CST Strain Gauge & Piezoelectric Sensors 2 — Questions and Answers
Question 1: What is the gauge factor (GF) of a typical metallic foil strain gauge?
- Approximately 0.5
- Approximately 2 (Correct answer)
- Approximately 50
- Approximately 200
Correct answer: Approximately 2
Metallic foil strain gauges have a gauge factor of approximately 2, meaning the resistance change is twice the mechanical strain.
Question 2: In a Wheatstone bridge load cell, what is the purpose of using four active strain gauges instead of one?
- To increase the output voltage range to measurable levels and cancel temperature effects (Correct answer)
- To reduce the cost of calibration
- To eliminate the need for an excitation voltage
- To allow the cell to operate without signal conditioning
Correct answer: To increase the output voltage range to measurable levels and cancel temperature effects
Four active gauges in a full bridge configuration doubles sensitivity and provides inherent temperature compensation through common-mode rejection.
Question 3: A piezoelectric load cell is NOT suitable for measuring which of the following?
- Impact forces
- Static dead loads held for extended periods (Correct answer)
- Rapidly changing dynamic forces
- High-frequency vibration forces
Correct answer: Static dead loads held for extended periods
Piezoelectric sensors cannot hold a static charge indefinitely; charge leaks over time, making them unsuitable for measuring sustained static loads.
Question 4: Which bonding defect in a strain gauge installation is most likely to cause creep error in a load cell?
- Using too little adhesive
- Incomplete curing of the adhesive (Correct answer)
- Applying the gauge at the wrong angle
- Using conductive adhesive instead of insulating adhesive
Correct answer: Incomplete curing of the adhesive
Incompletely cured adhesive remains viscoelastic and deforms under sustained load, causing the gauge to creep relative to the flexure element.
Question 5: What does the term 'creep compensation' refer to in load cell design?
- Adjusting excitation voltage to counteract drift
- Trimming the bridge with resistors to offset the mechanical creep of the flexure (Correct answer)
- Increasing the sampling rate of the indicator
- Applying a correction factor based on ambient temperature
Correct answer: Trimming the bridge with resistors to offset the mechanical creep of the flexure
Creep compensation involves adding small resistors or shunts to the bridge network to electronically counteract the mechanical creep characteristic of the elastic element.
Question 6: In a shear beam load cell, where are the strain gauges typically bonded?
- At the neutral axis where bending stress is zero
- On the outer surfaces at maximum bending stress locations
- At the point of maximum shear stress, 45° to the beam axis (Correct answer)
- On the end caps of the beam
Correct answer: At the point of maximum shear stress, 45° to the beam axis
Shear beam gauges are bonded at ±45° to the neutral axis where principal strains from shear are maximum, making them sensitive to shear rather than bending.
Question 7: What is the typical insulation resistance requirement between strain gauge circuits and the load cell housing per OIML R 60?
- At least 1 MΩ
- At least 5 GΩ
- At least 2,000 MΩ (Correct answer)
- At least 500 kΩ
Correct answer: At least 2,000 MΩ
OIML R 60 requires a minimum insulation resistance of 2,000 MΩ (2 GΩ) between the bridge circuit and the load cell body to prevent leakage errors.
CST Strain Gauge & Piezoelectric Sensors 2
What is the gauge factor (GF) of a typical metallic foil strain gauge?