Temperature transducers — RTD output variable: An RTD (resistance temperature detector) produces a change in which electrical quantity for a given change in temperature? The claim says it “produces a voltage.” Decide whether the claim is valid.
-
ACorrect
-
BIncorrect
-
CValid only with constant-current excitation
-
DValid only for platinum RTDs
-
EUndetermined without wiring configuration
Answer
Correct Answer: Incorrect
Explanation
Introduction / Context:Many temperature sensors convert thermal changes to electrical signals, but they do so via different primary variables. Thermocouples generate a voltage (Seebeck effect), whereas RTDs change resistance. Confusing these leads to wrong interface circuits and large measurement errors.
Given Data / Assumptions:
- RTD element (e.g., Pt100/Pt1000) has resistance that varies predictably with temperature.
- Measurement is typically made using a constant current source and measuring the resulting voltage drop, or via a Wheatstone bridge.
- No built-in thermoelectric junctions as in thermocouples.
Concept / Approach:The primary transduction in an RTD is resistive: R(T) increases with temperature (for platinum). Any observed voltage is a secondary effect produced by test current flowing through that resistance. Therefore, saying an RTD “produces a voltage” misstates the device principle; it “exhibits a resistance change,” and the instrumentation creates and measures voltage proportional to that resistance under known current.
Step-by-Step Solution:
Identify sensor class: RTD (resistive).Recall interface: excite with known current, measure V = I * R(T).Conclude the fundamental output variable is resistance, not an intrinsic voltage.Therefore, the claim is invalid for RTDs.Verification / Alternative check:RTD datasheets present R–T tables/curves, not Seebeck coefficients. Measurement ICs for RTDs provide precision current sources and ratiometric ADC inputs, affirming resistance-based operation.
Why Other Options Are Wrong:
- Correct: Confuses RTDs with thermocouples.
- “Valid only with constant-current excitation”: Even then, voltage is a measurement product, not the primary transduction.
- “Valid only for platinum RTDs”: All RTDs are resistive by definition.
- “Undetermined…”: Wiring affects error compensation, not operating principle.
Common Pitfalls:Mixing up thermocouple vs. RTD; overlooking lead-wire compensation (2-, 3-, 4-wire) which addresses resistance measurement accuracy.
Final Answer:Incorrect.