Reading an ISO/IEC 17025 Scope of Accreditation

What is a “Scope of Accreditation?”

In “calibration world” all organizations that are accredited to ISO/IEC 17025 have a scope of accreditation. “The Scope” is a handy tool to evaluate a calibration provider’s abilities. If you want to know if a company can calibrate a device then, it should be in their scope. If you want to know how well they can measure that item, then that is on their scope as well. Scopes of accreditation are also useful in comparing calibration providers. You may find out that when you compare scopes, you will understand why two different organizations have very different prices for the “same” calibration.

Calibration and measurement capabilities

Calibration and Measurement Capabilities or CMCs are numerical values that represent the best uncertainty that a calibration lab can publish for a given measurement. These are the values that the accreditation body (AB) has assessed the lab to, given their personnel, equipment, and processes. These values can be expressed as simple numerical values, or as a formula.

When we look at the expression of measurement uncertainty, we have two typical scenarios. Situations where the measurement uncertainty changes as the size of a measurement changes, and items where the measurement uncertainty exists independent from the measurement’s magnitude. We frequently have a combination of both types of error where we will use a formula to indicate a percentage of a measurement and a floor error specification

Measurement uncertainty expressed as a value

When measurement uncertainty is the same regardless of where in the range the value lies, we can show the uncertainty as a pure value. So, we may have a range for thermocouples expressed as 0 to 1732°C with a CMC of ±0.34°C anywhere within this temperature range. This base level uncertainty will be the same everywhere in the field of values from 0 to 1732°C.

The value from the CMC is not automatically the value you will receive. The performance of the UUT can impact the actual CMC that is reported. If you need the expressed uncertainty of measurement as close to the CMC as possible, you should include this request in your purchase order to ensure the calibration lab is aware of your requirements.

Measurement uncertainty expressed as a percentage

When a measurement uncertainty changes in relation to the range that is measured, a proportion of the field is frequently used. A simple example would be a voltage range of 10 to 100 VAC with an uncertainty of ±0.015%. When we want to know what the measurement uncertainty would be at 50 VAC, you can find what 0.015 percent of 50 is.

Multiply 50 VAC times 0.00015 (or 0.015%) to calculate the uncertainty of ±0.0075VAC

Like percentage, the scope could also contain a multiplier, ppm (parts per million), or ppb (parts per billion) figures. In all these cases, you just multiply the value in the range by the multiplier on the scope of accreditation.

Measurement uncertainty expressed Algebraically

Frequently, in the world of measurement, we deal with the two types of errors simultaneously. In these cases, we have a particular floor level of error as well as a certain level of uncertainty that increases with the magnitude of the measurement. In these cases, we will use a formula to express the uncertainty.

Typical scope line from a published scope of accreditation

In this case, we take the measurement we are interested in and plug it into our formula.

At 500 PSI, we can multiply 500 PSI by 0.037% to get 0.185 PSI and then add 0.51 to that to get a total uncertainty of ±0.695 PSI.

How scopes of accreditation are formatted

Most ABs have adapted similar formatting for scopes of accreditation. They are typically formatted to have tables divided by general to more specific types of items. In the example above we have a broad class of measurements “Mass” followed by a more specific class “Pressure.” Under pressure, we see the parameter listed as “Pressure Gages.”

The major parameters are typically divided up by categories like Amount of Substance, Acoustics, Ultrasound and Vibration, Electrical, Length, Mass, Thermodynamics, and Time and Frequency. So, when you are looking for a particular parameter on the scope of accreditation, it is easiest to try to find the item by starting with the broadest possible category in which the measurement parameter would fit, and then narrowing it down from there.