FAQ's: Measurement Uncertainty
FAQ's on Uncertainty
As a certified calibration technician and technical manager for a calibration...
What is a “Scope of Accreditation?”
In the “calibration world,” all organizations that are accredited to ISO/IEC 17025 have a scope of accreditation. “The Scope” is a very useful tool to evaluate a calibration provider’s abilities. If you want to know if a company can calibrate a device then, it should be on 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 too, 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
When measurement uncertainty is the same regardless of where in the range the value lies, we can express the uncertainty as a simple 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 range 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.
When a measurement uncertainty changes in relation to the range that is measured, a percentage of the range 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 simply multiply the value in the range by the multiplier on the scope of accreditation.
Frequently, in the world of measurement, we deal with the two types of errors simultaneously. In these cases, we have a certain 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 line item from a published scope of accreditation
Mass - Pressure
Parameter |
Range |
Expanded measurement of Uncertainty (+/-) |
Remarks |
Pressure Gages |
0 to 1000 PSI |
0.51 psig + 0.037 % of reading |
Dead Weight Tester |
In this case, we take the measurement we are interested in and plug it into our formula.
For 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.
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 general 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 a scope of accreditation it is easiest to try to find the item by starting with the widest possible category in which the measurement parameter would fit, and then narrowing it down from there.
If you are looking for detailed definitions of calibration terms you might want this FREE download.
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