Properties Characterizing Cryogenic Instrumentation

Two-Phase Systems. The combination of low boiling point and low heat of vaporization increases the possibility that cryogenic fluids become boiling, two phase systems. The influence of this fact upon pumping, liquid density, and level determination are obvious. Any sensor adding energy to the system is in fact creating a vapor-liquid interface at the very point of measurement. 

An added consequence is that when the system is at equilibrium in the two-phase region, the measurement of both temperature and pressure is redundant, in that the system has only a single degree of freedom. 

The fact that two phases usually do exist often means that two liquid levels can be measured. The level within a Stillwell, for example, will be lower than that of the surrounding bulk fluid. This results from the fact that the fluid within the Stillwell can be a single phase, more dense fluid, while the surrounding bulk fluid is often two phase, and hence less dense. 

Expansivity. Liquid hydrogen has a rather large coefficient of thermal expansion, when compared to ordinary fluids. Also, the vapor pressure curve of hydrogen is rather steep. As a consequence, the mere fact of pressurizing a tank of liquid hydrogen from one atmosphere to approximately two atmospheres causes the liquid to warm to about 23 K and the level to rise by about 5%. Thus, the liquid level of a closed hydrogen tank will rise upon pressurization, while the mass contents actually remain the same. 

Stratification. It is well established that cryogenic liquids experience thermal stratification. This condition tends to be a stable one, as the warmer, and less dense, fluid is at the top of the tank. This condition causes the ullage pressure and hence the required tank wall thickness to be greater than would be necessary if stratification were avoided. Also, these systems may be described as anisotropic, or nonhomogeneous, which presents a sampling problem to effect proper instrumentation. 

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