Sprinkler systems are the most effective manner to suppress a fire. The most simple sprinkler system has a pipe full of water and sprinkler heads. When heat triggers a sprinkler head, water is immediately discharged. This is called a WET SPRINKLER SYSTEM because the pipes are always full of water.
A wet sprinkler system has a water flow sensor. The most common sensors have an impeller that spins with water flow, however, there are many methods that can be used to determine water flow.
In locations where the pipes are located in unheated spaces and may potentially freeze, the pipes are not full of water. They are full of pressurized air. This is called a DRY SPRINKLER SYSTEM.
An air compressor maintains sufficient pressure to keep a valve closed. Because the surface area is larger on the air side, it takes less air pressure to hold back a water pressure even twice as high. Each system is different and the values illustrated are only for this example.
In the illustration above you can see the air pressure has dropped. At a specific set point, the air compressor turns on. The air pressure is restored, all is good.
When a sprinkler head opens, from fire or damage, the air rushes out lowering the pressure needed to keep the valve closed.
When the valve opens letting the water flow into the pipe, the air pressure gauge will match the water pressure reading.
A dry riser will also have a water flow sensor. Unlike the wet system with a mechanical device that moves with water flow, the dry system counts on the fact that if you have the water pressure inside the dry pipe, you must have a fire.
However, there is a flaw in that logic.
In cases of air compressor failure, or an air leak to large for the compressor to maintain the system, the air pressure drops in the same manner of an open sprinkler head, albeit at a slower rate.
If the air pressure drops to low, the fire alarm panel will signal a Supervisory Condition of "LOW AIR". The fire panel will start beeping and the monitoring station will call to notify the user of the LOW AIR alert. If the user fails to respond to a LOW AIR warning, the valve will eventually open.
Just like in a real fire event, water pressure on the dry side of the valve will trigger a DRY WATER FLOW event. Fire Trucks will roll.
LOW AIR Alerts should never be ignored or pushed back even a few hours. LOW AIR alerts should be addressed with the sprinkler contractor or maintenance company immediately.