Calibration method, using metrology standards, consists of comparing the measuring instrument numbers in relation to a reference and generating a certificate with the results of errors and uncertainties.
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Some general rules that can define calibration method:
ISO GUM
NIST - National Institute of Standards and Technology
The meaning of the word calibrate, according to metrology, is to compare values with standard instruments. But, this word is still used in some cases, like adjusting instrument values. Anyway, even before adjusting and after adjusting, the comparison with the reference is good to be done.
The calibration method is usually specific to each type of instrument.
The term method can also be considered a measurement procedure.
In the case of pressure measuring instruments, which can be pressure transducers, pressure gauges or safety valves, generally the method of standards suggests that the values are measured by going up and down the pressure several times.
Information is noted for each reference, both on the way up and on the way down.
In the method for temperature measuring instruments, the standard can be measured again for each temperature reading. Sometimes the reference standard varies with each temperature.
For scales, the method can use standard weights. Each weight can have error and uncertainty information. The uncertainty calculator shows the results of calibration. "Measurements are usually taken by repetitively replacing the weight.
In the calibration method for calipers, objects or blocks with standard dimensions can be used as a reference in measurements. Repeated measurements generate uncertainties.
In multimeters, each unit may have a specific calibration method. Voltage, current, resistance and frequency are four types of measurement. The ascent, descent and repetitions depend on the methods of the norms.
There are several official calibration methods defined for each type of instrument and also countries. And also, methods of formulas to calculate the results.
Depending on the feasible methods, confirmations of measurements should be determined. The frequency of performing calibrations can be customized for each case. Regarding the standards, the calibration methods help in the stability of the values. Instrument validation depends on those responsible for calibration. Several elements are defined in the methods considered acceptable in the realizations. Each operation has a basis, which is supportable in the calibration method. Calibration quality is important to facilitate measurements. The values read by the instrument must be acceptable with good quality in precision. The calibration speed can also be checked in the method. But the responsibility rests on the quality of the results. Presentations of results that are purchased with the standards are defined by the method on specific dates. And each type of instrument, for each area of work, can have a maximum tolerance. It is sometimes called the maximum allowable error. Stable measurements in calibration receive more attention on measurement error. If repeating values have a large variance, then standard deviation or the official expanded uncertainty can be used.
Calibration management helps laboratories operate according to standards.
Laboratories can define a development for verification and validation of methods.
An important item is to control and record environmental conditions. Sometimes the ambient temperature, pressure and humidity can interfere with the calibration results.
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Methods or procedures must be officially defined and placed on the calibration certificate. The definition of the use of resources to carry out the activities.
The validation of the methods depends on the appropriate procedures for the activities.
The method definition version must be placed on the certificate at each calibration date.
If the method should have additional details the version is updated in the next calibration certificate.
The activity must be planned and assigned to competent personnel with adequate resources.
Periodic critical analysis must be carried out to verify the needs of those who use the instruments.
Afterwards, modifications to the methods must be approved and authorized.
Validation of calibration methods can be performed by laboratories. Validation must be comprehensive to meet the needs of each application.
Comparisons can be made with results obtained by other methods.
The characteristics of the validated methods must be related to the needs of those who use the instruments. Performance also consistent with the requirements that were specified.
Laboratories may retain some validation records.
One of them is the determination of the performance characteristics of the methods.
In the case of sampling, the laboratory must have a method. This method must address the factors to be controlled. It must be available at the location where sampling is carried out.
The sampling method should describe sample selection, sampling plan, treatment of substance, material or product samples.
Records must include reference to the sampling method used.
By following the test method and instructions the laboratory meets the assessment of measurement uncertainty.