CST Vehicle Scale Systems & Applications 3

Question 1

What is the purpose of trim (summing) resistors in a vehicle scale junction box?

Accepted Answer

To balance individual load cell outputs so each cell contributes equally to the combined signal

Answer

Trim resistors adjust the sensitivity of individual load cell channels in the summing network, compensating for minor cell-to-cell sensitivity differences so each cell contributes an equal share to the total output.

Question 2

What minimum insulation resistance should be measured between load cell signal wiring and the scale structure (ground) to ensure proper operation?

Accepted Answer

1,000,000 ohms (1 MΩ) or higher

Answer

Load cell cables should exhibit insulation resistance of at least 1 MΩ between conductors and ground; lower resistance indicates moisture intrusion or insulation damage that introduces leakage currents and weighing errors.

Question 3

When performing an 'as-found' test on a vehicle scale, what information does the technician document?

Accepted Answer

The condition and accuracy of the scale before any adjustments or calibration are performed

Answer

An 'as-found' test records the scale's condition and accuracy before any work is performed, establishing a baseline that is compared to the 'as-left' results after calibration.

Question 4

Why is maintaining the correct excitation voltage critical in a multi-load-cell vehicle scale system?

Accepted Answer

Load cells are ratiometric devices whose output depends on excitation voltage, so incorrect voltage causes proportional weight errors

Answer

Strain gauge load cells are ratiometric — their output signal is proportional to both the applied load and the excitation voltage, so a deviation from the rated excitation voltage introduces a corresponding proportional error in the weight reading.

Question 5

What physical condition is most likely indicated when a vehicle scale shows a negative weight reading with no load on the deck?

Accepted Answer

Debris, frost heave, or an obstruction beneath the deck is lifting it and relieving the load cells

Answer

A below-zero reading with an empty deck usually means an upward force is being applied to the underside of the deck — commonly debris in the pit, frost heave under the foundation, or structural binding.

(CST) Certified Scale Technicians Practice Test

(CST) Certified Scale Technicians Practice Test

CST Vehicle Scale Systems & Applications 3