Calibration is the process of evaluating and adjusting the precision and accuracy of measurement equipment. Proper calibration of an instrument allows people to have a safe working environment and produce valid data for future reference.
Calibration refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. Instrument calibration is intended to eliminate or reduce bias in an instrument's readings over a range for all continuous values.
This figure provides an example of precision and accuracy; it also shows the result after calibration.
For this purpose, reference standards with known values for selected points covering the range of interest are measured with the instrument in question. Then a functional relationship is established between the values of the standards and the corresponding measurements. There are two basic situations:
The instrument reads in the same units as the reference standards. The purpose of the calibration is to identify and eliminate any bias in the instrument relative to the defined unit of measurement. For example, optical imaging systems that measure the width of lines on semiconductors read in micrometers, the unit of interest. Nonetheless, these instruments must be calibrated to values of reference standards if line width measurements across the industry are to agree with each other.
The instrument reads in different units than the reference standards. The purpose of the calibration is to convert the instrument readings to the units of interest. An example is densitometer measurements that act as surrogates for measurements of radiation dosage. For this purpose, reference standards are irradiated at several dosage levels and then measured by radiometry. The same reference standards are measured by densitometer. The calibrated results of future densitometer readings on medical devices are the basis for deciding if the devices have been sterilized at the proper radiation level.
The calibration method is the same for both situations stated above and requires the following basic steps:
Some people mix up field check and calibration. Field check is when two instruments have the same reading; this does not mean they are calibrated; it may be that both instruments are wrong. Let's use thermometer as an example; if a thermometer always read .25 degree higher, this error can not be eliminated by taking averages, because this error is constant. The easiest way to determine if it is accurate and fix it is to send the thermometer to a calibration laboratory. Another way to reveal constant errors is to have one or more similar thermometers. One thermometer is used and then replaced by another thermometer. If readings are divided among two or more thermometers, inconsistencies among the thermometers will ultimately be revealed.
Let's say if you are going to publish a paper and you submitted the paper with data obtained from an uncalibrated instrument. What if someone repeated you experiment and find out that your result is wrong? This will hurt your reputation in the field, decrease the reliablitiy on your future works.
Another example, if you are going to work with a chemical that will explode when it gets in contact with air temperature above 50°C. So you adjust the room temperature before you start working, then check the temperature with an uncalibrated thermometer. If the termometer gives a lower temperature than the true temperature, then you will be working in an unsafe environment. This example may be unrealistic, but there are many chemical and substance out there that require accurate and precise measurements in order to provide others a safe working environemnt.
Youden, W. J. Experimentation and Measurement. (1984)
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