What is a diaphragm or membrane valve ?
Diaphragm valves (or membrane valves) consists of a valve body with two or more ports, an elastomeric diaphragm which is just a specific material that has elastic properties. The other parts are a "weir" or saddle or seat - the diaphragm closes on this part of the valve. The valve body may be constructed from plastic, metal or other materials depending on the intended use.
Types of diaphragm valves ?
There are 2 main categories of diaphragm valves: one type seals over a "weir" (saddle) and the other seals over a seat, these are sometimes called "straight-through diaphragm" valves. In general, straight-through diaphragm valves are used in cases where the flow will be on or off. Whereas weir-type diaphragm valves are preferred for control or throttling applications. While diaphragm valves usually come in two-port forms.