CCT Calibration Techniques & Procedures 3 — Questions and Answers
Question 1: A calibration technician notices that repeated measurements of the same input produce results that cluster tightly but are consistently offset from the true value. This describes:
- Poor precision with good accuracy
- Good precision with poor accuracy (systematic bias) (Correct answer)
- Random error with no bias
- High hysteresis with low drift
Correct answer: Good precision with poor accuracy (systematic bias)
Tight clustering indicates good precision (repeatability), but a consistent offset from true value is a systematic error (bias), meaning accuracy is poor despite high repeatability.
Question 2: ISO/IEC 17025 requires that calibration laboratories maintain traceability to national measurement standards. The primary purpose of this requirement is to:
- Reduce the number of calibrations needed per year
- Ensure measurement results are comparable and compatible worldwide (Correct answer)
- Eliminate the need for uncertainty calculations
- Allow laboratories to set their own tolerance limits
Correct answer: Ensure measurement results are comparable and compatible worldwide
Traceability provides an unbroken chain of comparisons to national or international standards, ensuring measurements are universally meaningful and comparable.
Question 3: The guard band technique in calibration decision-making is used to:
- Extend calibration intervals without additional testing
- Account for measurement uncertainty when making pass/fail decisions near tolerance limits (Correct answer)
- Calibrate instruments at a single mid-range point only
- Replace the need for a calibration certificate
Correct answer: Account for measurement uncertainty when making pass/fail decisions near tolerance limits
Guard banding narrows the acceptance zone by the measurement uncertainty, reducing the probability of falsely accepting an out-of-tolerance instrument.
Question 4: When calibrating a digital multimeter's AC voltage function, the reference standard's frequency accuracy matters because:
- The multimeter's internal ADC samples at a fixed clock rate
- Many DMMs have frequency-dependent AC response that affects amplitude accuracy (Correct answer)
- AC voltage is measured via charge integration which requires precise timing
- The multimeter converts AC to DC using a frequency counter
Correct answer: Many DMMs have frequency-dependent AC response that affects amplitude accuracy
AC voltage accuracy in DMMs is specified over a frequency range, and errors can increase outside that range due to bandwidth limitations and RMS conversion circuitry characteristics.
Question 5: A calibration procedure calls for a 30-minute warm-up period before taking readings. Skipping this step most directly risks errors caused by:
- Hysteresis from mechanical wear
- Thermal drift as internal components stabilize to operating temperature (Correct answer)
- Electromagnetic interference during initial power-up
- Ground loop currents from cold solder joints
Correct answer: Thermal drift as internal components stabilize to operating temperature
Electronic components change their characteristics as they warm up; specifications are valid only after thermal equilibrium is reached, so skipping warm-up introduces thermally-induced drift errors.
Question 6: When calibrating a load cell, the technician applies dead-weight loads and records outputs. A 'creep' test involves:
- Rapidly cycling between zero and full load 10 times
- Holding a constant load and observing any change in output over time (Correct answer)
- Applying loads at an angle to test off-axis sensitivity
- Reversing the polarity of the excitation voltage
Correct answer: Holding a constant load and observing any change in output over time
Creep is the slow change in output signal under a constant applied load over time, and a creep test documents this characteristic by monitoring output drift at a fixed input.
Question 7: In liquid-in-glass thermometer calibration, the 'emergent stem correction' is needed when:
- The thermometer is used in a horizontal position
- Part of the liquid column extends above the calibration bath and is at a different temperature than the bath (Correct answer)
- The thermometer scale has unequal divisions
- The reference thermometer differs in diameter from the unit under test
Correct answer: Part of the liquid column extends above the calibration bath and is at a different temperature than the bath
When part of the thermometer stem is outside the calibration medium, that portion of liquid is at ambient temperature rather than bath temperature, introducing a predictable but significant error requiring correction.
CCT Calibration Techniques & Procedures 3
A calibration technician notices that repeated measurements of the same input produce results that cluster tightly but are consistently offset from the true value.
This describes: