CWT Measurement Standards & Accuracy 4 — Questions and Answers
Question 1: What is 'buoyancy correction' in precision weighing?
- Adjusting for the upward force of air on the weight and sample (Correct answer)
- Correcting for the weight of the scale platform
- Adjusting readings for humidity effects on the load cell
- Compensating for vibration from nearby machinery
Correct answer: Adjusting for the upward force of air on the weight and sample
Buoyancy correction accounts for the upward force of air displaced by the weight and the object being weighed, significant in high-precision applications.
Question 2: A technician notices that a scale's zero indicator drifts slowly upward over 30 minutes with no load. This is most likely caused by:
- An object resting on the platform
- Thermal warm-up of the electronic components (Correct answer)
- An incorrect span calibration setting
- Exceeding the scale's maximum capacity previously
Correct answer: Thermal warm-up of the electronic components
Electronic components, especially load cells and amplifiers, require a warm-up period; zero drift during this time is a common and expected phenomenon.
Question 3: Under NIST Handbook 44, a 'division' (d) and a 'verification scale interval' (e) may differ. What does a larger 'e' value relative to 'd' indicate?
- The scale has enhanced display resolution beyond its verified accuracy (Correct answer)
- The scale is improperly calibrated
- The scale lacks a digital display
- The scale is operating at maximum capacity
Correct answer: The scale has enhanced display resolution beyond its verified accuracy
When e > d, the display shows finer increments than the verified accuracy class requires, meaning displayed resolution exceeds verified accuracy.
Question 4: Which test specifically verifies that a scale's reading does not change when the load is shifted from one side of the platform to another?
- Sensitivity test
- Shift or eccentric load test (Correct answer)
- Repeatability test
- Tare test
Correct answer: Shift or eccentric load test
The shift or eccentric load test places the same weight at different positions on the platform to check for position-dependent reading errors.
Question 5: What is the primary purpose of NCWM's Publication 14?
- Defining ASTM weight classes
- Providing guidelines for engine-block weighing
- Establishing requirements for net-content labeling in commercial transactions (Correct answer)
- Setting calibration intervals for laboratory balances
Correct answer: Establishing requirements for net-content labeling in commercial transactions
NCWM Publication 14 provides guidelines and test procedures for checking net contents of packaged goods in commerce.
Question 6: A CWT candidate needs to calibrate a 500 lb capacity floor scale. Which ASTM weight class is most appropriate for the test weights?
- Class 1
- Class 4
- Class 6 (Correct answer)
- Class 7
Correct answer: Class 6
ASTM Class 6 weights are commonly specified for field use with commercial scales, balancing accuracy needs with practicality for field conditions.
Question 7: When a scale's reading fails to return to zero after a load is removed, this is called:
- Hysteresis error (Correct answer)
- Span error
- Repeatability error
- Resolution error
Correct answer: Hysteresis error
Hysteresis error occurs when the scale's reading path differs between increasing and decreasing loads, often manifesting as a non-zero reading after load removal.
CWT Measurement Standards & Accuracy 4
What is 'buoyancy correction' in precision weighing?