The Thermomolecular Pressure Difference Effect
The thermomolecular pressure difference is one of the several phenomena which are significant when pressure measurements are made under purely gaseous conditions, particularly when large differences in temperature exist between the place in which pressure
- PDF / 442,615 Bytes
- 16 Pages / 439 x 666 pts Page_size
- 85 Downloads / 152 Views
The Thermomolecular Pressure Difference Effect
10.1
Introduction
The thermomolecular pressure difference is one of the several phenomena which are significant when pressure measurements are made under purely gaseous conditions, particularly when large differences in temperature exist between the place in which pressure measurements have to be carried out and a reference place, where temperature is generally room temperature and where precise pressure measurements can be made by primary or transfer standards. Such phenomena need careful consideration and corrections have to be applied. Some corrections are specific of the experimental apparatus employed. An example is the dead-space correction, which is directly proportional to the volumes and the pressure differences considered and inversely proportional to the reference temperature of the experiment. This correction depends to a large extent on the different temperature gradients between the site used as a reference and the place where the pressure measurements are physically made. Other corrections depend on the dynamics of pressure variations. An example is provided by the adiabatic heating and cooling effects, which also play an important role in gas pressure measurements (see Sect. 8.2). Other corrections concern the purity of the fluid used. There are, for example, gas impurities that heavily affect the reproducibility of some of the most important fixed points (see Sect. 3.1.1, 4.1). Still, other corrections relate to the conditions of use of the primary and secondary pressure and temperature standards. An example is the aerostatic or gas head correction, (see Sect. 7.1.3.3), which is directly proportional to pressure and inversely proportional to temperature and, as the dead-space correction, depends to a high degree on temperature distribution between the operation levels, as temperature changes produce gas density changes. There is an effect, evident when a large temperature difference exists between the two ends of a tube connecting the gas volume to the pressure-measuring device, which cannot be corrected in a simple way. This is the thermomolecular pressure difference
F. Pavese, G. Molinar Min Beciet, Modern Gas-Based Temperature and Pressure Measurements, International Cryogenics Monograph Series, DOI 10.1007/978-1-4419-8282-7_10, © Springer Science+Business Media New York 2013
447
448
10 The Thermomolecular Pressure Difference Effect
effect, also called thermal transpiration, a physical phenomenon that produces a measurable pressure difference. This effect is made evident by the two kinds of molecule collisions occurring in a pressurized gas in a container. At higher pressures, in viscous regime, nearly all collisions occur between gas molecules. At very low pressures, in the Knudsen regime, there are only gas collisions of molecules against tube surface. As pressure is decreased and the change from the viscous to the Knudsen regime is taking place, there is an intermediate region in which the magnitude of the thermomolecular pressure differ
Data Loading...
