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10 steps to define Uncertainty 

10 steps to calculate uncertainty

Here are 10 broad steps to define Measurement Uncertainty, let’s discuss them one by one briefly. 

You can also Download our One Page Excel Calculation format for simplifying Calculations. 

  1. Knowledge of Metrology 
  2. The clarity in the Measurement Process 
  3. Identify Uncertainty Components 
  4. Clarity of Type A/B Uncertainty 
  5. Standard Uncertainty Calculation 
  6. Uncertainty Distribution 
  7. Combined Uncertainty Calculation 
  8. Degree of Freedom Calculation 
  9. Decide the k value 
  10. Final Expanded Uncertainty 

1. Knowledge of Metrology

Before starting Uncertainty measurement, anyone should know the basic concept of Metrology & Terminologies Used in Measurement. 

To know more about the Engineering Metrology, Join this online Course ‘Engineering Metrology & 3D Measurement

2. Clarity in Measurement Process 

Clarity means you should know the measurement method. 

How measurement is complete? 

What instruments require to complete measurement? 

Those are the basic questions you should know.  

3. Identify Uncertainty Components 

The best way to define Uncertainty Components is by the SWIPE method. 

  • S: Standard= All Uncertainty components related to Standard like Uncertainty, Accuracy 
  • W: Work piece= All Uncertainty components related to workpiece like the least count 
  • I: Instrument= All Uncertainty components related to Equipment/Instruments like Uncertainty, Accuracy 
  • P: Person or Process= Mostly covered in Type A Uncertainty, You can also consider least count here 
  • E: Environment Factors= Some of the common components are Soaking difference, Difference in thermal coefficient of expansion of material between Devices under Calibration & Master Equipment used, Overall Temperature Measurement system uncertainty in the lab, etc.  

4. Clarity of Type A/B Uncertainty 

According to the Vocabulary in Metrology (VIM), Type A Uncertainty is the “evaluation of a component of measurement uncertainty by a statistical analysis of measured quantity values obtained under defined measurement conditions.”

We simply Analysis Type A Uncertainty by Repeating the Measurement on the same point & condition. 

According to the Vocabulary in Metrology (VIM), Type B Uncertainty is the “evaluation of a component of measurement uncertainty determined by means other than a Type A evaluation of measurement uncertainty.”

Most of the type B uncertainty data come from as we discussed in the SWIPE concept.  

5. Standard Uncertainty Calculation

Standard measurement uncertainty (SD) is divided by the absolute value of the measured quantity value. 

CV = SD/x or SD/mean value. 

Check Calculation in Uncertainty Excel Sheet 

6. Probability Uncertainty Distribution 

Probability Distribution shows the relationship between the outcome of an event and its frequency of occurrence.

In Mechanical measurement, two types of distribution are commonly used 

  • Normal
  • Rectangular

Check Calculation in Uncertainty Excel Sheet 

7. Combined Uncertainty Calculation 

Combine Type A & Type B uncertainty components in the Uncertainty Budget

Check the Uncertainty Budget Calculation in our Uncertainty Calculation format

8. Degree of freedom

Degrees of freedom refer to the maximum number of logically independent values, which are values that have the freedom to vary, in the uncertainty Components.

9. Decide the k value 

The coverage factor, or ‘k’ value, determines the confidence in the data points within a certain standard deviation value

For k=1, there is a confidence that 68% of data points lie within one standard deviation, while k=2 means confidence that 95% of the data points would lie within two standard deviations. 

For more information, check our online course “Basics of Uncertainty Measurement

10. Final Expanded Uncertainty 

Expanded uncertainty is the last calculation when estimating uncertainty in measurement.

Check out the full calculation from instrument information to Expanded Uncertainty in our Uncertainty Measurement Excel sheet

Useful resources: 

  1. Online Course on Metrology 
  2. Online Course on Uncertainty 
  3. Download the Uncertainty Calculation sheet 

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