Thermocouple measurement practice: which instrument is preferred to read the small output EMF produced by a thermocouple temperature sensor?

Introduction / Context:
Thermocouples generate small direct-current voltages (often a few millivolts) proportional to temperature difference at the junction. Accurately measuring these tiny EMFs requires an instrument with both high sensitivity (millivolt range) and high input resistance to avoid loading the sensor.

Given Data / Assumptions:

• Thermocouple EMF magnitude: typically 1–50 mV depending on type and temperature.
• Source resistance can be several ohms to tens of ohms.
• Desired instrument should not load the thermocouple and must resolve millivolts.

Concept / Approach:

Choose an instrument whose input resistance is large compared with the source resistance and whose range matches millivolt levels. A high-resistance milli-voltmeter provides both: millivolt scaling and minimal loading, yielding accurate readings or conditioning for further processing.

Step-by-Step Solution:

Verification / Alternative check:

Modern practice often uses instrumentation amplifiers with mega-ohm inputs feeding ADCs; functionally this emulates a high-resistance millivoltmeter, confirming the choice.

Why Other Options Are Wrong:

• High-resistance ammeter measures current, not suitable for mV EMFs.
• High-resistance voltmeter (on higher ranges) may lack millivolt sensitivity.
• Moving-iron AC voltmeters are for AC and are insensitive at millivolt DC levels.

Common Pitfalls:

• Using general-purpose voltmeter with insufficient resolution, leading to large quantization or scale errors.

Final Answer:

high resistance milli-voltmeter