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2. Measurement Process Characterization
2.3. Calibration
2.3.6. Instrument calibration over a regime

Data collection

Data collection The process of collecting data for creating the calibration curve is critical to the success of the calibration program. General rules for designing calibration experiments apply, and guidelines that are adequate for the calibration models in this chapter are given below.
Selection of reference standards A minimum of five reference standards is required for a linear calibration curve, and ten reference standards should be adequate for more complicated calibration models.

The optimal strategy in selecting the reference standards is to space the reference standards at points corresponding to equal increments on the y-axis, covering the range of the instrument. Frequently, this strategy is not realistic because the person producing the reference materials is often not the same as the person who is creating the calibration curve. Spacing the reference standards at equal intervals on the x-axis is a good alternative.

Exception to the rule above - bracketing If the instrument is not to be calibrated over its entire range, but only over a very short range for a specific application, then it may not be necessary to develop a complete calibration curve, and a bracketing technique (ISO 11095) will provide satisfactory results. The bracketing technique assumes that the instrument is linear over the interval of interest, and, in this case, only two reference standards are required -- one at each end of the interval.
Number of repetitions on each reference standard A minimum of two measurements on each reference standard is required and four is recommended. The repetitions should be separated in time by days or weeks. These repetitions provide the data for determining whether a candidate model is adequate for calibrating the instrument.
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