CWT Weighing Equipment Operation & Calibration 2 — Questions and Answers
Question 1: What is the purpose of a 'span calibration' on a weighing instrument?
- To set the zero reference point
- To adjust the sensitivity so the instrument reads correctly at a known non-zero load (Correct answer)
- To measure the total capacity of the scale
- To determine the resolution of the display
Correct answer: To adjust the sensitivity so the instrument reads correctly at a known non-zero load
Span calibration adjusts the instrument's sensitivity/gain so that it accurately reads a known test weight applied above zero.
Question 2: Which environmental condition most commonly causes 'thermal drift' errors in load cell-based scales?
- High humidity
- Vibration from nearby machinery
- Temperature changes affecting load cell output (Correct answer)
- Electromagnetic interference from power lines
Correct answer: Temperature changes affecting load cell output
Temperature changes alter the electrical resistance and mechanical properties of load cells, causing their output to drift from the calibrated value.
Question 3: A technician places test weights on a scale and finds the reading is 0.5 lb low at mid-span but accurate at full capacity. This is most likely caused by:
- A faulty display unit
- Non-linearity or a linearity calibration error (Correct answer)
- An incorrect zero setting
- Overloading the scale
Correct answer: Non-linearity or a linearity calibration error
A reading that is off at one point but correct at another indicates a linearity error, meaning the instrument's response is not proportional across its range.
Question 4: What is the recommended practice before performing a calibration on a vehicle scale in cold weather?
- Calibrate immediately to capture the cold baseline
- Allow the scale to stabilize at ambient temperature for at least one hour (Correct answer)
- Reduce the test weight load by 10% to account for temperature
- Spray the load cells with antifreeze
Correct answer: Allow the scale to stabilize at ambient temperature for at least one hour
Allowing the scale to stabilize ensures that thermal gradients within the structure have equalized, providing a stable baseline for accurate calibration.
Question 5: Under NIST Handbook 44, what is the term for the difference between the actual value applied to a weighing device and the indicated value?
- Uncertainty
- Error (Correct answer)
- Tolerance
- Deviation
Correct answer: Error
Error is defined as the indicated value minus the actual value (or vice versa depending on convention) and is the fundamental measurement of instrument accuracy.
Question 6: When calibrating a floor scale using substitution weighing, what is the main advantage over direct weighing?
- Substitution weighing requires fewer test weights to verify full capacity (Correct answer)
- It eliminates the need for an indicator
- It avoids the need to zero the scale between tests
- It is only applicable to platform scales
Correct answer: Substitution weighing requires fewer test weights to verify full capacity
Substitution weighing uses a small set of reference weights plus a known substitute load, reducing the total number of calibrated test weights needed to verify full capacity.
Question 7: A digital weighing indicator displays 'OVERLOAD' even though the applied load is within the scale's rated capacity. The most likely cause is:
- The load cell output exceeds the indicator's input range due to mismatched sensitivity (Correct answer)
- The display font size is too large
- The power supply voltage is too high
- The platform is too clean
Correct answer: The load cell output exceeds the indicator's input range due to mismatched sensitivity
If the load cell's mV/V output does not match the indicator's expected input range, the indicator may interpret normal loads as overloads.
CWT Weighing Equipment Operation & Calibration 2
What is the purpose of a 'span calibration' on a weighing instrument?