The importance of establishing a national measurement standard was recognized early on when our forefathers drafted the Constitution. Article 1, Section 8, gave Congress the power to put in place a standard of Weights and Measures.  It was understood then, as it is now, in an industrial nation such as the United States, the standardization of weights and measures is an essential part of life for all of society and commerce.

A key component of this decree became the need to develop a well-defined system of units that would be recognized throughout the globe. The birth of the International System of units (SI) commonly known as the metric system, was agreed to, and adopted on May 20th, 1875, as the universally accepted standard. Seventeen countries including the United States were represented in Paris at The International Treaty of the Meter on the day which is known as World Metrology Day! It was on this date that the International Bureau of Weights and Measures (BIPM) was officially created. This organization is responsible for bringing about the unification of a measurement system, which would establish and maintain units and standards linked to associated physical constants.

Though the standards can change as technology improves, the concept is the same, choose a measure and establish an associated value.  In ancient times these measures were as simple as a person’s hand or foot (for length measurements), but as we have evolved there became a need for measurement units with less variance, and uncertainty. Today, most units of measure are bound by the laws of physics.  These measures are what are referred to as the standards. To each of these standards is assigned a unit of measure which can be recognized across the globe, traceable to a defined physical constant. 

Like the need to define each unit and standard, it was also necessary to clearly define the most frequently used terms in the field of metrology.  The International Vocabulary of Metrology (VIM) was originally developed in 1984 under the auspices of the International Bureau of Weights and Measures (BIPM).  In 1997 the Joint Committee for Guides in Metrology (JCGM) was formed under the direction of BIPM to oversee the International Vocabulary of Metrology – Basic and general concepts and associated terms (VIM), and the Guide to the Expression of Uncertainty in Measurement (GUM).  

The VIM is the most universal attempt to standardize terminology across different fields of science, legislature, commerce, and trade. The VIM vocabulary is meant to be a worldwide reference for scientists and engineers involved in planning or performing measurements, as well as for governmental bodies, trade associations, accreditation bodies, and professional societies. Since its first publication, VIM has undergone several revisions, which have expanded the terminology through examples and notations. The latest is the third edition, released in 2012.

The basic premise of the VIM is to eliminate any misunderstanding in the communication of fundamental terminology as it relates to metrology. One of the primary topics covered by VIM centers on defining calibration and all the associated components. In a calibration laboratory, we often find ourselves explaining the difference between calibration, verification, validation, certification, and adjustment, because misguidedly industry exchanges one term with the other. This was recognized universally as BIPM set forth with VIM to dispel any confusion between these terms and many others.  Notably, all these terms are important to understand, yet critically different.  It is imperative, for many reasons, which all parties involved in the calibration of an instrument be assured they are speaking clearly about the same process.

As defined in VIM Chapter 2.39, “calibration [is the] operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication.”

In clearer terms, calibration is the process of comparison of an instrument’s readings to a known traceable standard to identify the disparity. The calibration process shall be performed under well-defined conditions with known uncertainties, using a process and standard such as that defined by a professional society, or an international organization. 

It is further noted in VIM 2.39 that “Calibration should not be confused with adjustment of a measuring system, often mistakenly called self - calibration, nor with the verification of calibration”. Affirming that the calibration process is the actual comparison of the indicator readings to the known traceable standard, and it is not the action associated with an adjustment or repair performed prior to the calibration. Once a calibration is performed, the system error and uncertainty can be determined based on the relation or variance of the indicator readings and the known standards value.

It is often stated that a piece of equipment is “in-calibration”. Other than the fact that the calibration period has not expired, it is expected that there is a specific criterion defining the requirements to which it conforms.  Frequently, equipment is calibrated to be in compliance with the manufacturer’s specification, commonly expressed as the maximum permissible error or MPE. Where the MPE is the extreme value of measurement error, with respect to a known reference. Moreover, a maximum permissible error and/or a target uncertainty may be specified, and the measurement procedure and the measuring system should then be chosen in order not to exceed these measuring system specifications. Conversely, out-of-calibration equipment is equipment beyond its cycle time and/or equipment with existing specifications that cannot be verified (often through test). One can think of an equipment’s calibration cycle as the time between calibrations.  Whether it is defined by the duration, or frequency of use, the cycle time is established using statistical data outlining proof of stability. Verification of calibration should normally be the first step in any measurement process.

