A light-emitting device assembly which can be used in many applications has a contact carrier, at least one light-emitting device, a heat sink and at least one securing member. A flow rate of the purge control valve having a larger flow rate size is controlled in a step mode, and a flow rate equal to or smaller than a variation quantity in said step mode is controlled by the purge control valve having a smaller flow rate size. The method may also include analyzing the extracted formation water to identify the first species of microorganism that promotes the biogenic production of the combustible gas. The first transistor and the third transistor include an oxide semiconductor material in channel formation regions.