We have used a Johnson-Noise Thermometer (JNT) with a Quantized Voltage Noise Source (QVNS) as a calculable reference to determine the ratio of temperatures near the Zn freezing point to those near the Sn freezing point. The temperatures are derived in a series of separate measurements comparing the synthesized noise power from the QVNS with that of Johnson noise from a known resistance. The synthesized noise power is digitally programmed to match the thermal noise powers at both temperatures and provides the principle means of scaling the temperatures. This produces a relatively flat spectrum for the ratio of spectral noise densities, which is close to unity in the low-frequency limit. The data are analyzed as relative spectral ratios over the 4.8 kHz to 450 kHz range averaged over a 3.2 kHz bandwidth. A three-parameter fit model is used to account for differences in time constants that are inherently temperature dependent. A drift effect is observed of approximately -6 microK/K per day in the results and an empirical correction is applied to yield a relative difference in temperature ratios of -11.5 microK/K 39 microK/K with respect to the ratio of temperatures assigned on the International Temperature Scale of 1990 (ITS-90). When these noise thermometry results are combined with results from acoustic gas thermometry at temperatures near the Sn freezing point, a value of T T90 = 7 mK 27 mK for the Zn freezing point is derived.
Citation: International Journal of Thermophysics
Pub Type: Journals
ITS-90, Johnson noise, Josephson junction arrays, noise thermometry, pseudo noise, temperature, temperature measurement