Not to be confused with calibration, verification per VIM 2.44, is “objective evidence that a given [process or measurement system] fulfils specified requirements”. Where the requirements may often be the manufacturers specification. In its most simple form, a verification of a system could be the actions of an inspector using a check-standard (control-standard) prior to taking any measurements to determine if the system is operating properly and within specification. However, the verification of a calibration process is more rigorous and parallels the actions of an auditor during an acceptance evaluation. Additionally, a validation is defined in VIM 2.45 as a “verification that the specific requirements are adequate for its intended use”. Thus, not every verification is a validation. For example, a measurement procedure, ordinarily used for the measurement of a gage block, may be validated also for the measurement of a scale bar. 

To circumvent intense scrutiny into all the parameters involved in a verification of a calibration process, a corporation will require that the calibration lab be accredited to an internationally recognized standard. In doing so, they are assured that the laboratory is following all the necessary procedures to ensure their test and calibration process is binding. ISO/IEC 17025, developed by the International Organization for Standardization is such that, where an accreditation under this standard will corroborate that all the required steps have been taken to ensure technical competency and consistency, using practices recognized worldwide. Without this accreditation, regulatory authorities and suppliers typically will not accept calibration or test results from a calibration laboratory.

It is important here to note, that if a system shows up at your laboratory not operating within the required specification, further investigation should be taken into the accuracy of your measurements during prior use. Strictly speaking, if adjustments are necessary to get the system to operate within the defined calibration specifications, all previous measurements could be subject to error. Under these circumstances, the system calibration report should indicate the equipment’s “as-found” results (pre-adjustment), and the equipment’s “as-left” results (post adjustment). Where the as-found results will identify the systems performance during any pre-calibration use, and the as-left results will authorize the use moving forward.

At this point you may be asking yourself, what do I do if the system calibration is not within the required specifications, or if it does not comply with the OEM’s specified MPE?  On occasion, a repair or adjustment is needed to get a system back into operation.  Again, this adjustment is not considered part of the “calibration” of the equipment as specifically stated in VIM 3.11 and noted here, “Adjustment of a measuring system should not be confused with calibration”. It is however important to acknowledge that the equipment may have been operating outside of the specification prior to its adjustment.  Care must then be taken to trace back the use of this equipment to a state of operation when it was within specification, such as a previous verification. The definition for adjustment is provided in VIM 3.11 as the “set of operations carried out on a measuring system so that it provides prescribed indications corresponding to given values of a quantity being measured”. Moreover, if any adjustment is needed, it will then be necessary to calibrate the equipment making certain that the adjustments now have the system operating within the required specification before it can be released for use.

With the help of VIM, it is hopeful that the reader understands the difference between calibration, verification, validation, and adjustment.  This leaves us to the last item to discuss, “certification”.  Where calibration, verification, validation, and adjustment are all actions taken on an object, artifact, process, or piece of equipment to guarantee measurement integrity, certification is not. Certification is best defined as the past tense depiction of a subject that was previously calibrated. Accordingly, VIM 5.14 states that if a reference material is certified it has been issued a document by an authoritative body, providing specified values with associated uncertainties that are traceable using valid procedures. Thus, concluding that only certified reference materials (CRM), can be used for calibration or for verifying measurement trueness. 

In conclusion, calibration is the process of testing or checking an instrument against a known standard. To authenticate this process, the calibration must use certified standards that are traceable to an international recognized physical constant approved by the International Bureau of Weights and measures (BIPM). Though a calibration will typically not be a direct comparison to this constant, it will likely use a certified reference material or artifact that follows a chain traceable back to an international standard. For further detail on this topic and more, refer to the International Vocabulary of Metrology – Third Edition.