Relevance of heat conduction in the correction and interpretation of high temperature, fine wire thermocouple measurements

The ultimate goal of this article is to highlight the relevance of heat conduction along the fine wires of a thermocouple, even if its junction presents a bead-like shape. In the thermocouple heat transfer balance, needed to derive the gas temperature from that of the sensor, the ‘choice’ between the correlations for convection to a sphere (because of that apparent shape) or a cylinder still generates considerable doubts. In this work, a 2-D, two-color pyrometer has been used to obtain temperature profiles along thermocouples with different bead/wire size ratios in conditions representative of combustion environments. The results prove that commercial beads (2–3 times larger than the wire) do not generate any gradient, and that a Nu(Re) correlation for a cylinder should be implemented, since the temperature in the junction is actually dominated by heat conduction from/to the adjacent wires, except for very large beads. Furthermore, some gradients found in the wires, around those large beads and elsewhere, have been compared with simple models that adequately reproduce their reach, considering both the conduction along the metal wires and convection from the gas. A criterion for neglecting heat conduction errors in thermocouple measurements is given, in general relaxing those commonly cited in the literature. However, the relevance of carefully considering this criterion is highlighted, for its implication in the gradients measurable in the gas and the spatial resolution attainable. These recommendations are extensible to other techniques based on thermocouples.