CST Scale Installation & Calibration Procedures 3 — Questions and Answers
Question 1: A technician is calibrating a hopper scale using the substitution method. What does this method involve?
- Using certified test weights to establish span, then replacing them with bulk material of known weight (Correct answer)
- Substituting a new indicator for the old one without recalibrating load cells
- Running the scale with no hopper to establish a baseline
- Replacing all load cells simultaneously to avoid mismatch errors
Correct answer: Using certified test weights to establish span, then replacing them with bulk material of known weight
Substitution calibration uses certified weights to set span, then replaces them with material so the scale can be used for self-calibration checks against material weight.
Question 2: According to OIML R 76, what is the maximum permissible error (MPE) at initial verification for a Class III scale at 500 e?
- ±1.0 e (Correct answer)
- ±0.5 e
- ±1.5 e
- ±2.0 e
Correct answer: ±1.0 e
For Class III scales between 50 e and 2000 e, the MPE at initial verification is ±1.0 e.
Question 3: A weigh-in-motion (WIM) system requires site calibration after installation. Which vehicle parameter must be controlled during the calibration pass?
- Vehicle speed must be within the specified calibration speed range (Correct answer)
- Vehicle fuel tank must be full
- Vehicle tires must be inflated to exactly 80 PSI
- Vehicle engine must be idling, not under load
Correct answer: Vehicle speed must be within the specified calibration speed range
WIM systems are speed-sensitive because dynamic load factors change with velocity; calibration passes must be made within the system's designated speed window.
Question 4: When leveling a bench scale using adjustable feet, which instrument is preferred to verify level?
- A precision spirit level or electronic inclinometer accurate to 0.1° or better (Correct answer)
- A carpenter's bubble level
- A laser level pointed at the wall
- Visual inspection of the indicator housing
Correct answer: A precision spirit level or electronic inclinometer accurate to 0.1° or better
Precision spirit levels or electronic inclinometers provide the accuracy needed to ensure the scale platform is within the manufacturer's tilt specification, typically ±0.5°.
Question 5: During installation of a tank weighing system, 'live load' isolation is achieved by:
- Using flexible conduit, hoses, and piping connections that do not transmit force to the tank (Correct answer)
- Grounding all metal piping to the tank frame
- Mounting the indicator directly on the tank wall
- Filling the tank to 50% capacity before zeroing
Correct answer: Using flexible conduit, hoses, and piping connections that do not transmit force to the tank
Rigid connections to a weighed vessel introduce parasitic forces that corrupt the weight reading; flexible connections isolate the tank so only its true weight is measured.
Question 6: What is the function of 'check weights' (also called 'shift weights') during a floor scale calibration?
- To verify repeatability by placing and removing a known load multiple times (Correct answer)
- To add extra load beyond rated capacity to test overload protection
- To calibrate the scale at exactly mid-range only
- To adjust the linearity correction in the indicator
Correct answer: To verify repeatability by placing and removing a known load multiple times
Repeatability tests using shift weights confirm that the scale returns consistent readings for the same load applied multiple times.
Question 7: A 10,000 lb capacity scale uses 4 load cells rated at 5,000 lb each. What is the system's combined rated output if each cell is 3 mV/V?
- 3 mV/V (the system output equals one cell's output because cells are in parallel) (Correct answer)
- 12 mV/V
- 1.5 mV/V
- 6 mV/V
Correct answer: 3 mV/V (the system output equals one cell's output because cells are in parallel)
Load cells wired in parallel present the same mV/V output to the indicator as a single cell because voltage is common across parallel branches.
CST Scale Installation & Calibration Procedures 3
A technician is calibrating a hopper scale using the substitution method.
What does this method involve?