CST PM & Calibration Schedules 2 — Questions and Answers
Question 1: Under NIST Handbook 44, what is the maximum permissible error (MPE) tolerance class typically applied during a periodic calibration of a Class III commercial scale?
- ±0.5 scale division
- ±1 scale division (Correct answer)
- ±2 scale divisions
- ±3 scale divisions
Correct answer: ±1 scale division
NIST HB 44 specifies that Class III devices must be within ±1 scale division at the time of inspection or calibration.
Question 2: A PM checklist for a floor scale specifies checking 'zero return after load removal.' What condition would indicate a failed zero return?
- Display shows exactly 0.0 after removing the test weight
- Display settles to a non-zero value beyond the zero-return tolerance (Correct answer)
- Display flickers momentarily before settling to zero
- Display shows a negative value then self-corrects to zero
Correct answer: Display settles to a non-zero value beyond the zero-return tolerance
A failed zero return occurs when the indication does not return to within the accepted zero tolerance after a load is removed.
Question 3: Which calibration interval is most commonly recommended for a checkweigher used in a high-speed food packaging line operating 24/7?
- Annually
- Semi-annually
- Monthly or per shift (Correct answer)
- Every 5 years
Correct answer: Monthly or per shift
High-throughput checkweighers in continuous operation should be verified each shift or at minimum monthly to catch drift caused by vibration and wear.
Question 4: During a scheduled PM on a load cell–based tank scale, a technician notices the cable jacket is cracked near the junction box. What is the correct action?
- Wrap with electrical tape and continue service
- Log the finding and replace the cable before returning to service (Correct answer)
- Increase calibration frequency to compensate
- Degrease the crack and seal with silicone
Correct answer: Log the finding and replace the cable before returning to service
A cracked cable jacket exposes signal wires to moisture and EMI, which can corrupt readings; the cable must be replaced before the scale is returned to service.
Question 5: What is the primary purpose of performing an 'as-found' calibration check before making any adjustments during a PM visit?
- To bill the customer for extra time
- To establish a baseline and document the scale's condition before adjustment (Correct answer)
- To warm up the indicator electronics
- To satisfy the requirement to use all test weights
Correct answer: To establish a baseline and document the scale's condition before adjustment
An as-found check documents how the scale was performing before the technician intervened, providing traceability and evidence of drift.
Question 6: A calibration certificate for test weights used in a PM must include which of the following to maintain traceability?
- The technician's personal opinion of weight accuracy
- A reference to national or international measurement standards (Correct answer)
- The color of the weight set
- The weight manufacturer's marketing name
Correct answer: A reference to national or international measurement standards
Traceability requires that calibration certificates reference an unbroken chain back to national or international standards such as NIST.
Question 7: An indicator's internal reference calibration (span calibration) is adjusted using certified test weights. After adjustment, the span error at full capacity is –0.3 divisions. What should the technician do?
- Accept the result if it is within MPE for the device class (Correct answer)
- Re-adjust until error is exactly 0.0 divisions
- Replace the load cell immediately
- Reduce the scale's capacity rating by 10%
Correct answer: Accept the result if it is within MPE for the device class
A span error within the maximum permissible error for the applicable device class is acceptable and does not require further adjustment.
CST PM & Calibration Schedules 2
Under NIST Handbook 44, what is the maximum permissible error (MPE) tolerance class typically applied during a periodic calibration of a Class III commercial scale?