In this paper, we introduce a new pneumatic temperature control technique and its application to precision thermometry. The method controls temperature by adjusting gas pressure through the unique thermohydraulic linkage of the pressure-controlled loop heat pipe (PCLHP). Due to this temperature-pressure linkage, the PCLHP-based pneumatic temperature control achieves exceptional control speed, stability, and precision. To fully understand this method, we systematically investigated the effects of various influencing parameters, such as heat load, sink temperature, and rate of pressure change, on the stability of temperature control. In addition, we successfully achieved closed-type pneumatic temperature control using a mechanically-driven gas pressure controller. We also developed a hybrid PCLHP that incorporates a heat pipe liner into the isothermal region to further improve the temperature uniformity of the pneumatically-controlled temperature field. With this technique, we significantly improved the accuracy of the fixed point of the International Temperature Scale of 1990 by using inside nucleation of the freezing temperature of tin and determining the liquidus temperature of tin. In this paper, we summarize the results of these diverse efforts in characterizing the pneumatic temperature control technique, along with theoretical analyses.

In this work, precise gas pressure control based on a closed pneumatic circuit was achieved with a mechanically driven gas pressure controller (MDGPC), consisting of a variable-volume bellows chamber and linear actuator. The linear actuator was employed to change an axial dimension of the bellows chamber with the proportional (P) and proportional-integral (PI) controls for fast, stable, and precise pressure control of the gas inside the bellows chamber. The pressure control stability and resolution of the MDGPC were approximately 1.5 Pa and 10 Pa for the P control and 1 Pa and 5 Pa for the PI control, respectively. Despite the more stable and precise control characteristics of the PI control method, overshoots and undershoots observed during the set-point pressure changes and recoveries from pressure disturbances rendered it unsuitable for the MDGPC control method. In contrast, the MDGPC operated under the P control did not show any significant overshoots or undershoots when the set-point pressure abruptly changed or when the MDGPC was exposed to pressure disturbances. Therefore, it was concluded that a fast, precise, and stable gas pressure control in a closed manner was attainable with the MDGPC under the P control.

In this work, recent advances in temperature control techniques and the resulting contemporary progress in precision thermometry are addressed together with a broad review of traditional temperature control methods. Particular emphases are placed on clarification of the nature of temperature control and its classification, and the relevant technical issues are addressed based on this clarification and classification. Being a thermodynamic quantity having the same dimension as energy, temperature of an object is traditionally controlled by means of the changing rate of energy (Heat) transfer; however, this approach has led to a slow, less stable, and uneven temperature field due to inherent limits caused by finite properties of materials. To overcome this problem, thermodynamic characteristics of two-phase heat transfer devices, such as heat pipes and loop heat pipes, have been extensively employed where high-speed nature of fluid flow was exploited to realize a uniform temperature field, and unique thermodynamic linkage between saturation temperature and pressure was successfully applied to attain a fast, stable, and predictable temperature control of a finite-sized isothermal space. Representative examples and applications are provided in the context of unique features of the introduced contemporary temperature control techniques, which caused significant scientific strides in the related fields.