Calibration

Definition and Importance of Calibration
– Calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy.
– The outcome of the comparison can result in no significant error being noted on the device under test, a significant error being noted but no adjustment made, or an adjustment made to correct the error to an acceptable level.
– The calibration standard is normally traceable to a national or international standard held by a metrology body.
– The formal definition of calibration by the International Bureau of Weights and Measures (BIPM) involves establishing a relation between quantity values and corresponding indications with associated measurement uncertainties.
– The calibration process is purely a comparison and does not include any subsequent adjustment.
– National Metrology Institutes (NMIs) maintain primary standards of measurement to provide traceability to customers’ instruments by calibration.
– Examples of NMIs include NPL in the UK, NIST in the United States, and PTB in Germany.
– The Mutual Recognition Agreement allows for traceability from any participating NMI, eliminating the need for companies to obtain traceability from the NMI of their country.
– NMIs establish an unbroken chain from top-level standards to instruments used for measurement.
– The need for known accuracy and uncertainty has led to the establishment of national laboratories and consistent and comparable standards internationally.
– Calibration and subsequent measurements should be traceable to internationally defined measurement units to improve quality and gain acceptance from outside organizations.
– Traceability is achieved through a formal comparison to a standard related to national or international standards.
– Quality management systems require an effective metrology system, including periodic and documented calibration of all measuring instruments.
– ISO 9000 and ISO 17025 standards specify the level of traceable actions and how they can be quantified.
– Calibration values are often accompanied by a traceable uncertainty statement to a stated confidence level.

Instrument Calibration Process
– Calibration may be required for a new instrument, after repair or modification, when moving to a different location, or after a specified time period or usage.
– Calibration may be necessary before and/or after a critical measurement or after an event such as exposure to shock, vibration, or physical damage.
– Sudden changes in weather or questionable observations may also prompt calibration.
– Calibration may be specified by customer requirements or instrument manufacturer recommendations.
– Calibration often involves adjusting the output or indication of a measurement instrument to agree with the value of the applied standard.
– The calibration process begins with the design of the measuring instrument, which should be capable of holding a calibration through its calibration interval.
– Calibration ensures the quality of measurement and the proper working of the instrument.
– The exact mechanism for assigning tolerance values and calibration intervals varies by country and industry type.
– The manufacturer assigns the measurement tolerance, suggests a calibration interval, and specifies the environmental range of use and storage.
– The using organization assigns the actual calibration interval based on specific measurement requirements.
– Calibration methods can be manual or automatic.
– Manual calibration involves multiple steps, such as connecting the device under test to a reference gauge and comparing readings.
– Automatic calibration uses specialized equipment, such as an electronic control unit and pressure transducer, with data collection facilities for automation.
– Manual calibration requires manual record-keeping, while automatic calibration streamlines data gathering.
– Both methods have their advantages and are suitable for different types of devices.
– Calibration procedures capture all the steps needed for a successful calibration.
– Manufacturers or organizations may provide calibration procedures that meet specific requirements.
– Calibration procedures can be found in clearinghouses like the Government-Industry Data Exchange Program (GIDEP).
– The traceability of calibration is established by repeating the process with transfer standards or certified reference materials.
– Metrology considers other factors during calibration process development.

Calibration Standards
– ISO 17025 does not provide clear recommendations for calibration intervals.
– ANSI/NCSL Z540 states that equipment should be calibrated at periodic intervals for acceptable reliability.
– ISO-9001 requires calibration or verification of measuring equipment at specified intervals or prior to use.
– MIL-STD-45662A mandates calibration at periodic intervals for acceptable accuracy and reliability, with the option to adjust intervals based on previous calibrations.
– Calibration intervals may be agreed upon with the customer or regulated by legal requirements.

Accuracy Ratio
– The goal is to have a standard with less than 1/4 of the measurement uncertainty of the device being calibrated.
– The 4:1 ratio, originally 10:1, is commonly used for calibration.
– Maintaining a 4:1 accuracy ratio with modern equipment is challenging.
– If the accuracy ratio is less than 4:1, the calibration tolerance can be reduced or the accuracy of the device being calibrated can be adjusted.
– Informal calibration processes can lead to tolerance stacks and post-calibration issues.

Historical Development of Calibration
– The words ‘calibrate’ and ‘calibration’ entered the English language during the American Civil War.
– Ancient civilizations like Egypt, Mesopotamia, and the Indus Valley used angular gradations for construction.
– Calibration was associated with precise division of linear distance and angles.
– Linear measurements and weighing scales supported commerce and technology until AD 1800.
– Standardization attempts like the Magna Carta led to the establishment of the Metric system.
– Early measurement devices had direct units, such as length using a yardstick and mass using a weighing scale.
– During the reign of Henry I, a yard was defined as the distance from the tip of the King’s nose to the end of his outstretched thumb.
– The Assize of Measures in the reign of Richard I standardized measurements throughout the realm.
– The Mètre des Archives from France and the establishment of the Metric system further standardized measurements.
– Calibration errors, like zeroing errors, can be adjusted by the user.
– The Mercury barometer and water-filled manometers were early pressure measurement devices.
– The Industrial Revolution led to the adoption of indirect pressure measuring devices.
– The Bourdon tube, invented by Eugène Bourdon, is an example of an indirect reading instrument.
– Pressure instruments Source:  https://en.wikipedia.org/wiki/Calibration