Measurement Uncertainty with case study – ISO 17025

Case Study

A laboratory is required to estimate the uncertainty of the mass of a small piece of metal. The metal is weighed using a balance that has a stated uncertainty of 0.01 g. The laboratory repeats the measurement 10 times and obtains the following result

10.00 g
10.01 g
10.00 g
10.02 g
10.01 g
10.00 g
10.01 g
10.00 g
10.02 g

Data Set

The following data set can be used to estimate the uncertainty of the measurement:

* Balance stated uncertainty: 0.01 g
* Sample mass: 10 g
* Standard deviation of repeated measurements: 0.005 g
* Uncertainty of measurement standards: 0.001 g

Equations

The following equations can be used to estimate the uncertainty of the measurement:

• Uncertainty of the balance:
  • u_b = 0.01 g
• Uncertainty of the sample mass:
  • u_s = 0.005 g
• Uncertainty of the measurement standards:
  • u_m = 0.001 g
• Overall uncertainty of the measurement:
  • u = u_b + u_s + u_m
  • u = 0.02 g

Conclusion

The overall uncertainty of the measurement is estimated to be 0.02 g. This means that the true mass of the sample is likely to be within the range of 10.00 g to 10.02 g.

Factors That Could Affect the Uncertainty of the Measurement

The following are some of the factors that could affect the uncertainty of the measurement:

• The skill of the operator
• The environmental conditions
• The age and condition of the balance

By taking these factors into account, the laboratory can improve the accuracy and reliability of their measurements.