Temperature control and stabilization in the cryodielectrometer for temperatures 0.5 – 300 K
To meet requirements, which arise while design of equipment for research the dielectric parameters of low-loss materials in the millimeter waveband, the necessity of high-precision temperature stabilization and measurement appear. Despite the fact that a lot of attention [1,2] is devoted to this task already, it reminds still actual, due to a plenty of specific features, which each given measuring complex possess.Thermometers intended to measure low and very low temperatures, as a rule use the temperature dependence of certain physical properties of basis substances. There is known quite large amount of temperature controllers [3-6], which are able to operate in a wide temperature range. However, some of them operate in direct current DC-mode [4], and others, which are used AC-mode, intended for operation in too narrow temperature range ≈ 1 K [3, 6]. Besides, they are able to use only one type of sensor and possess, in our opinion, too complicated feedback scheme for the temperature stabilization [5]. In this paper we consider the scheme of the temperature regulator-stabilizer with two temperature sensors having both positive and negative temperature coefficients of resistance (TCR). It designed for low temperature measurements on the cryodielectrometer “TORNADO” in the temperature range 0.5-300 K. As well results of the low-temperature radiospectroscopy experiment using the device designed are given.
Precise temperature controller for resistance thermometers
We describe here a stable and sensitive temperature controller to be used with resistance thermometers. The temperature resolution provided by this instrument was ΔT≂20 μK at room temperature and ΔT≂1 μK at liquid 4He temperature.
Behaviour of shear viscosity near the critical point of carbon dioxide
SummaryAccurate measurements of shear viscosity near the critical point of carbon dioxide, performed down to(T−Tc)/Tc∼-10−6 with an improved version of the vibrating-wire viscometer, are presented in this paper. Both density and viscosity can be measured at the same time, so that gravitational effects can be controlled. The experimental results, obtained at a vibrating frequency of 1600 Hz, suggest a finite value of the viscosity at the critical point. This result, in apparent contradiction with the theoretically predicted divergent behaviour may be explained qualitatively by a frequency effect on the measured viscosity.RiassuntoIn questo lavoro si presentano misure accurate della viscosità di scorrimento attorno al punto critico dell'anidride carbonica, eseguite fino a(T−Tc)/Tc∼-10−6 con una versione perfezionata del viscosimetro a filo vibrante. Sia la densità che la viscosità possono essere misurate contemporaneasmente, così gli effetti gravitazionali possono essere controllati. I risultati sperimentali, ottenuti ad una frequenza di vibrazione di 1600 Hz, suggeriscono un valore finito della viscosità al punto critico. Questo risultato, in apparente contraddizione con un comportamento divergente previsto teoricamente, può essere spiegato qualitativamente con un effetto di frequenza sulla viscosità misurata.РезюмеПриводятся результаты точных измерений сдвиговой вязкости вблизи критической точки диоксида углерода, проведенные при (T−Tc)/Tc∼-10−6 с помощью улучшенного варианта вискозиметра с колеблющимся проводом. Одновременно могут быть измерены плотность и вязкость, так что можно контролировать гравитационные зффекты. Экспериментальные результаты, полученные при частоте 1600 Гц, предполагают конечную величину вязкости в критической точке. Этот результат, противоречащий теоретически предсказанному поведению, может быть качественно обьяснен, как влияние частоты на измеренную вязкость.