CST Analog & Digital Scale Technology 2 — Questions and Answers
Question 1: Which type of load cell output signal is most commonly used in modern digital scales?
- 4–20 mA current loop
- Millivolt per volt (mV/V) (Correct answer)
- 0–10 V DC voltage
- Frequency modulated pulse
Correct answer: Millivolt per volt (mV/V)
Strain gauge load cells produce a millivolt-per-volt (mV/V) output proportional to applied load, which is the standard in digital weighing systems.
Question 2: An analog pointer scale exhibits a consistent 2% over-reading across its entire range. This is most likely caused by:
- Worn pivot bearings
- Incorrect span calibration (Correct answer)
- Excessive friction in the knife-edge
- A bent indicator needle
Correct answer: Incorrect span calibration
A uniform percentage error across the full range indicates a span (gain) calibration error rather than a zero or mechanical fault.
Question 3: In a digital indicator, what does the term 'resolution' refer to?
- Maximum load capacity of the scale
- Smallest increment the display can show (Correct answer)
- Time required to stabilize a reading
- Accuracy class of the weighing instrument
Correct answer: Smallest increment the display can show
Resolution is the smallest change in load that the indicator can detect and display, expressed as the number of display divisions.
Question 4: Which wiring configuration is used to compensate for resistance changes due to lead wire temperature fluctuations in remote load cell installations?
- 2-wire connection
- 3-wire connection
- 4-wire sense connection
- 6-wire sense connection (Correct answer)
Correct answer: 6-wire sense connection
A 6-wire connection uses separate sense lines that allow the indicator to measure and compensate for voltage drop caused by lead wire resistance changes.
Question 5: What is the purpose of a 'shunt calibration' resistor in a load cell system?
- Protects the load cell from overvoltage
- Simulates a known load by shifting bridge output a fixed amount (Correct answer)
- Filters high-frequency electrical noise
- Balances multiple load cells in parallel
Correct answer: Simulates a known load by shifting bridge output a fixed amount
A shunt calibration resistor is placed across one bridge arm to produce a predictable mV/V shift, allowing electronic span verification without physical weights.
Question 6: A digital bench scale displays an unstable, flickering reading even with no load on the platform. The most likely cause is:
- A/D converter failure
- Electrical interference or ground loop (Correct answer)
- Overloaded load cell
- Incorrect number of display divisions programmed
Correct answer: Electrical interference or ground loop
Unstable no-load readings on a digital scale are typically caused by electrical noise or ground loops entering the signal path.
Question 7: What is meant by 'creep' in a strain gauge load cell?
- Gradual mechanical wear of the flexure element
- Slow drift of the output signal under constant applied load over time (Correct answer)
- Temperature-induced zero shift
- Hysteresis between loading and unloading cycles
Correct answer: Slow drift of the output signal under constant applied load over time
Creep is the change in load cell output that occurs over time while a constant load remains applied, caused by stress relaxation in the elastic element.
CST Analog & Digital Scale Technology 2
Which type of load cell output signal is most commonly used in modern digital scales?