You may have a properly maintained calibration lab, well-trained personnel, high-end measuring equipment but the traceability of the reference standard you use is in question, then the calibration performed could be rejected.
As a beginner once in calibration, my only understanding about the meaning of Traceability in calibration is the unbroken chain of calibration where my reference standard is calibrated by a more accurate standard that is linked all the way to International Standards or SI. It is not wrong to understand this way, but it is not enough.
Have you ever thought if there is a difference between a traceable standard and a traceable calibration?
How do we really know that it is linked to International Standards through an unbroken chain of calibration?
What are the ‘evidence of traceability’ that shows us our calibration is a traceable calibration?
In this post, I will share with you the following:
The formal definition of Metrological Traceability
What are Calibration Traceability Chain and Traceability Pyramid
Why do we need to have traceability in our calibration?
The 2 Implementation Requirements for Metrological Traceability as Per ISO 17025
How to Ensure that We Have Received a Traceable Calibration?
How to Ensure Lab Has Traceability if it is not Accredited- Traceability Audit Requirements
What Are the Evidence of Traceability in a Calibration Certificate
When Does Traceability Become Invalid?
Definition of Traceability
This is the formal definition based on VIM (International Vocabulary of Metrology)
Metrological traceability (VIM 3 clause 2.41)
Property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.
As per VIM, for a shorter terminology, Metrological traceability can be termed as just ‘Traceability’ as long as it will not conflict to other definitions on a specific subject.
In a much simpler understanding, traceability is the series of comparison with the instrument’s measurement results (measured value and uncertainty) to the value of a higher accuracy standard that is linked to a more accurate standard until it reaches International Standards or SI.
I just want to emphasize that Traceability is referring to the “result of measurement”, NOT the instrument, the standard, or even the calibration performed.
If an instrument is traceable, this means that the results of measurement we have are the same wherever we go. With varying tolerable errors as it goes down to a traceability chain. For example on a 1kg standard weight, the 1 kg we have is also 1kg wherever we go, thus any product or service resulted by using this standard weight will be acceptable anywhere it goes
As a practical example of traceability in calibration, let us consider a digital weighing scale.Below are the steps where we have an unbroken chain of calibration:
The Digital Weighing Scale is calibrated by an M1 grade standard weight maintained by the user.
This M1 standard weight is calibrated by comparing its mass to a class F1 standard by a 3rd party Calibration Lab.
This F1 standard weight is calibrated by comparison to Class E1 weights from a National Lab.
Class E1 is compared against the primary standard that is directly linked to SI
The series of connections is linked by what we call the ‘traceability chain’. There is also a series of comparisons where accuracy is getting better as it goes up on the top chain, it is in order following the shape of a pyramid, a ‘traceability pyramid’. Read more below.
What Are Calibration Traceability Chain and Traceability Pyramid? Metrological traceability chain is “the sequence of measurement standards and calibrations that are used to relate a measurement result to a reference”
Traceability can be explained well by using a figure to represent the “unbroken chain of comparison” from a higher level going to a lower level (or lower level going to higher level). This figure is the traceability chain and traceability pyramid.
The traceability chain represents that every standard is linked to each other. When one chain is cut along the way, for example, overdue calibration, it means traceability is also disconnected in that part. This is because we can no longer trace it back to the top chain.
The traceability chain links every reference standard used in an order of higher to lower accuracy values (starting from the top going down) until it reaches the lowest chain. These hierarchies of accuracy are well represented by the Traceability pyramid.The traceability pyramid presents the hierarchy of every reference standard and the size (magnitude) of the provided uncertainty. The higher the location of the reference standard in the pyramid, the more accurate and smaller the measurement uncertainty.To further explain Traceability in calibration, I integrated both the traceability chain and traceability pyramid. This combination clearly shows the unbroken chain of calibrations presented by the chain and the hierarchy of standards with corresponding measurement uncertainty as presented by the pyramid, which is getting smaller as it goes up the traceability chain.
The order in the traceability chain and traceability pyramid from bottom to top can be shortened by jumping directly to a higher level of standard. This will improve your measurement uncertainty but of course, it will result in a higher cost of calibration.
In every step or sequence of the Traceability chain where we relate a measurement result to a reference, this process must be under control to ensure that the quality of transferred measurement results are maintained.
