While XRS is in space, the liquid helium coolant is slowly evaporating. Since the XRS instrument works only as long as the helium coolant lasts, the XRS team here on earth wants to keep track of how much is left. The technique that will be used is called mass gauging, since it measures the mass of liquid helium present.
The principle of the mass gauge is simple. A heater in the helium tank adds a measured amount of heat to the helium. The heat pulse spreads out through the entire liquid helium bath. The more helium is present, the more the heat spreads out, and the smaller the temperature rise. Thus, at the beginning of the mission, when the helium tank is nearly full, the temperature rise will be much smaller than it will be towards the end of the mission, when only a small amount of liuqid helium is left.
The heat pulse measuring technique is especially well suited to superfluid helium, the coolant used in the XRS mission. The thermal conductivity of superfluid helium is high enough that the heat pulse spreads through the helium bath quickly enough to make the measurement possible and convenient.
The heat pulse is small, to avoid heating the helium bath any more than necessary. In ground tests of the XRS mass gauging system, the liquid helium temperature increased by about 0.01 Kelvin.
Mass gauging has been used successfully on other space missions in the past, for example:
Links
Cryogenics & Fluids Home Page
|
|
