The closed loop recirculation is a proven method of directly monitoring hydrogen in molten aluminium. A small volume of carrier gas, usually nitrogen, is brought in contact with the melt by means of an immersed probe, and is continuously recirculated in the closed loop until its hydrogen content reaches equilibrium with the vapor pressure of H in the melt. The H concentration in the gas is measured and converted into a reading of the gas concentration in the metal. This method is fast, reproducible, and accurate, and can be used online on the factory floor.
The amount of H in the gas loop of the instrument is determined by a thermal conductivity sensor, which provides high reproducibility and a broad measurement range.