Inertia of liquid microthermometer readings

Abstract

Currently the need for micro thermometers with low inertia and accordingly with the small diameter of the capillary is increasing, and information about their inertia, which is necessary, in particular, for medicine, becomes insufficient. Micro thermometers with liquid-phase sensitive elements are made on the basis of capillaries filled with liquid. Important is the type of liquid (water, alcohol, mercury, etc.), the angle of inclination of the thermometer capillary, and its internal diameter. The time of their thermal inertia is rather small, since the thermal equilibrium of the thermometer and the controlled object is set extremely fast. However, in reality, the movement of a liquid in a capillary of a thermometer, which determines the readout of the temperature value, due to the change in the thermodynamic conditions during the measurement, is not so rapid to satisfy the metrology. The experience of studying the spontaneous penetration of liquids in porous micro and nano channels of natural structures is limited mainly by works that capture the significance of the influence of initial conditions. The features of filling capillaries of different internal diameters at different angles of inclination are considered in the paper. It enables us to work out the technology of manufacturing and application of these thermometers, and thus to predict the duration of setting the micro thermometers’ indications not limited by thermal inertia. For this purpose, the Washburn’s and Bosanquet equations are considered. Rate of filling the capillary is affected by fluid viscosity, surface tension, length of the filled capillary, and its diameter. Since the viscosity of the liquid increases slightly with pressure and decreases exponentially with temperature, it is inversely proportional to the rate of liquid penetration or to the rate of moving meniscus. We have analyzed the impact of these factors on the micro thermometers’ performance that promotes their efficient production and application.