Thermal Time Constant Measurements for Ge Film, GaAs Thermometers at Low Temperatures

In this experiment, thermal time constant measurements were taken from a system consisting of a heater, He gas, and a sensitive, low temperature resistance thermometer. The resistance thermometer [3] was made of a Ge film deposited on a GaAs substrate. The data from this experiment will be used to design a mass measurement system for hydrogen storage tanks in micro-gravity environments. INTRODUCTION NASA would like to develop a more efficient way of determining the mass of hydrogen fuel remaining on board the space shuttle. The hydrogen is stored in a large (2,500L) tank located inside the shuttle. Although the amount of fuel remaining could be determined from history of use, experience has shown this is not practical. The hydrogen inside the tank consists of both liquid and vapor phases. In a micro-gravity environment, the exact liquid/vapor configuration of the hydrogen is not known. Depending on the recent motion of the shuttle, the configuration could range from many small drops, to a single glob. The current method used to determine the amount of hydrogen fuel remaining requires the shuttle to be accelerated for approximately 15 sec, at which point, a standard level meter can be used to detect the single liquid/vapor meniscus. Four possible methods have been considered to gauge the liquid mass in a tank: optical, sonic, pressurization, and temperature. It is the last approach that we are taking. The temperature approach takes advantage of the large difference in heat capacities of liquid and vapor phases [4]. The heat capacities are independent of the shuttle’s motion or gravitational field. By using small, sensitive thermometers and injecting a known quantity of heat, we can calculate the change in temperature (∆T). Using the volumetric heat capacities of the liquid and vapor states and knowing that the volume of the tank (V) is equal to the sum of the gas and liquid volumes (Vg , Vl ), we determine the volume of the liquid state Eq.(1). We then calculate the mass of liquid at 20.4K and 10^5 bar (1atm). l l g g Q T C V C V ∆ = + tank/liters 1.5 0.017 joules l Kelvin Q V V liters T   = −   ∆   (1) There are two considerations for this method: (1) whether a thermometer system can be built with the necessary sensitivity, and (2) whether the measurement can be made fast