Experimental study of the furnace effect with the copper point blackbody

Abstract

The furnace effect is a major uncertainty source in the realization of high-temperature fixed points in metal–carbon systems. The cause of this effect is as yet unexplained, and understanding it is a high-priority task for the inclusion of fixed points in future high-temperature metrology. In this paper, the furnace effect is studied experimentally and systematically with three furnaces, two cells, and two radiation thermometers. The copper point is chosen for the fixed point of investigation in order to eliminate the influence of the properties of the furnace. Correction for drift and the size-of-source effect of the thermometer is applied to achieve the required measurement accuracy. As a result, the furnace effect has been demonstrated quantitatively for the first time as the difference in radiance temperature of the same cell observed in different furnaces by the same thermometer. The experimental results show that it is related to the dimension of the furnace and the design of the fixed-point blackbody cell.

Investigation on the cause of the furnace effect of high-temperature fixed points

“…The facilities utilized in this investigation is basically of the same type as the ones described in the previous investigation. For details the readers are referred to [14] and the references therein. Here, only what is essential is presented.…”

“…All three furnaces used in this study were of the same types as had been used in the previous study [14], and have Jouleheating heater elements indirectly heating the furnace tubes. The '3-zone' furnace (MAT60SCII manufactured by Matels Inc.) has a 1 m long 60 mm inner-diameter alumina furnace tube containing the graphite block, for thermal homogenization, with the fixed-point blackbody cell inside.…”

“…The diagrams of the three furnaces are reproduced from [14] and are shown in figure 1. All furnaces were operated under Ar gas flow and with no window at the front port.…”

“…In an article co-authored by the author, a systematic investigation has been conducted at the copper point to identify the cause of the furnace effect [14]. The Cu point was chosen to eliminate the effect of the furnace thermal properties (thermal inertia and gradient) that will manifest itself through deformation of the melting curve for the HTFP: for a copper point the effect should show as a kink in the freezing plateau, which makes the plateau duration shorter but keeps the freezing temperature, assigned to the flat part of the plateau, unaffected.…”

On the uncertainties in the realization of the kelvin based on thermodynamic temperatures of high-temperature fixed-point cells

“…The "furnace effect" is where differences are seen with the same fixed-point cell in different furnaces which are larger than can be explained by the uncertainty assessment [18]. The furnace effect is an area that requires more work to understand its origins and magnitude.…”