This paper represents the relation between moisture content and dew point of SF6 gas using the Magnus method. The method shows that the main circuit breaker in the high voltage gas insulated switchgear (HVGIS) is the most fragile compartment. The relationship of dew point to moisture content of SF6 in HVGIS is linear with the rate of change coefficient of 0.1258 0C/ppmv. Furthermore, the study shows pressure can be used to forecast the SF6 gas lifetime by using Time to Failure (TTF) method. The results explain the average remain lifetime values of SF6 gas is 21 years. The highest predicted value is on the Busbar bay (bus 2), reaching 42 years, and the lowest value is on the Karet-2 bay (Main CB), namely 11 years. This alternative method is capable showing reliable results compare with existing statistic method used in located HVGIS.