ISO 15971:2008涉及天然气和天然气替代品热值的测量？分离方法，即不涉及气体成分测定或由此计算的方法。ISO 15971:2008描述了用于此目的的各种仪器的操作原理，并为这些仪器的选择、评估、性能评估、安装和操作提供了指南。 热值可以基于质量、摩尔或更常见的基于体积来表示。在标准参考条件下，基于体积的天然气优越热值的工作范围通常在30 MJ/m3和45 MJ/m3之间（参见ISO 13443）。沃泊指数的相应范围通常在40mJ/m3和60mJ/m3之间。ISO 15971:2008既不认可也不质疑任何商业制造商对仪器性能的声明。其中心论点是，任何特定应用（根据一组具体操作要求定义）的适用性只能通过精心设计的实验测试计划来评估。为这些测试的适当内容提供了指南。

ISO 15971:2008 concerns the measurement of calorific value of natural gas and natural gas substitutes by non？separative methods, i.e. methods that do not involve the determination of the gas composition, nor calculations from it. ISO 15971:2008 describes the principles of operation of a variety of instruments in use for this purpose, and provides guidelines for the selection, evaluation, performance assessment, installation and operation of these. Calorific values can be expressed on a mass basis, a molar basis or, more commonly, a volume basis. The working range for superior calorific value of natural gas, on the volume basis, is usually between 30 MJ/m³ and 45 MJ/m³ at standard reference conditions (see ISO 13443). The corresponding range for the Wobbe index is usually between 40 MJ/m³ and 60 MJ/m³. ISO 15971:2008 neither endorses nor disputes the claims of any commercial manufacturer for the performance of an instrument. Its central thesis is that fitness-for-purpose in any particular application (defined in terms of a set of specific operational requirements) can be assessed only by means of a well-designed programme of experimental tests. Guidelines are provided for the proper content of these tests.