Metrology Monday! #72 The ISO/IEC 17025 Method vs. Procedure Controversy

The inspiration for today’s article comes from fellow metrologist Gerome Bhola.? I find that a lot of people struggle with the question that he asked.? “Can you clarify the difference between a calibration method and a calibration procedure for me as per ISO17025? We are having issues with our ISO consultants and lab personnel on this.”

Let’s start with the definitions in the VIM.? The first step in the development of a calibration procedure is to define the measurand that we intend to measure.? The VIM definition of Measurand is simply “Quantity intended to be measured.”? An example of this could be the reading of a temperature indicating device for thermocouple probes at 100 degrees Celsius.

The next step is to define the measurement principle, which the VIM defines as the “phenomenon serving as a basis of a measurement.” Proceeding from our first example, this could be the Thermoelectric effect applied to the measurement of temperature.

Now we are ready for the measurement method, which the VIM defines as a “generic description of a logical organization of operations used in a measurement.” There are only five fundamental measurement methods associated with calibration.? They are Direct Measurement, Differential Measurement, Transfer Measurement, Ratio Measurement, and Indirect Measurement.

Direct Measurement – Where the measuring instrument comes into contact with the quantity being measured.

Differential Measurement – The reference and the device under test are measured through opposition of a detector.

Transfer Measurement – Characterizing a device with a reference in order to improve the uncertainty of a direct measurement device.? It is possible to make transfer measurements in different units from the measurand, for example using an AC-DC transfer standard where and AC voltage source is characterized with a detector that reads DC millivolts.

Ratio Measurement – Determination of a dimensionless quantity based on measurements on the input and output of a device, such as an RF attenuator.

Indirect Measurement – Finding the value of interest from other values.? For example, finding the value of resistance by measuring voltage and current.

I have yet to find a calibration measurement method that does not fit into one of these categories.? If anybody can prove me wrong by counterexample, I would be very interested in seeing it.? If you are interested in learning more about calibration measurement methods, please see Fluke’s book Calibration: Philosophy in Practice, Chapter 5.

Now on to the last VIM definition measurement procedure.? This is defined as a “detailed description of a measurement according to one or more measurement principles and to a given measurement method, based on a measurement model and including any calculation to obtain a measurement result.”

Now, here is where the controversy comes in.? ISO/IEC 17025 is a document for both testing and calibration laboratories.? In the world of testing, they are not generally aware of the VIM or any of its definitions.? The testing community has used the term method generally as metrologists would use the term procedure.? Sometimes, the term test method lies somewhere between method and procedure, such as ASTM test methods that provide both general principles and generic procedures.

ISO/IEC 17025 references the VIM as a normative reference in section 2.? When we did the 2017 revision of the standard, I tried to get the writing committee to take on the issue of the term Method not being in line with the definition in the normative reference.? I would say that between the combination of the magnitude of the task to correct this issue, the number of testing experts that were comfortable with the term as is, and the general exhaustion of jet lag at the meeting, I was not as successful in addressing this issue as I would have liked.? I was able to get one small victory with the agreement to add the note in 7.2.1.1 in the section on selection and verification of methods.? “Note: “Method” as used in this document can be considered synonymous with the term “measurement procedure” as defined in ISO/IEC Guide 99.” (the ISO name for the VIM).

So there you have it.? Although the VIM defines method and procedure quite differently, for the purpose of ISO/IEC 17025:2017, they are the same thing.? Next week I will address the consequences of this when touching on ISO/IEC 17025 requirements for validation of methods.? #MetrologyMonday #FlukeMetrology??