relationship between values indicated by a measuring instrument or
measuring system, and the corresponding standard or known values
derived from the standard.
• The performance characteristics of instruments are obtained through
the static calibration.
• It is a process to check the instrument against known standard.
• The deviation from the known standard values is error in the
instrument.
• This helps in identification of actual error in the instrument and hence
enhancing the accuracy of the device.
• Calibration procedure involve a comparison of an instrument with:
• a primary standard
• a secondary standard with the higher accuracy than the instrument to be calibrated
• An instrument of known accuracy
• Calibration of instruments and processes is essential for checking their performances against
known standards.
• It is desired to calibrate all the instruments used for the measurement to some reference
instruments which have a higher accuracy.
• The reference instrument in turn must be calibrated against very high accuracy instrument or
against primary standard.
• It is essential to recalibrate the instruments from time to time after using it for certain period of
time.
Benefits of Calibration
• Cost savings
• Reliable measurements
• Compliance
• Detection of ageing instruments
• Process and profit improvement
• Error calculation
Source: Fluke calibration-https://as.flukecal.com/
Calibration in our daily lives
Source: Fluke calibration-https://as.flukecal.com/
Terms related to calibration
• Calibration procedure: ‘A documented, verified, and validated process
that describes a set of operations in accordance to a given method.’
• Calibration provider: ‘Laboratory or facility including personnel that
performs calibration in an established location.’
• Errors: ‘Result of a measurement minus the true value of the
measurand.’
• Reference standard: ‘Measurement standard having the highest quality
available in an organization.’
• Uncertainty of measurement: ‘Dispersion of values that can be
attributed to the measurand.
Objective of Calibration
Calibration is done for the two main purposes as follows:
• If the instrument reads in the same units as the reference standards, the
purpose of calibration is to eliminate biases and minimize the
instrumental errors.
• If the instrument reads in different units than the reference standards, the
calibration serves the purpose of converting the instrument readings to
the units of interest.
The following examples show both input and output standards used in
the calibration of pressure:
During the calibration, the following assumptions are generally made:
• Handling procedures are consistent.
• Test item and reference standard respond in the same manner in the test
environment.
• Random errors associated with the measurement are independent.
• Multiple measurements form a distribution curve with the same standard
deviation.
• The test item and reference standards are stable during the measurement.
• The bias of the test item and reference standard are the same, and hence it
can be eliminated by taking the difference between the two readings
• Once the calibrations are carried out, ideally the difference is zero or can be
explained by appropriate charts and statistical relationships.
• Calibration is repeatable, yielding the same results
Advantages of Calibration
• Calibration is a process of testing and comparing the errors of
measurement instruments and processes with accepted standards in
order to detect and correct variations in performance.
• Therefore, calibration assures that devices and processes meet expected
performance specifications within universally acceptable levels and
accuracy. Hence, calibration has the number of benefits.
Summary
• It determines whether measurements made before the calibration were valid.
• It gives confidence that the future measurements will be accurate.
• It assures consistency and compatibility with those made elsewhere.
• It leads to repeatability and reproducibility assessments of the instruments
and processes.
• It provides confidence that products meet their specifications, thus reducing
legal liability.
• Without calibration, the product quality may be poor, thus opening up legal
challenges and high failure rates of the products, thus increasing costs.
• It increases efficiency by ensuring that measurements are correct.
• In the process industry, calibration of devices assures that the processes are
well controlled and that the products meet expected specifications
