CST Troubleshooting Techniques & Repair Methods 3 — Questions and Answers
Question 1: When using a summing card (junction box) to diagnose individual load cells, what measurement confirms a shorted load cell?
- Higher than specified bridge resistance
- Lower than specified bridge resistance (Correct answer)
- Correct mV/V output at rated load
- Normal insulation resistance
Correct answer: Lower than specified bridge resistance
A shorted load cell will exhibit lower than specified bridge resistance because additional current paths reduce the effective resistance.
Question 2: A platform scale indicator shows 'ERR-2' and will not accept a zero command. According to typical manufacturer conventions, what does this type of error most likely indicate?
- A/D converter failure
- Weight is outside the acceptable zero range (Correct answer)
- Communication port fault
- Low battery voltage
Correct answer: Weight is outside the acceptable zero range
Most scale indicators use error codes like 'ERR-2' or similar to indicate that the current weight is outside the permissible zero band for the zero command.
Question 3: A scale technician notices that a scale reads correctly in the morning but reads high by several graduations in the afternoon. What phenomenon is most likely responsible?
- Electromagnetic interference from nearby equipment
- Thermal expansion of the platform structure causing mechanical stress on load cells (Correct answer)
- Power supply fluctuations
- Operator error in reading the display
Correct answer: Thermal expansion of the platform structure causing mechanical stress on load cells
Thermal expansion of the platform during the warmer part of the day can induce mechanical stress on load cells, causing artificially elevated readings.
Question 4: When replacing a sealed load cell, a technician must apply sealant around the cable entry point. What is the primary purpose of this step?
- To increase the load cell's rated capacity
- To prevent moisture from wicking into the load cell along the cable (Correct answer)
- To electrically insulate the cable from the mounting hardware
- To dampen vibration transmitted through the cable
Correct answer: To prevent moisture from wicking into the load cell along the cable
Sealing the cable entry prevents moisture from entering the load cell via capillary action along the cable, which would degrade insulation resistance and cause signal errors.
Question 5: A retail price-computing scale fails its shift test (constant load test). The reading drifts more than the permissible tolerance over 30 minutes. Which component is most likely to blame?
- The display backlight
- A load cell exhibiting excessive creep (Correct answer)
- The printer mechanism
- The keyboard membrane
Correct answer: A load cell exhibiting excessive creep
The shift test measures reading stability over time under a constant load; excessive creep in a load cell causes the output to slowly drift and fail this test.
Question 6: A technician finds that a scale's output increases when the load cell cable is flexed near the junction box. What does this symptom indicate?
- Normal cable capacitance effect
- An intermittent open or high-resistance connection in the cable (Correct answer)
- Correct load cell function
- Junction box ground fault
Correct answer: An intermittent open or high-resistance connection in the cable
An intermittent open or high-resistance connection causes the signal to change when the cable is physically disturbed, confirming a wiring fault at or near that location.
Question 7: A crane scale consistently under-reads by a fixed amount regardless of the load applied. Zero is correct with no load. What is the most probable cause?
- Load cell sensitivity mismatch
- Incorrect deadweight or tare configuration in the indicator (Correct answer)
- Span calibration error
- Overload damage to the load cell
Correct answer: Incorrect deadweight or tare configuration in the indicator
A fixed absolute under-read at all loads while zero is correct indicates an incorrectly programmed tare or deadweight value subtracting from the gross reading.
CST Troubleshooting Techniques & Repair Methods 3
When using a summing card (junction box) to diagnose individual load cells, what measurement confirms a shorted load cell?