CST Repeatability & Linearity Testing 2 — Questions and Answers
Question 1: When performing a repeatability test, the test load should be applied and removed how many times according to NIST HB 44?
- 3 times minimum
- 5 times minimum (Correct answer)
- 7 times minimum
- 10 times minimum
Correct answer: 5 times minimum
NIST HB 44 requires a minimum of 5 repeated load applications to properly evaluate repeatability error.
Question 2: A scale tested for linearity shows increasing error as load increases. This pattern is best described as:
- Hysteresis error
- Span error
- Progressive nonlinearity (Correct answer)
- Zero drift
Correct answer: Progressive nonlinearity
Progressive nonlinearity describes errors that systematically increase or change in magnitude across the measurement range.
Question 3: Which of the following best defines the 'best straight line' method used in linearity analysis?
- A line drawn through zero and full-scale readings only
- The line that minimizes the maximum deviation of any data point (Correct answer)
- A line connecting the first and last calibration points
- The line produced by averaging all scale readings
Correct answer: The line that minimizes the maximum deviation of any data point
The best straight line minimizes the maximum deviation (peak error) between actual scale output and the ideal reference line.
Question 4: During a linearity test on a 500 lb capacity scale, readings at 100, 200, 300, and 400 lb are all within tolerance, but the 250 lb reading is out of tolerance. This indicates:
- A zero offset error
- A mid-range nonlinearity (Correct answer)
- A span calibration error
- A dead-load drift problem
Correct answer: A mid-range nonlinearity
An out-of-tolerance reading at a mid-range point while endpoints are correct indicates a mid-range nonlinearity in the load cell or mechanism.
Question 5: Repeatability testing on a conveyor belt scale should be performed at what percentage of the scale's maximum capacity?
- 25% of max capacity
- At least 50% of max capacity (Correct answer)
- Exactly 100% of max capacity
- Between 10% and 20% of max capacity
Correct answer: At least 50% of max capacity
Conveyor belt scale repeatability tests are typically conducted at loads of at least 50% of maximum capacity to produce meaningful results.
Question 6: A technician finds that a scale's repeatability error exceeds the tolerance. The FIRST corrective step should be to:
- Replace the load cell immediately
- Check for mechanical interference or binding (Correct answer)
- Recalibrate the span
- Replace the indicator
Correct answer: Check for mechanical interference or binding
Mechanical interference or binding is the most common cause of poor repeatability and should be investigated before replacing components.
Question 7: For NTEP-certified scales, linearity tolerance is typically expressed as a multiple of:
- The scale division (d)
- The verification scale division (e) (Correct answer)
- The maximum capacity (Max)
- The minimum capacity (Min)
Correct answer: The verification scale division (e)
NTEP linearity tolerances are expressed in terms of the verification scale division (e), which may equal or be larger than d.
CST Repeatability & Linearity Testing 2
When performing a repeatability test, the test load should be applied and removed how many times according to NIST HB 44?