GRTgaz operates one of the longest high-pressure natural gas transmission networks in Europe, with 32,414 kilometres of pipes. The composition of natural gas, which is a mixture of alkanes and inert compounds, may vary on an hourly basis depending on its origins. Hence, its Gross Calorific Value* (GCV) varies as well. As GCV is used for billing along with the volume of gas delivered, GRTgaz uses gas chromatographs at control points to accurately determine it. Calibration gases from Messer are used to calibrate these analysers. These gases are produced in the Messer specialty gas laboratory in Zwijndrecht, Belgium. Messer also provides high purity operating gases, mainly helium, for the functioning of these chromatographs.