MEASURE FOR MEASURE
Calibration: What Is It?
by Graeme C. Payne
When members of just about any profession talk among themselves, they usually use specialized terms they all understand—jargon. There is usually consensus among them about the meaning of the terms, but they often forget newcomers or interested parties from other fields may not have the same understanding. There are also cases in which the common use of a word is different from or even opposite its technical definition. “Metrology” and “calibration” are two such words.1
The International Vocabulary of General and Basic Terms in Metrology (VIM) is an internationally accepted document that provides technical definitions of metrology, calibration and many other measurement related terms.2
Why do we need to refer to this technical glossary instead of a common desktop dictionary? One reason is the VIM is a listed authoritative reference in ISO 9000 and ISO 10012. Another reason is technical dictionaries and glossaries define the accepted technical meanings of terms, while common dictionaries merely record the ways words are used or misused in a language and most of their meanings.
According to the VIM, metrology is “the science and practice of measurement.”3 Metrology is important in some way to every human endeavor. More specifically, it is critical to all the physical, chemical and biological sciences and the technologies and manufacturing processes that flow from them.
In daily life, metrology affects us in commerce and law enforcement and in regulated industries such as healthcare and aviation. Whenever you purchase a gallon of gasoline or a pound of onions, drive by an officer with a radar gun on the highway or have an electrocardiograph taken, you have directly interacted with metrology by means of a calibrated measuring instrument. One practical application of metrology—a subset of the whole field—is the system of ensuring measurements conform to certain defined relationships.
A Process of Comparison
Broadly speaking, calibration is the process of determining the relationship between the readings obtained by a measuring instrument or system and the applicable units of some defined system of measurement. According to records uncovered by archaeologists, people have been doing this for at least 5,000 years. At first, units of measure were often based on things such as the volume of grain that could be held in two hands (cup) or the distance between the point of the Pharaoh’s elbow and longest fingertip plus the width of his palm (cubit). Now, our defined set of measurement references is known as the International System of Units (SI).4
Calibration quantifies the relationship between the readings of a micrometer, voltmeter, thermometer, weighing balance, mass spectrometer or graduated cylinder and the relevant units in the SI system. Paraphrasing the formal definition in the VIM, the instrument’s readings are compared to the values of a measurement standard under controlled and specified conditions.5
Each measurement result can be related to the SI units by the property of traceability, which accounts for the known or estimated uncertainty of the measurement process.6 The measurement standard has, in most cases, gone through the same process. Calibration is repeated at regular intervals to provide continued assurance the instrument’s performance is suitable for its use.
Calibration is essentially a process of comparison. An instrument is used to measure or is measured by a calibration standard, and the result is compared to two things: the known value and uncertainty of the standard and the performance specifications required by the customer. The concept is simple, but the work is in the details. Some of the details include:
- The assigned value of the measurement standard, which is usually determined from its calibration history.
- The known uncertainty of the standard, which comes from several places, including the historical reports of calibration and the internal statistical process control (SPC) methods many calibration labs have for their measurement systems. Labs that have an effective measurement SPC system know how their systems perform in that location, so their uncertainty values are likely to be more realistic—not always better, just more realistic.
- The environment of the calibration activity, which almost always includes temperature and relative humidity. Depending on the measurement, other influences such as absolute barometric pressure, the local gravitational vector, electromagnetic fields or building vibration may also have measurable effects.
- The methods and equipment used to make the comparisons.
- The uncertainty of the measurement system relative to the published performance specifications of the item being calibrated or the customer’s requirements, if different.
All the details have to be evaluated to provide a measure of the relationship between the measurements made with the instrument and the reference SI values. That relationship is the uncertainty, and its documentation defines traceability, which is the characteristic of the measurement result that demonstrates its accuracy in terms of the SI. Taken together, the relationship and its documentation indicate the quality of the measurements that can be made with that instrument when it is used correctly.
A calibration is performed using a calibration procedure, which is a documented, validated and controlled method for making the comparisons. The procedure may be a written paper or electronic document, or it may be a particular test program on an automated calibration system.
Many calibration procedures are written in conformance to the guidelines in Recommended Practice 3: Preparation of Calibration Procedures (NCSL RP-3), which defines the procedures’ purpose and content.7
The purpose of a calibration procedure is to determine and document the measurement relationships of the item being calibrated. It should define the parameters to be measured, the measurement standards to be used and the data to be collected. It should also list any safety precautions and preliminary steps and make note of the particular method of the comparisons, the calibration environment and anything else that is important for the items covered by the procedure.
A calibration procedure is written with the fundamental premises that the item being calibrated is in good working order and the person performing the calibration is trained and qualified and understands the scientific and physical principles of the measurements.8
What About Adjustment?
Up to this point, I haven’t said anything about adjustment. That’s because it’s not part of the formal definition of calibration, nor is it part of the description of a calibration procedure in NCSL RP-3. The results (data) of a calibration procedure may indicate a need for adjustment or other repair, but taking such action is a separate process. After the adjustment or repair is complete, the calibration procedure should always be repeated to verify the proper measurement relationship has been re-established.
There are two reasons adjustment is not part of the formal definition of calibration:
- The historical calibration data on an instrument can be useful when describing the normal variation of the instrument or a population of substantially identical instruments. That information can also be used for process improvement. For example, it can be used to evaluate the reliability of the instruments and change the recalibration interval. If the instrument is adjusted before the data from a full calibration run have been collected, then there is no historical value, and it cannot be used to improve the system.
- Any set of similar measurements may be considered a statistical process, and a single measurement from that process is a random sample from the probability density function that describes it. Without other knowledge, there is no way to know if the sample is within the normal variation limits. The history gives us that information. If the measurement is within the normal variation and not outside the specification limits, there is no reason to adjust it. In fact, making an adjustment could just as likely make it worse as it could make it better. W. Edwards Deming discusses the problem of overadjustment in chapter 11 of Out of the Crisis.9
The most common uses of the word “calibration” outside the metrology community include the concept of adjusting the instrument, and most customers expect it. In addition, many manufacturers have calibration procedures in their manuals that are not performance comparisons but, instead, are the test, alignment or adjustment procedures used for a new or repaired unit that is in an unknown condition.
Remember, a true calibration procedure assumes the instrument is in good working order. So, while acknowledging the formal technical definition, calibration providers also have to recognize the practical realities of business and work with the common definition as well.
REFERENCES AND NOTES
- This article is the first in a series of three. The July column will examine some of the different views of calibration, and the September column will discuss the importance of calibration.
- International Vocabulary of Basic and General Terms in Metrology, second edition, International Organization for Standardization, 1993. It is commonly referred to as the VIM, an acronym taken from the French title, Vocabulaire International de Termes Fondamentaux et Généraux de Métrologie.
- VIM, section 2.2, see reference 2.
- SI is an acronym taken from the French name, Le Systéme International d'Unités.
- VIM, section 6.11, see reference 2.
- VIM, section 6.10, see reference 2.
- Recommended Practice 3: Preparation of Calibration Procedures, NCSL International (formerly National Conference of Standards Laboratories), 1990.
- W. Edwards Deming, Out of the Crisis, Massachusetts Institute of Technology, 1982.
GRAEME C. PAYNE is the president of GK Systems Inc., a technical consulting company near Atlanta. A Senior Member of ASQ, Payne has been working in electronic calibration and product testing since 1981. He is a certified quality engineer, calibration technician and quality technician. He is also the chair-elect of the Measurement Quality Division and a member of NCSL International.