Q. There is a “bob-bob-bob” sound of air.

A. It is not abnormal. It is the sound of air moving in and out of the fuel tank and fuel sub tank.

When the equipment is in use, the room temperature is close to 20°C. However, when the room temperature drops after the fire is extinguished, the air in the fuel tank cools and contracts, changing the balance with atmospheric pressure, causing a “bob” or “drip” sound when air enters the fuel tank. If the room temperature drops slowly, the sound may be heard after an hour or two. The sound is caused by changes in room temperature. The fuel sub tank, which is the part that receives fuel, has enough space to prevent fuel from overflowing.