CWT CWT Troubleshooting & Maintenance 2 — Questions and Answers
Question 1: What is the purpose of a 'repeatability test' performed on a weighing device?
- To verify that the scale gives consistent readings when the same load is applied multiple times under the same conditions (Correct answer)
- To check that the scale returns to zero after each weighing
- To confirm the scale handles its maximum rated load safely
- To test the scale's performance under temperature extremes
Correct answer: To verify that the scale gives consistent readings when the same load is applied multiple times under the same conditions
A repeatability test involves placing the same test weight on the scale multiple times and comparing the readings to ensure the device produces consistent, reproducible results.
Question 2: A crane scale used in industrial weighing shows drift — its reading slowly increases over several minutes even though the suspended load has not changed. What is the most probable cause?
- Thermal effects causing the load cell to drift as it warms up (Correct answer)
- The load is slowly swinging and adding dynamic force
- The indicator's battery is running low
- The scale has exceeded its rated capacity
Correct answer: Thermal effects causing the load cell to drift as it warms up
Thermal drift occurs when a load cell's output changes as its temperature stabilizes after being exposed to a sudden temperature change or after a load is first applied.
Question 3: Which of the following best describes 'creep' as a load cell specification parameter?
- The change in load cell output occurring over time under a constant load (Correct answer)
- The maximum rated capacity of the load cell
- The minimum detectable load the cell can sense
- The cell's sensitivity expressed in millivolts per volt
Correct answer: The change in load cell output occurring over time under a constant load
Creep is the change in a load cell's output signal that occurs over time while a constant load remains applied, and excessive creep can cause weighing results to slowly change during a transaction.
Question 4: A technician is troubleshooting a scale that works correctly in the morning but reads high by afternoon. What environmental factor should be investigated?
- Temperature rise causing thermal expansion or load cell drift (Correct answer)
- Increased foot traffic near the scale
- Afternoon power grid voltage fluctuations
- Changes in humidity affecting the display contrast
Correct answer: Temperature rise causing thermal expansion or load cell drift
Systematic errors that worsen as the day warms up are classic signs of temperature-related effects on load cells, electronics, or the mechanical structure of the scale.
Question 5: When should a CWT recommend replacing a load cell rather than attempting to recalibrate the scale?
- When the load cell shows physical damage, corrosion, or electrical leakage that cannot be corrected by adjustment (Correct answer)
- When the scale fails a single shift test
- When the calibration interval has been exceeded by more than one week
- When the indicator display shows occasional flicker
Correct answer: When the load cell shows physical damage, corrosion, or electrical leakage that cannot be corrected by adjustment
Physical damage, moisture ingress, severe corrosion, or insulation resistance failures in a load cell indicate a hardware defect that cannot be resolved by electronic recalibration.
Question 6: What does a low insulation resistance reading between a load cell's signal wires and its cable shield typically indicate?
- Moisture or contamination has compromised the load cell or cable, causing signal errors (Correct answer)
- The load cell needs a higher excitation voltage
- The indicator's input impedance is too low
- The load cell is operating at maximum capacity
Correct answer: Moisture or contamination has compromised the load cell or cable, causing signal errors
Low insulation resistance between signal conductors and the cable shield indicates that moisture or contamination has entered the load cell or cable, which can cause erratic, noisy, or offset readings.
CWT CWT Troubleshooting & Maintenance 2
What is the purpose of a 'repeatability test' performed on a weighing device?