This solution explores the characteristics of thermocouple measuring instruments to identify the incorrect statement among the options.

Understanding Thermocouple Fundamentals

Thermocouples function based on the Seebeck effect. When two dissimilar metals are joined at two junctions, and these junctions are at different temperatures, a voltage is produced. This voltage ($V$) is approximately proportional to the temperature difference ($\Delta T$) between the junctions, following the relation $V \approx \alpha \Delta T$, where $\alpha$ represents the Seebeck coefficient specific to the metals used. The voltage generated is typically quite small, measured in millivolts ($mV$).

Detailed Analysis of Statements

• Statement 1: They are very useful as transfer instruments

  This statement is accurate. Thermocouples have relatively stable characteristics and a broad temperature range, enabling their use as reliable instruments for transferring temperature calibration data.

• Statement 2: They are incapable of taking any overload

  The assertion that they are incapable of handling "any" overload is a strong claim. While significant overloads can damage thermocouples, they might tolerate minor overloads. This statement's absolute nature makes it questionable, but it's not the primary incorrect statement identified.

• Statement 3: Power losses are very low

  This statement is identified as the incorrect one. Although thermocouples operate with low voltage outputs and are connected to high-impedance circuits (leading to minimal current, $I$), they possess intrinsic resistance. This includes the thermocouple wires themselves ($R_{tc}$) and potentially extension wires ($R_{ext}$). Power dissipation occurs due to these resistances, calculated as $P = I^2 R_{total}$. While these losses are often small, labeling them as "very low" can be considered inaccurate, especially in high-precision applications where even minor energy losses are significant. The presence of non-zero resistance inherently leads to power dissipation.

• Statement 4: Their calibration does not change with time or temperature

  This statement is factually incorrect. Thermocouple calibration is known to drift over time due to factors like aging, oxidation, and repeated thermal cycling. Recalibration is typically required periodically to ensure accuracy.

Considering the options, statement 3 is identified as the incorrect statement because the term "very low" regarding power losses is context-dependent and potentially misleading, as power dissipation is an inherent consequence of the circuit's resistance.