CCT Calibration Standards & Regulations 3 — Questions and Answers
Question 1: What is the correct definition of 'measurement uncertainty' as used in calibration standards?
- The probability that a measurement is wrong
- A parameter associated with the result of a measurement that characterizes the dispersion of values (Correct answer)
- The difference between two repeated measurements
- The tolerance allowed by the manufacturer
Correct answer: A parameter associated with the result of a measurement that characterizes the dispersion of values
Per GUM (Guide to the Expression of Uncertainty in Measurement), uncertainty is a parameter characterizing the dispersion of values that could reasonably be attributed to the measurand.
Question 2: Which document provides the internationally accepted methodology for evaluating and expressing measurement uncertainty?
- ISO 9001
- JCGM 100:2008 (GUM) (Correct answer)
- ANSI/NCSL Z540.3
- MIL-STD-45662A
Correct answer: JCGM 100:2008 (GUM)
JCGM 100:2008, commonly known as the GUM (Guide to the Expression of Uncertainty in Measurement), provides the internationally accepted framework for uncertainty evaluation.
Question 3: Under ANSI/NCSL Z540.3, what is the maximum acceptable probability that a measurement result exceeds the tolerance limits?
- 1%
- 2% (Correct answer)
- 5%
- 10%
Correct answer: 2%
ANSI/NCSL Z540.3 requires that the probability of false acceptance (accepting an out-of-tolerance item as in-tolerance) does not exceed 2% per measurement.
Question 4: What does the 'Test Uncertainty Ratio' (TUR) represent?
- The ratio of the measurement range to the instrument's accuracy
- The ratio of the tolerance of the unit under test to the uncertainty of the measurement standard (Correct answer)
- The ratio of systematic error to random error
- The ratio of calibration frequency to instrument drift rate
Correct answer: The ratio of the tolerance of the unit under test to the uncertainty of the measurement standard
TUR is the ratio of the tolerance of the unit under test (UUT) to the expanded uncertainty of the measurement process used to calibrate it.
Question 5: Which of the following best describes a Type A evaluation of measurement uncertainty?
- Uncertainty derived from manufacturer specifications
- Uncertainty evaluated by statistical analysis of a series of observations (Correct answer)
- Uncertainty assigned by the accreditation body
- Uncertainty taken from calibration certificate data
Correct answer: Uncertainty evaluated by statistical analysis of a series of observations
Type A evaluation of uncertainty uses statistical methods applied to a series of repeated observations, such as calculating the standard deviation of a measurement set.
Question 6: What is the role of a 'reference standard' in a calibration hierarchy?
- It sets the legal limits for equipment tolerance
- It provides a known, traceable value against which other standards or instruments are compared (Correct answer)
- It documents calibration procedures for technicians
- It certifies that a laboratory is accredited
Correct answer: It provides a known, traceable value against which other standards or instruments are compared
A reference standard provides a traceable, known measurement value used to calibrate transfer standards or working standards, forming part of the traceability chain.
Question 7: What is 'in-tolerance probability' in the context of ANSI/NCSL Z540.3 compliance?
- The chance that the instrument will remain in tolerance until its next calibration
- The probability that the actual value of a measured parameter is within its specified tolerance (Correct answer)
- The percentage of passing calibrations in a lab's history
- The likelihood that the reference standard is accurate
Correct answer: The probability that the actual value of a measured parameter is within its specified tolerance
In-tolerance probability (or end-of-period reliability) is the probability that a parameter's true value lies within its specified tolerance limits at the time of calibration.
CCT Calibration Standards & Regulations 3
What is the correct definition of 'measurement uncertainty' as used in calibration standards?