In the sprinkler system plumbing, the air pressure gauge and the water pressure gauge show the pressure of two different sprinkler systems. These two systems are the dry sprinkler pipes and the wet sprinkler pipes.
The systems are separated by a moveable metal plate called a clapper. The clapper is inside an assembly sometimes called a clapper valve, or dry system valve.
The clapper is being held in place by the pressurized air in the dry sprinkler system; so air, basically, is holding back the water.
Normally, when the water isn't in the dry system part of the sprinkler system, the air is pressurized and presses against one side of the clapper. Water, which is also pressurized, pushes against the other side of the clapper.
The important part to this is that on one side of the clapper, pressurized air pushes against the whole area, while on the other side of the clapper, water only pushes against a small area. Even though the pressure of the air is lower than the pressure of the water, because the area of push against the clapper by the air is far greater than the area of push by the water, the pressurized air keeps the clapper from moving out of the way of the water.
By opening a sprinkler head, heat from a fire lets the air out of the dry sprinkler system. The air pressure then drops and the air no longer holds the clapper in place. The water pressure, no longer opposed by air pressure, pushes the clapper out of the way and the dry system is flooded; the dry system has become wet.
The gauges are showing the pressure of two separated sprinkler systems. To answer the question, the only condition where the air pressure gauge and the water pressure gauge show the same pressure is when the clapper of the valve no longer separates the dry sprinkler system from the wet sprinkler system.
Douglas Krantz
