Weighing System Installation & Verification Flashcards

In a multi-load cell weighing system, what is the primary function of the summing board (junction box)?

Answer: To combine and trim individual load cell outputs into a single balanced signal

The summing board parallels the individual load cell signal wires and provides trim potentiometers to equalize each cell's contribution so the combined output accurately represents the total load.

What does 'return to zero' (RTZ) performance testing verify in a weighing system?

Answer: That the indicator reads zero (within the allowable tolerance) after a test load is removed

RTZ testing applies a test load, records the reading, removes the load, and confirms the indicator returns to within the zero tolerance band, verifying no residual signal offset.

What is 'creep' in the context of load cell performance specification?

Answer: A change in load cell output signal that occurs over time while a constant load remains applied

Creep is a time-dependent drift in the load cell's output under sustained constant load, specified as a percentage of full scale over a defined period (typically 30 minutes).

Which grounding practice is most important when installing an electronic weighing system to prevent measurement errors?

Answer: Connect all metallic structural members, load cell housings, cables, and the indicator to a single common earth ground point

A single-point common earth ground eliminates ground loops—differences in ground potential between components that inject spurious voltages into the millivolt-level load cell signal.

What is the purpose of trim potentiometers installed in a load cell summing network?

Answer: To balance and equalize individual load cell signal outputs so each cell contributes equally to the total reading

No two load cells have identical sensitivity; trim pots allow the technician to adjust each cell's contribution to the sum so the system reads accurately regardless of where the load is applied.

When verifying a belt conveyor (in-motion) scale, which test directly confirms the system's totalization accuracy?

Answer: Material test — feeding a precisely weighed reference quantity of material through the system and comparing to the totalizer

A material test (also called a reference material or bulk weigh test) is the definitive check: a known mass of material passes through the scale and the totalizer result is compared to the certified reference mass.

What is 'hysteresis error' in a weighing system or load cell?

Answer: The difference in output readings obtained when the same load is approached from increasing load versus decreasing load

Hysteresis occurs because the elastic elements in a load cell do not return along the exact same stress-strain path during unloading as they followed during loading, producing slightly different outputs for the same load.