Difference between Active and Passive Instruments:
Classic Examples Explained
Passive example – Bourdon tube pressure gauge: The fluid pressure itself straightens the curved Bourdon tube, and that movement turns the pointer through gears. No battery or mains supply is needed; all the energy comes from the measured pressure.
Active example – float-type fuel level gauge: A float rises and falls with the fuel level and moves the wiper of a potentiometer. The potentiometer is supplied by the car battery. The fuel level only controls how much of that battery voltage reaches the meter, so the output energy comes from the external source.
Active vs Passive Instruments: Comparison Table
| Feature | Passive instrument | Active instrument |
|---|---|---|
| Source of output energy | The measured quantity itself | An external power source, controlled by the measured quantity |
| External power supply | Not needed | Needed |
| Resolution / sensitivity | Limited | High; can be improved by amplification |
| Loading of measured system | Can be significant | Usually small |
| Design and cost | Simple and cheap | More complex and costly |
| Output can be transmitted | Usually local reading only | Yes, e.g. 4–20 mA or digital signal |
| Examples | Bourdon pressure gauge, liquid-in-glass thermometer, PMMC meter | Float-type fuel gauge, pressure transmitter, digital multimeter |
Exam Trap: Active and Passive Transducers
Be careful: for transducers, the words are often used the other way round. An active transducer is self-generating and needs no external power (for example a thermocouple, piezoelectric crystal or photovoltaic cell). A passive transducer needs external power to produce an output (for example a strain gauge, RTD, thermistor or LVDT). Always check whether a question is asking about instruments or transducers.
Practice MCQs for Job and University Exams
1. In a passive instrument, the output energy comes from:
(a) An external power supply
(b) The quantity being measured
(c) A battery inside the instrument
(d) An amplifier
Answer: (b)
2. Which of the following is a passive instrument?
(a) Digital multimeter
(b) 4–20 mA pressure transmitter
(c) Bourdon tube pressure gauge
(d) Float-type fuel gauge with potentiometer
Answer: (c)
3. A float-type fuel level gauge using a potentiometer and the car battery is an example of:
(a) Passive instrument
(b) Active instrument
(c) Null-type instrument
(d) Self-generating transducer
Answer: (b)
4. The main advantage of active instruments over passive instruments is:
(a) Lower cost
(b) No power supply needed
(c) Better resolution
(d) Simpler design
Answer: (c)
5. Which transducer is self-generating (active transducer)?
(a) Strain gauge
(b) RTD
(c) Thermocouple
(d) LVDT
Answer: (c)
6. Which transducer needs an external power source (passive transducer)?
(a) Piezoelectric crystal
(b) Photovoltaic cell
(c) Thermocouple
(d) Thermistor
Answer: (d)
Frequently Asked Questions (FAQs)
Q: Is an analog voltmeter active or passive?
A: A PMMC analog voltmeter is passive, because the current from the measured circuit itself produces the pointer deflection. A digital voltmeter is active, because it needs a battery or supply to work.
Q: Is a transmitter an active instrument?
A: Yes. A pressure or temperature transmitter uses an external supply (usually 24 V DC) to produce its 4–20 mA output signal.
Q: Why are active instruments more expensive?
A: They need a power supply, signal-conditioning electronics and often amplifiers, which add cost and complexity.
Q: Which type should I choose?
A: Use a passive instrument for simple, local readings where low cost and no power supply matter. Use an active instrument when you need high resolution or need to send the reading to a control system.
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