Applies to the simultaneous determination of the loss of mass at 550 °C which is the sum of the masses of water that has not undergone any reaction and of the hydrogen fluoride formed by pyrohydrolysis and the mass of water released at 550 °C determined by the Karl-Fischer Method (direct electrometric titration). Makes reference to ISO 760. The difference between these two determinations is due to the formation of hydrogen fluoride by pyrohydrolysis (2 HF correspond to 1 H2 O). Calculation of the water content from the two results obtained. The diagram of the test apparatus is shown in a figure.