The core difference — where the water is
Every standpipe answers one question: when a firefighter opens a hose valve on the fire floor, how fast does water arrive, and what makes it arrive? A wet system already has water in the pipe, so the answer is "instantly." A dry system has to get water into the pipe first — automatically, on command, or by a pumper — before anything comes out of the valve.
That single difference drives where each type is used, how it is maintained, and, critically, what a flow test has to do to prove it works.
Wet standpipes
Automatic wet — the piping is filled with water at all times and connected to a water supply that can meet the system demand on its own. Open a hose valve and water flows immediately at the required pressure. This is the simplest system to use and the most common in heated buildings with an adequate supply.
Manual wet — the piping is also kept filled with water, but only for pressure signaling and to speed the first water; the connected supply is NOT sufficient to meet the firefighting demand. A fire-department pumper connected to the FDC must supply the required flow and pressure. It looks "wet" but it is not self-sufficient — the pump on scene is what makes it work.
Dry standpipes
Automatic dry — the piping holds pressurized air or nitrogen. Opening a hose valve drops the pressure and trips a dry-pipe or deluge-type valve, which admits water automatically. This is the answer where a riser runs through unheated space and a wet pipe would freeze.
Semiautomatic dry — the piping is dry, and water is admitted only when someone activates a remote control device (for example at the hose station). It is not charged until that action is taken.
Manual dry — the piping is empty and has no permanent water supply at all. The system depends entirely on a fire-department pumper connected at the FDC to charge it and deliver the demand. It is the oldest and simplest arrangement, and completely useless without a capable pump on scene.
Why wet vs dry decides the test — and the fire response
On a wet automatic system, water is already there, so a flow test largely verifies that the supply delivers the required flow and pressure at the remote outlets. On dry and manual systems, the test also has to prove the mechanism that gets water into the pipe — the dry-pipe valve trips, the remote control charges the system, or, on manual systems, that a pumper can deliver the full demand through the FDC.
That last point is the one owners underestimate. A manual standpipe cannot be proven — or used in a fire — without a pump capable of producing the designed flow at the designed pressure. FLOW runs pumper apparatus specifically so manual and dry standpipes can be charged to their full 2½-inch demand at the FDC, which a hydrant connection alone frequently cannot do. Confirm the type from the drawings before scoping any test.