What defines a dry pipe sprinkler system?

A dry pipe sprinkler system is characterized by the use of pressurized air or nitrogen within the pipes instead of water. This design is specifically implemented in environments where temperatures can drop to freezing levels, which may cause water within a traditional wet pipe system to freeze and potentially rupture the pipes.

In a dry pipe system, the pipes are filled with air or nitrogen, maintaining pressure that keeps the valve closed until a fire breaks out. When a fire activates a sprinkler head, the pressurized air is released, dropping the pressure and allowing a valve to open, flooding the system with water sourced from an external supply. This ensures that water only enters the pipes when it is absolutely necessary, thereby preventing the risk of freezing in colder climates.

The other options describe characteristics that do not align with the definition of a dry pipe system. For example, having water in the pipes or involving a mixture of gas and water does not accurately reflect the operation or design of dry pipe systems. Therefore, understanding that pressurized air or nitrogen is central to the operation of a dry pipe sprinkler system is crucial when studying fire protection systems.