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When compared to a traceable reference, calibration ensures that the value provided by a measuring device is precise and repeatable. However, it cannot be assumed that the instrument will always deliver precise and repeatable readings after it has been calibrated. Therefore, instrument recalibration is necessary to determine how far off from a reference value or standard the instrument is, as well as to ensure that the variation is adequate for the measurement over time.
Recalibration is also necessary under the following two conditions:
• when the instrument undergoes routine maintenance
• when the instrument goes out of order and is repaired
While frequent calibration would accomplish the stated objective, it must be considered that calibration comes at a large cost. Therefore, selecting the appropriate calibration frequency, which strikes a balance between risk and cost, becomes an essential measuring task.
Factors Influencing Recalibration
The time between calibrations of measuring equipment is influenced by a number of factors. The important factors are:
• manufacturer’s recommendations
• the volume and intensity of usage
• the measurement’s accuracy and precision
• the possibility that a measuring device would exceed its tolerance while being used
• equipment kind, wear and drift propensity, and manufacturer’s recommendations
• circumstances of the environment, such as temperature, vibration, radiation, etc.
• data trends derived from historical calibration records
• history of maintenance and services
• frequency of cross-checking against other measurement tools or reference standards
• handling, storage, and risk management practices
• training level of serving personnel
The purpose of a periodic calibration, therefore, is:
• to calculate the measurement uncertainty related to the measurement instrument’s or reference standard’s departure from a reference value
• to reassure that the reference standard or measuring equipment can still be used to achieve the measurement uncertainty
While frequent calibration would accomplish the stated objective, it is important to consider the substantial cost of calibration. Therefore, selecting the right calibration frequency—which strikes a balance between cost and risk—becomes an essential measuring task. Techniques for assessing the interval must be created after the first calibration has been established to ensure that neither the danger of the instrument being out of calibration nor the cost of doing so rises. Several of these processes have been documented in international standards. Two often utilized techniques are the control chart method and the staircase or calendar-time approach.
