TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Fluid Fermenting Cellar: Another form of the fluid fermenting cellar is a soft cover marsh gas pool built on the ground, where the soft cover marsh gas pool is composed of an open marsh gas pool built on the ground and a fluid pool cover; the upper edge of the open marsh gas pool built on the ground is embedded with an annular recess; the fluid pool cover is made of a flexible sealing film, the edge of the soft cover is an end-to-end annular aerating sealing ring, whose shape and size match that of the annular recess; the aerating sealing ring of the soft cover edge is combined with the annular recess embedded on the open marsh gas pool, after aerating the aerating sealing ring, the open marsh gas pool and the soft cover being firmly connected to form a sealed space, and the fermenting raw material fermenting and producing gas in the soft cover marsh gas pool, in which the top of the soft cover marsh gas pool wall is provided with a marsh gas outlet pipe and the bottom part is provided with a discharging port. Preferred Method: The methods above make use of an accordion air bellow typed automatic control fluid air storage sac, and comprises using the lifting of the accordion air bellow typed fluid air storage sac to trigger a programmed switch which controls the start and stop of an air pump connected to the fluid air storage sac; when the marsh gas entering the fluid air storage sac and reaching a certain height, trigging the programmed switch to start the air pump to pump out the marsh gas in the fluid air storage sac, and meanwhile the fluid air storage sac falling off; when the fluid air storage sac falls off to a certain position, triggering another contact of the programmed switch to stop the air pump and stop pump; the marsh gas continuing to enter the fluid air storage sac and raise up; then repeating the steps to make the fluid fermenting cellar or the soft cover marsh gas pool produce the marsh gas in an extremely low pressure.