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Concepts of Calibration

What is Calibration?

Instrument calibration is one of the primary processes used to maintain instrument accuracy. Calibration is the process of configuring an instrument to provide a result for a sample within an acceptable range. Eliminating or minimizing factors that cause inaccurate measurements is a fundamental aspect of instrumentation design.



The Importance of Calibration

The figure above depicts how a properly performed calibration can improve product performance. Ideally, a product would produce test results that exactly match the sample value, with no error at any point within the calibrated range. This line has been labeled “Ideal Results”. However, without calibration, an actual product may produce test results different from the sample value, with a potentially large error. 


Calibrating the product can improve this situation significantly. During calibration, the product is “taught” using the known values of Calibrators 1 and 2 what result it should provide. The process eliminates the errors at these two points, in effect moving the “Before Calibration” curve closer to the Ideal Results line shown by the “After Calibration” curve. The Error At Any Point has been reduced to zero at the calibration points, and the residual error at any other point within the operating range is within the manufacturer’s published linearity or accuracy specification.


Factors Affecting Calibration




Proper instrument calibration is important to prevent potential error sources from degrading the result. Several factors can occur during and after a calibration that can affect its result. Among these are:

Using the wrong calibrator values: It is important to closely follow the instructions for use during the calibration process. Disregarding the instructions and selecting the wrong calibrator values will “teach” the instrument incorrectly, and produce significant errors over the entire operating range. While many instruments have software diagnostics that alert the operator if the calibrators are tested in the incorrect order (i.e. Calibrator 2 before Calibrator 1), the instrument may accept one or more calibrators of the wrong value without detecting the operator error.

Calibrator formulation tolerance: It is important to use calibrators that are formulated to tight tolerance specifications by a reputable manufacturer. There is a tolerance associated with formulating a calibrator/control due to normal variations in the instrumentation and quality control processes. This tolerance can affect the mean value obtained when using the calibrator. For example, if the calibrators have nominal values of 50 and 850 mOsm/kg H2O, and were manufactured toward the low end of their allowable range, the net effect might be to lower the calibration curve by approximately several mOsm/kg H2O over the calibrated range. As an example, Figure 3 illustrates what can happen in a situation where Calibrator 2 is assumed to be at its nominal value, say 850 mOsm/kg H2O, when the true formulated value is 846. The calibration process “teaches” the instrument incorrectly that 846 is actually 850, thus raising the Actual Results to curve higher than it would be if the instrument were “taught” that Calibrator 2 was 846 mOsm/kg H2O, or Calibrator 2 had an actual formulation value of 850 mOsm/kg H2O.

Sample preparation technique: As in the case of normal testing, a good sample preparation technique is essential to obtaining the best performance from the calibration process. A similar situation to that depicted in Figure 3 can occur if good sample preparation techniques are not followed when providing the calibrator samples. Conditions such as pipetting different sample volumes, allowing air bubbles in the samples, or preparing the samples too early so that evaporation occurs, can all increase the variation in the results obtained from the calibrators tested in the calibration process. This increased variation can result in mean values for the calibrators that vary by several mOsm/kg H2O from what they should be, erroneously shifting the calibration curve, resulting in increased errors for all results.

Ambient temperature effects: It is important to periodically calibrate an instrument at a temperature close to that at which it will be operated. Even when a calibration is performed properly, other factors can affect the accuracy of results. Environmental factors, such as the ambient temperature, can introduce errors that may not be readily evident when testing samples with unknown values. Components, such as electronics, used in an instrument may be affected by changes in operating temperature. If an instrument is calibrated at one temperature and then operated at a significantly different temperature, the temperature-induced error can also degrade the results’ accuracy.

Concepts Concerning Calibration

In measurement technology, calibration signifies the determination of the measurement error of the complete measuring instrument. No technical intervention takes place on the measuring instrument during calibration. On indicating measuring instruments, the measurement error is determined by calibration, comparing the value displayed with the correct value for the measurement in question.

Adjustment

A measuring instrument (or aggregate of mass) is set and adjusted in such a way that the measurement errors are as small as possible, or that the sum of the measurement errors does not exceed the error tolerances. Adjustment, therefore, requires an intervention that permanently changes the measuring instrument or aggregate of mass.

