CWT Weighing Equipment Operation & Calibration 5 — Questions and Answers
Question 1: What is 'hysteresis' in the context of load cell performance?
- The time delay between applying a load and the display stabilizing
- The difference in output between loading and unloading at the same applied load (Correct answer)
- The long-term drift of a load cell over months of use
- The sensitivity of a load cell to side loads
Correct answer: The difference in output between loading and unloading at the same applied load
Hysteresis is the phenomenon where a load cell gives slightly different outputs for the same applied load depending on whether the load is increasing or decreasing.
Question 2: According to NIST Handbook 44, what is the maximum allowable tolerance for a Class III commercial scale (e.g., a retail grocery scale) at 500 scale divisions?
- 0.5 division
- 1 division (Correct answer)
- 1.5 divisions
- 2 divisions
Correct answer: 1 division
For Class III devices, NIST HB 44 specifies a maintenance tolerance of ±1 division (d) for loads up to 500d.
Question 3: A moisture analyzer uses an internal balance mechanism combined with a drying heater. To ensure weighing accuracy, the instrument should be:
- Operated only at full capacity for each sample
- Leveled and allowed to warm up per manufacturer specifications before use (Correct answer)
- Calibrated only with wet samples
- Used without a tare to avoid interference
Correct answer: Leveled and allowed to warm up per manufacturer specifications before use
Moisture analyzers contain sensitive internal balances that require proper leveling and thermal stabilization before accurate measurements can be made.
Question 4: A forklift scale (on-board weighing system) shows erratic readings when the forklift engine is running. The most likely cause is:
- Overloaded hydraulic system
- Electromagnetic interference or vibration from the engine affecting load cell signals (Correct answer)
- Incorrect tare programmed in the indicator
- Display too far from the load cell junction box
Correct answer: Electromagnetic interference or vibration from the engine affecting load cell signals
Engine vibration and EMI from ignition/alternator systems are common sources of signal noise in on-board vehicle weighing applications.
Question 5: Which of the following correctly describes the relationship between 'accuracy class' and 'number of scale intervals (n)' for a weighing instrument?
- Higher accuracy class instruments have fewer allowable scale intervals
- Higher accuracy class instruments are permitted more scale intervals (finer resolution) relative to capacity (Correct answer)
- The number of scale intervals has no relationship to accuracy class
- Accuracy class only applies to test weights, not weighing instruments
Correct answer: Higher accuracy class instruments are permitted more scale intervals (finer resolution) relative to capacity
OIML and NIST classify weighing instruments such that higher accuracy classes (I, II) are permitted more scale intervals than lower classes (III, IIII), reflecting their superior resolution capability.
Question 6: When a CWT technician seals a legally approved scale after calibration, the seal primarily serves to:
- Protect the scale from rain and moisture
- Prevent unauthorized adjustment of the metrological parameters (Correct answer)
- Increase the scale's maximum legal capacity
- Identify the manufacturer of the instrument
Correct answer: Prevent unauthorized adjustment of the metrological parameters
Official seals prevent tampering with calibration adjustments and provide evidence of unauthorized access, protecting the integrity of legal-for-trade measurements.
Question 7: During a calibration of a belt conveyor scale, the technician should apply test loads using:
- Only static weights placed directly on the belt
- A simulated load or a weigh-belt test using certified material or chain weights traveling at operating speed (Correct answer)
- Handheld weights lowered onto the belt from above
- Water filled bags that conform to the belt shape
Correct answer: A simulated load or a weigh-belt test using certified material or chain weights traveling at operating speed
Belt conveyor scales must be tested dynamically — with material or calibrated chain weights moving at operational belt speed — because their accuracy depends on motion-related factors.
CWT Weighing Equipment Operation & Calibration 5
What is 'hysteresis' in the context of load cell performance?