ISO 22975-2:2016 specifies definitions and test methods for durability and performance of heat-pipes for solar thermal application. ISO 22975-2:2016 is applicable to heat-pipes for use with evacuated tubes, including glass-metal sealed evacuated tubes and double-glass evacuated tubes, as well as with flat plate collectors. ISO 22975-2:2016 provides test methods for determining durability of the heat-pipe, including high temperature resistance and freeze resistance. ISO 22975-2:2016 also provides test methods for measuring performance of the heat-pipe, including starting temperature, temperature uniformity and heat transfer power of the heat-pipe. ISO 22975-2:2016 is only applicable to gravity heat-pipes.