CCT Calibration Techniques & Procedures 4 — Questions and Answers
Question 1: A calibration standard with a stated uncertainty of ±0.01% and a 95% confidence level means:
- The standard is always within ±0.01% of the true value
- There is a 95% probability that the true value falls within ±0.01% of the standard's reported value (Correct answer)
- The standard will be within tolerance 95% of the time it is used
- Only 5% of readings taken with this standard will be invalid
Correct answer: There is a 95% probability that the true value falls within ±0.01% of the standard's reported value
Uncertainty at a stated confidence level means the true value lies within the stated interval with that probability, not that it is guaranteed to be within that interval.
Question 2: Which method is most appropriate for calibrating a high-accuracy platinum resistance thermometer (PRT) in the range of 0°C to 660°C?
- Comparison in an oil bath against a calibrated thermocouple
- Fixed-point calibration at ITS-90 defined phase transition temperatures (Correct answer)
- Dry block calibration with a blackbody radiation source
- Calibration against a bimetallic reference standard
Correct answer: Fixed-point calibration at ITS-90 defined phase transition temperatures
ITS-90 fixed points (such as the freezing points of tin, zinc, and aluminum) provide the most accurate reference temperatures and are the accepted method for primary PRT calibration.
Question 3: When documenting a calibration, the minimum traceability statement on the certificate must include:
- The name of the technician and their years of experience
- The reference standards used, their calibration certificates, and the accrediting body (Correct answer)
- The customer's intended use of the instrument
- A list of all possible sources of error
Correct answer: The reference standards used, their calibration certificates, and the accrediting body
Traceability documentation must identify the standards used and link them to an unbroken chain back to national or international standards through their certificates and the body that accredited them.
Question 4: A calibration technician is using a fluke 5522A multi-product calibrator to calibrate an oscilloscope. To verify the oscilloscope's vertical gain accuracy, the technician should output:
- A DC voltage precisely set to a known amplitude
- A sine wave at a known frequency and amplitude (Correct answer)
- A square wave with a defined rise time
- A ramp waveform at the oscilloscope's bandwidth limit
Correct answer: A sine wave at a known frequency and amplitude
Vertical gain accuracy is verified by comparing the oscilloscope's displayed amplitude of a sine wave to the calibrator's precisely known output amplitude at a specific frequency.
Question 5: The 4:1 test accuracy ratio (TAR) rule specifies that the calibration standard's accuracy should be at least 4 times better than the tolerance of the instrument being calibrated. This ratio exists primarily to:
- Reduce the number of calibration points required
- Limit the probability that standard uncertainty contributes significantly to conformance decision errors (Correct answer)
- Ensure the standard can always drive the UUT to full scale
- Comply with OSHA workplace safety regulations
Correct answer: Limit the probability that standard uncertainty contributes significantly to conformance decision errors
A 4:1 TAR ensures the standard's error is small relative to the UUT tolerance, reducing the risk of incorrectly accepting or rejecting the UUT due to standard uncertainty.
Question 6: Environmental conditions recorded during calibration (temperature, humidity, pressure) are required primarily because:
- They satisfy customer contract requirements regardless of technical impact
- Many instruments' characteristics vary with environment, affecting measurement results and uncertainty (Correct answer)
- Environmental logging is required only for electrical calibrations
- They allow the calibration interval to be extended
Correct answer: Many instruments' characteristics vary with environment, affecting measurement results and uncertainty
Temperature, humidity, and pressure affect both the standard and the UUT; recording conditions allows corrections to be applied and uncertainty contributions from environment to be quantified.
Question 7: When calibrating a precision resistor using the substitution method, the reference and unknown resistors are measured sequentially with the same bridge/meter to:
- Eliminate errors caused by lead resistance in the connecting cables
- Cancel out systematic errors of the measuring instrument itself (Correct answer)
- Test whether the unknown resistor is frequency-dependent
- Verify the temperature coefficient of the reference resistor
Correct answer: Cancel out systematic errors of the measuring instrument itself
In substitution, the measuring instrument reads both resistors under identical conditions, so any instrument bias cancels when the ratio of readings is taken, yielding a more accurate comparison.
CCT Calibration Techniques & Procedures 4
A calibration standard with a stated uncertainty of ±0.01% and a 95% confidence level means: