Calibration

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Calibration is the comparison, between the measurements of an instrument, and the measurements of a standard reference in the laboratory.

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In metrology, the calibration certificate is generated with the error and uncertainty between the standard and the tested. The instruments can be pressure, temperature, caliper, valve, etc..

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Details:

  • Calibration
    • Metrology
      • Measurement
      • Physical quantity
      • Unit of measurement
      • National System of Units
      • Laboratory
      • Quality

First, errors and uncertainties are consulted in the standard instrument, which is the reference. This information must be on the calibration certificate of the standard instrument. This will be used in the calculation of inheritance of uncertainties and errors. The inheritance of these errors and uncertainties helps in traceability.

Afterwards, the measurements of the instrument to be calibrated must be compared with the standard. There are two ways:

1 – Set the value of the standard as a reference, and then measure through the instrument to be calibrated.

2 – Fix the value of the instrument to be calibrated, and then measure the value of the standard as a reference.

Measurements must be taken several times. This information generates the mean, uncertainty, error. And yet, with inheritance of the uncertainties and errors of the pattern. The name of the measurement through the standard is entered as VC (Conventional Value). Also, the name of the measurement through the instrument to be calibrated is sometimes set as VI (Indicated Value).

Measurements are also from the lower range to the upper range.

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Tolerance

The comparison between the values ​​generates error and uncertainty. Error and uncertainty together can be used as a way of knowing if the instrument is within tolerance.
Tolerance is specific for each instrument within its area of ​​use.
Some instruments may have a high tolerance, others a low tolerance.
Example: the electrical voltage of a specific equipment can be established, for example, close to 12V, within a range of 11.5V to 12.5V. To measure this voltage, a voltmeter can be used. But this voltmeter, used to measure this voltage, has an error and uncertainty. So this error + uncertainty, of this voltmeter, must be less than the allowed range.
That’s why it’s important to calibrate, which within the metrology area, means comparing the values ​​read with an official reference. After generating the certificate, with the calculations of errors, uncertainties and inheritances, then there is a way to help find out if the error + maximum uncertainty is within the tolerance range.

Equipment measurement readings can be automated. For example: MODBUS Equipment

Types of Calibrations

For each type of instrument, calibration is performed in a different way.
For example, a caliper, nominal blocks are generally used.
Then the size of the blocks is measured several times, and these values ​​are recorded. But the nominal blocks must also have a calibration certificate. Why the errors and uncertainties of the standards are used in the calculations too. They are legacies of errors and uncertainties. This traceability is important to guarantee the results.
After being measured several times, then the average is calculated. Then it is compared with the pattern, but with an error inherited from the pattern as well. Then the uncertainty of the mean is calculated. This uncertainty is accompanied by the degrees of freedom. In parallel with the k factor. But the uncertainty is also influenced by the uncertainty of the standard certificate.
Sometimes the ambient temperature, pressure and humidity can influence the calculation. In any case, they should be used as a reference in the calibration.

Resolution

Another important factor is the resolution of the instrument. Each instrument can have various ranges, units and resolutions.
The resolution, can be the amount of maximum digits that are used as division.
For example, if the resolution is 0.01, then the values ​​can be 12.51, 31.73, 55.37, 17.11, etc.
The resolution interferes with the uncertainty calculated in the instrument calibration. The probability distribution curve has several types. It can be normal distribution, rectangular distribution, triangular distribution, etc. These distributions interfere with uncertainty calculations.
In the case of resolution, in digital equipment, in general the distribution can be rectangular.
But analog instruments may have other types of distribution, in the case of resolution.
Another important item is the fixed reference when the calibration data is collected. The reference can be the official standard instrument set, and the instrument to be calibrated is read and compared. The reference can be the instrument to be calibrated, being fixed, and using official standard read and compared. This interferes with several items. Mainly in the error calculation. Changing the reference type reverses the error.

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