CST Troubleshooting Techniques & Repair Methods 2 ā Questions and Answers
Question 1: A digital scale displays a stable reading but consistently shows values 2% above the certified value across its entire range. What is the most likely cause?
- Faulty A/D converter
- Incorrect span calibration (Correct answer)
- Worn load cell bearings
- Electromagnetic interference
Correct answer: Incorrect span calibration
A consistent percentage error across the entire range indicates an incorrect span calibration rather than a hardware fault.
Question 2: When troubleshooting a scale that returns to zero correctly but gives erratic readings under load, what should be checked first?
- Power supply voltage
- Load cell excitation voltage
- Mechanical binding or restriction of the platform (Correct answer)
- Indicator display board
Correct answer: Mechanical binding or restriction of the platform
Erratic readings under load that disappear at zero typically indicate a mechanical issue such as binding, side loading, or debris under the platform.
Question 3: A technician measures the mV/V output of a load cell and finds it is within specification, but the indicator still reads incorrectly. What is the next logical diagnostic step?
- Replace the load cell
- Check the indicator's internal calibration and input range settings (Correct answer)
- Verify the scale's legal-for-trade stamp
- Inspect the junction box wiring
Correct answer: Check the indicator's internal calibration and input range settings
If the load cell signal is correct, the fault lies in the indicator's processing ā checking its calibration and input range settings is the next step.
Question 4: A load cell's insulation resistance between the cable shield and signal wires measures 50 MĪ© when it should exceed 1000 MĪ©. What condition does this indicate?
- Normal aging of the cable
- Moisture intrusion or cable damage (Correct answer)
- Overloaded load cell
- Incorrect bridge voltage
Correct answer: Moisture intrusion or cable damage
Low insulation resistance between shield and signal wires indicates moisture intrusion or cable damage, which can cause unstable or drifting readings.
Question 5: A vehicle scale gives a correct reading for the first axle but a significantly different total when both axles are on. Which component is most suspect?
- One or more load cells contributing to only part of the platform (Correct answer)
- The indicator's power supply
- The printer interface
- The calibration weights
Correct answer: One or more load cells contributing to only part of the platform
When a partial-platform reading is correct but the full-platform reading is not, a load cell covering the unloaded section is likely faulty or disconnected.
Question 6: During a zero-return test, a scale repeatedly drifts positive by 0.5 divisions after each weighing cycle. What is the most probable cause?
- Temperature drift in the indicator
- Material buildup under the platform (Correct answer)
- Incorrect span calibration
- Load cell creep
Correct answer: Material buildup under the platform
Progressive positive zero drift after each cycle indicates material accumulating under the platform and adding tare load.
Question 7: A technician replaces a load cell in a multi-cell floor scale. After replacement, the summed output is correct at full capacity but the cornerload test fails. What must be performed?
- Replace all remaining load cells
- Perform a cornerload or section adjustment (Correct answer)
- Recalibrate the span only
- Check the junction box fuses
Correct answer: Perform a cornerload or section adjustment
After replacing a single load cell, a cornerload or section adjustment must be performed to rebalance the contributions of each cell to the total output.
CST Troubleshooting Techniques & Repair Methods 2
A digital scale displays a stable reading but consistently shows values 2% above the certified value across its entire range.
What is the most likely cause?