Traceable measurement and uncertainty analysis of the gross calorific value of methane determined by isoperibolic calorimetry

As methane is the major component of natural gas and non-conventional gases as biogas or mine gases, its energy content has to be measured accurately regardless of its production site for fiscal trading of transported and distributed natural gas. The determination of calorific value of fuel gases with the lowest uncertainty can only be performed by direct method with a reference gas calorimeter. To address this point, LNE developed a few years ago an isoperibolic reference gas calorimeter according to the Rossini's principle. The energy content H-s of methane of purity 99.9995% has been measured to 55 507.996 kJ kg(-1) (890.485 kJ mol(-1)) with an expanded relative uncertainty of 0.091% (coverage factor k = 2.101 providing a level of confidence of approximately 95%). These results are based on ten repeated measurements and on the uncertainty assessment performed in accordance with the guide to the expression of uncertainty in measurement (GUM). The experimental setup and the results are reported here and for the first time, the fully detailed uncertainty calculation is exposed.