Instrumentation basics: the electrical output produced by a thermocouple due to the Seebeck effect is typically which electrical quantity?

Difficulty: Easy

Correct Answer: a voltage

Explanation:


Introduction / Context:
Thermocouples are ubiquitous temperature sensors because they are rugged, inexpensive, and cover wide temperature ranges. They consist of a junction of two dissimilar metals. When a temperature difference exists between the measurement (hot) junction and the reference (cold) junction, a small electromotive force is generated—this is the Seebeck effect.


Given Data / Assumptions:

  • Two dissimilar metals form a junction.
  • There is a temperature gradient between junctions.
  • The sensor is connected to a high-impedance measuring instrument.


Concept / Approach:
The Seebeck effect states that a temperature difference across a conductive loop of two different materials generates a voltage proportional to the temperature difference (within a specified range). Practical thermocouple signals are in the millivolt range and require cold-junction compensation and amplification for accurate measurement. Resistance and capacitance changes characterize other sensors (RTDs, thermistors, capacitive humidity sensors) and are not the native output of a thermocouple.


Step-by-Step Solution:
Identify the sensing principle: Seebeck effect.Map principle to output: electromotive force (voltage) appears across the leads.Confirm that instrumentation reads millivolt-level signals with compensation.Select “a voltage” as the correct electrical quantity.


Verification / Alternative check:
Thermocouple calibration tables express output in millivolts versus temperature, reinforcing that voltage is the primary signal. Measurement systems often specify input ranges in millivolts for different thermocouple types (K, J, T, etc.).


Why Other Options Are Wrong:
Resistance is the domain of RTDs/thermistors. Capacitance is unrelated to thermocouples. Current is not directly generated; any current drawn is due to the measuring circuit and should be negligible to avoid error. “None” is incorrect because a well-defined quantity—voltage—exists.


Common Pitfalls:
Forgetting cold-junction compensation; loading the thermocouple with low-impedance meters; ignoring noise and using long unshielded runs without proper filtering.


Final Answer:
a voltage

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