Study of an Annular Two-Phase Thermosyphon Used as an Isothermal Source in Thermometry

Abstract

The definition of a temperature scale, independent of the substance thermometric qualities used, was proposed by Kelvin in 1898 and was named the thermodynamic temperature scale, which coincides with a thermometer scale filled with a perfect gas. However, achieving this scale is practically impossible; therefore, another temperature scale was developed, the nearest possible to the thermodynamic one and fully characterized to remain convenient and maintain its precision.Taking into account all these specifications, the international temperature scale (ITS) was created; it is updated every 20 years. This scale is based on a primary set of fixed points of temperature, for instance: the triple point of water, the melting point of gallium, the freezing point of zinc among many others. One of the calibration methods is based on the use of cells containing the fixed points cited in the ITS.The other method of calibration is the comparison method, in which both the thermometer to be calibrated and the reference thermometer (calibrated one) are placed in an isothermal bath. The American Society for Testing and Materials (ASTM) established the calibration procedure to be followed when using this method [1]. For this purpose, a highly stable isothermal reservoir is required in order to avoid the socalled bar effect; this thermal phenomenon produces heat gains or losses due to the axial thermal gradient that appears when one end of the bar is in contact with the thermal source and transfers heat as a fin [2]. This situation leads to significant errors in temperature measurements, according to European Association of National Metrology Institutes EURAMET [3] and [4]. Reiss was a pioneer: he suggested using heat pipes to compensate the error induced by the bar effect in the calibration of thermometers [5].Sostmann [6] and Bienert [7] proposed the use of heat pipes in thermometry; they pointed out that heat pipes are normally designed to transport the maximum heat flow with the minimum possible temperature difference. However, in thermometry applications, the heat transport capacity is less relevant than the device's isothermal behavior.Tamba et al. evaluated a water heat pipe, controlling the operation pressure, when it was used as a comparative furnace in order to calibrate a standard platinum resistance thermometer (SPRT) in a range of 338.15 K to 430.15 K, in a direct comparison against the reference SPRT, the combined uncertainty was estimated as 3.1 mK. These results show that • Immersion depth of the temperature sensor is 21 cm. Study of an Annular Two-Phase Thermosyphon Used as an Isothermal Source in Thermometry• Higher values of heat supplied promoted isothermicity.• Device efficiency was higher than 94%.• The system accomplished all the requirements for calibration of temperature instruments using the comparative method.

Heats pipes for temperature homogenization: A literature review

“…The simplest design is a thermosyphon with an annular geometry, over its full length [53] (Figure 10.a) or over a partial length [54] (Figure 10.b). The involved surfaces are therefore smaller than for previous furnaces.…”

“…The involved surfaces are therefore smaller than for previous furnaces. Sanchez-Silva et al [54] developed a copper/water annular thermosyphon to calibrate thermal sensors by a comparative method. Reproducibility issues of experimental tests reveal that the direct use of such thermosyphon may be difficult when the required reference temperature is close to the ambient.…”