Thermodynamic Temperature Amplification by Means of Two Coupled Gas‐Controlled Heat‐Pipes

Two gas‐controlled heat‐pipes, one working with mercury and the other with sodium, have been connected to the same helium pressure line. At any given pressure, the high temperature of the sodium heat‐pipe is thermodynamically related to the low temperature of the mercury heat‐pipe. Pressure control well within 1 ppm is obtained in both devices by using a 25 Ω SPRT inserted in the thermometer well of the mercury heat‐pipe. A computer controls the helium pressure by operating input/output electrovalves and a variable volume. With such a system, helium pressures between 8 kPa and 200 kPa are controlled with a sensitivity well beyond the resolution of currently available pressure gauges, thus reaching a temperature stability in the mercury heat‐pipe at a level of about 0.2 mK. The very high reproducibility of the 25 Ω SPRT used in the mercury heat‐pipe between 240 °C and 400 °C, within 2⋅10⁻⁷, is transferred with this amplification system to higher temperatures, thus obtaining in the sodium heat‐pipe between 660°C and 961°C a reproducibility better than can be obtained with high temperature standard platinum resistance thermometers. An accurate measurement of the thermodynamic relationship between the ITS‐90 temperatures of the two substances at their liquid‐vapour transitions at identical pressure values has been done and the results are hereby presented. The reproducibility of this “temperature amplifier” is then compared with measurements at the fixed points.