Pressurized water tanks also supply water to sprinkler systems. They're often located in rural locations where city water is unavailable and the demand on the sprinkler system is relatively light. Tank capacity is generally 5,000 to 20,000 gallons. Two-thirds of the tank is water, and one-third is air or inert gas. The air pressure in the tank is approximately 100 psi (pound-force per square inch).
Pressure tanks have very limited capacity and are designed to suppress room fires. Once the water in the tank is gone, there's no more water available to the sprinkler system. Well pumps, which don't have sufficient capacity to keep the sprinkler system flowing at the designed rates, usually fill the tanks. Monitoring of the water supply is critical. For a pressure tank to work properly, it's important to monitor and maintain tank water level, water temperature, and air pressure.