This document provides means for ensuring that samples of natural gas and natural gas substitutes that are conveyed into transmission and distribution grids are representative of the mass to which they are allocated. NOTE To ensure that a particular gas is taken into account in the standard, please refer to Annex A. This document provides comprehensive information on the way that samples can be contaminated, altered, modified or degraded and methods, means and procedures for ensuring that the sample remains representative from the start of the sampling process to the point where the sample is presented to the analytical device. This document is primarily intended for sampling at sites and locations where interchangeability criteria, energy content and network entry conditions are measured and monitored and is particularly relevant at cross border and fiscal measurement stations. It serves as an important source for control applications in natural gas processing and the measurement of trace components. This document does not address the safety issues associated with gas sampling. This standard is applicable to natural dry gas (single phase - typically gas transiting through natural gas pipelines) sampling only. On occasion a natural gas flow can have entrained liquid hydrocarbons. Attempting to sample a wet natural gas flow introduces the possibility of extra unspecified uncertainties in the resulting flow composition analysis. Sampling a wet gas (two or three phases) flow is outside the scope of this standard.