Method of determining with great accuracy the rate of leakage from the case of an electrical apparatus

A method of determining the leakage rate of a dielectric gas under pressure inside a case of an electrical apparatus, the method comprising:

• performing, over a certain period of time, measurements of the temperature and of the pressure of the gas inside the case by means of a temperature sensor and a pressure sensor fixed on the case via the outside;
• determining a series of density values for the gas on the basis of corresponding pairs of measured pressure and temperature values;
• and
• determining the leakage rate on the basis of said series of density values, wherein, simultaneously with measuring a pair of temperature and pressure values for the gas inside the casing, further comprising:
• measuring a current passing through the apparatus located inside the casing, and a climate parameter concerning a medium in which the case is located, in order to select an index from a set of different indices and assign the selected index to the density value corresponding to said pair of measured temperature and pressure values, each index corresponding to an index range which contains the measured value of the current and to an index range which contains the measured value of the climate parameter,
• wherein on the basis of the density values having a same index, a leakage rate corresponding to said index is determined, and wherein on the basis of the leakage rates corresponding respectively to the different indices, the leakage rate of the gas from the case is determined.