BEE Measurement and Instrumentation 3 — Questions and Answers
Question 1: Which transducer converts displacement into a proportional electrical voltage using magnetic coupling?
- Strain gauge
- LVDT (Correct answer)
- Thermistor
- Piezoelectric sensor
Correct answer: LVDT
The Linear Variable Differential Transformer (LVDT) produces an AC output voltage proportional to the core's displacement from its null position.
Question 2: In a Wheatstone bridge, balance is achieved when:
- P/Q = R/S (Correct answer)
- P×Q = R×S
- P+Q = R+S
- P−Q = R−S
Correct answer: P/Q = R/S
Bridge balance requires P/Q = R/S (or equivalently P×S = Q×R), producing zero galvanometer deflection.
Question 3: A thermocouple generates an EMF based on the:
- Piezoresistive effect
- Seebeck effect (Correct answer)
- Hall effect
- Faraday effect
Correct answer: Seebeck effect
The Seebeck effect causes a voltage to develop at the junction of two dissimilar metals when a temperature difference exists.
Question 4: Which of the following instruments is used to measure power in a three-phase, three-wire unbalanced load using only two wattmeters?
- One-wattmeter method
- Two-wattmeter method (Correct answer)
- Three-wattmeter method
- VAR meter method
Correct answer: Two-wattmeter method
The two-wattmeter method correctly measures total three-phase power in any three-wire system regardless of load balance.
Question 5: Creep in an energy meter refers to:
- Slow rotation under rated load
- Continuous rotation of the disc with no load applied (Correct answer)
- Backward rotation of the disc
- Vibration of the moving parts
Correct answer: Continuous rotation of the disc with no load applied
Creep is the slow continuous rotation of the energy meter disc when voltage is applied but no load current flows, causing measurement error.
Question 6: The gauge factor (GF) of a strain gauge is defined as:
- ΔR/R divided by strain (ε) (Correct answer)
- ΔR multiplied by strain (ε)
- R divided by ΔR
- Strain divided by ΔL
Correct answer: ΔR/R divided by strain (ε)
Gauge factor GF = (ΔR/R)/ε represents the fractional change in resistance per unit strain, typically 2–2.5 for metallic gauges.
Question 7: An oscilloscope probe with a 10× attenuation factor is used. The screen displays 3 V. The actual signal voltage is:
- 0.3 V
- 3 V
- 30 V (Correct answer)
- 300 V
Correct answer: 30 V
A 10× probe attenuates the signal by 10 before it reaches the oscilloscope, so the actual voltage is 3 V × 10 = 30 V.
BEE Measurement and Instrumentation 3
Which transducer converts displacement into a proportional electrical voltage using magnetic coupling?