If the two thermocouples cannot be considered as twins and linear, the power measurement accuracy will be obviously compromised.
This situation is shown in Figure 3.7 where the two thermocouples are supposed to have two quite different nonlinear characteristics.
In this case, the voltage measured by voltmeter will be Den instead of De.
Wattmeters based on thermal principle allow high accuracy to be achieved in critical cases of highly distorted wide-band spectrum signals.