Official Verification

The official verification of a measuring instrument (or aggregate of mass) incorporates the tests to be carried out by the responsible verification authority, by the verification specifications, and by the seal. By this testing, it is determined whether the measuring instrument submitted complies with the verification specifications, that is whether it satisfies the demands to be placed on it in terms of its qualities and technical measurement characteristics, and in particular it is verified that the sum of measurement errors does not exceed the error tolerances. Using the seal it is certified that, at the time of testing, the measuring instrument satisfied these requirements and that, due to its qualities, it is to be expected that it will remain within the indicated tolerance range throughout the verification validity period if operated following the rules of technology. Law regulates those measuring instruments that require compulsory verification and those that are exempt.

Traceability

A procedure to refer measurement results to national and international standards through an unbroken chain of calibrations.

National Standard

A Standard that, by national decree, is recognized in a country as the basis for determining the values of all other standards of the measurement value in question.

Reference Standard

A Standard, generally of the highest available accuracy in a certain location or organization, from which measurements are derived.

Service Standard

A Standard that is routinely used to calibrate or test aggregates of mass, measuring instruments, or reference materials.

Measurement Uncertainty

Parameter assigned to the measurement result that identifies the mean variation of the values that may reasonably be assigned to the measurement value.

Smallest Possible Measurement Uncertainty

The measurement uncertainty assigned to a laboratory as the smallest possible measurement uncertainty for each measurement value and determining measurement ranges, based on the assessment of the measurement uncertainty budget and, if applicable, comparative measurements carried out.


Calibration Hierarchy

To ensure metrological traceability, a calibration made at a local level, such as an industrial internal calibration (also called in-house calibration), must be linked to a national standard by an unbroken chain of calibrations, with each step explicitly supported by appropriate documentation (ILAC P10:01/ 2013). The measurement uncertainty necessarily increases along the sequence of calibrations, starting from the national level down to the local level. Therefore, the prerequisite of metrological traceability is the establishment of a calibration hierarchy as shown in Fig. 1. 

The calibration hierarchy is defined in the International Vocabulary of Metrology as follows. 

Calibration hierarchy: sequence of calibrations from a reference to the final measuring system, where the outcome of each calibration depends on the outcome of the previous calibration. 

Note 1: Measurement uncertainty necessarily increases along the sequence of calibrations. 

Note 2: The elements of a calibration hierarchy are one or more measurement standards and measuring systems operated according to measurement procedures. 

Note 3: For this definition, the “reference” can be a definition of a measurement unit through its practical realization of a measurement procedure or a measurement standard. 

Note 4: A comparison between two measurement standards may be viewed as a calibration if the comparison is used to check and, if necessary, correct the quantity value and measurement uncertainty attributed to one of the measurement standards (JCGM 200: 2012, Definition 2.40).


Organizations Performing Calibration: Different organizations perform calibrations (Fig. 1): national metrology institutes, accredited calibration laboratories, and in-house calibration laboratories (ILAC P10:01/ 2013). In each step of the calibration chain, measurement standards are used to calibrate the measuring equipment for the next step. For example, an accredited laboratory can calibrate a company’s working standard against a reference standard. Accredited laboratories must fulfill the requirements of (ISO 17025: 2005). Above the national organizations, at the international level, decisions concerning the International System of Units (SI) and the realization of the primary standards are taken by the Conférence Générale des Poids et Mesures (CGPM). The development and maintenance of primary standards are coordinated by the Bureau International des Poids et Mesures (BIPM), which also organizes inter comparisons on the highest level. The International Laboratory Accreditation Cooperation (ILAC) promotes laboratory accreditation and the recognition of competent calibration and test facilities around the world. 

Calibration Interval: Calibrations must be repeated at appropriate intervals (ILAC G24: 2007). A measurement instrument, for example, should be periodically recalibrated because changes in its characteristics can occur during its use and after some time. Recalibration on appropriate intervals ensures detection of these possible changes. The length of these intervals will depend on several variables, such as the 

  • Uncertainty required 

  • Frequency of use 

  • Way of use 

  • Environmental conditions at the use 

  • Stability of the equipment




Conclusion

Calibration is essential to improving a company’s bottom line and maintaining a superior reputation. This Blog outlines the Important concepts of calibration, the Importance of Calibration, depending on factors that affect calibration severely, Official Verification, Concept of Traceability, National, Reference, and Service standards. We have also tried to shed some light on the topic of Calibration Hierarchy and Calibration Interval.


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