Diaphragm valve

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A diaphragm valve is a pilot operated valve. It is very similar to a piston valve in theory, but replaces it's hard cylinder with a flexible diaphragm.


The diaphragm is sometimes made of a simple piece of pliable material, for instance, thick neoprene (about 1/4" of the rubber is common) or a section of tupperware lid. However, there is often a problem with the material not being strong enough. In this case, the center of it is often reinforced with a pair of washers; one a bit bigger than the barrel's diameter on the pilot side, and one a bit smaller than it on the barrel/chamber side.

These valves are often used on coaxials and over/unders. In an over/under, the valve is built in a "T" fitting. The barrel is put coaxially through one end, the pilot and diaphragm are put in the opposite one, and a chamber is connected to the perpendicular opening with a 90° elbow.


top=pressurized, bottom=firing


  1. Air is added by filling behind the diaphragm (C).
  2. The diaphragm (E) is flexed forwards, and seals against the barrel (A). (Alternatively, one can use a spring to move the piston forwards, which allows one to fill [slowly at first, to allow the pressure to equalize] from the chamber.)
  3. More air is added, and leaks around the diaphragm (or through a small equalization hole) and into the chamber (B), filling it to the desired pressure.
  4. The pilot valve (D) behind the diaphragm is opened, the pressure in the pilot volume (C) drops, and the higher pressure in the chamber area pushes the diaphragm away from the barrel.
  5. The air flows around from the chamber, and into the barrel, propelling the projectile.

The performance of these valves can be calculated with GGDT. If your valve honks, it is probably a good idea to invest in a better pilot valve.

The following is contributed by Technician1002 A sprinkler valve is a commercial diaphragm valve with an electric pilot solonoid. These are relatively inexpensive in smaller sizes and are popular with spudders who don't wish to build a fast valve.

Sprinkler valves come in 2 configurations.

  1. One is configured like a barrel sealer with the outlet in the center of the diaphragm. These can be identified by the flow source feeding to the outer area of the diaphram and the outlet is fed from the center of the diaphragm. These have the equalization hole at the edge of the diaphragm to pressurize the pilot area.
  2. The second configuration is used in most sprinkler valves. The flow is revered so they resemble a chamber sealer valve in operation. These are identified by the source side going to the center of the diaphram. The diaphram on these have the equalization hole in the center of the diaphram to pressurise the pilot area. This latter type is a favorite with spudders. The large area outside the valve seat make these open quickly when modified for a large pilot valve

Both sprinkler valve configurations use a pilot valve that opens a path from the pilot area to the discharge side of the valve. This is often an electric solonoid, but may be manual or motor driven.

To stop honking and improve the speed, both types are commonly modified by removing the original pilot valve and plugging up the port to the discharge side. A new larger pilot is added by drilling a hole into the pilot area for a blowgun or ball valve to vent the pilot area to the outside much faster than the original pilot was designed to do.