The 25 Series was first introduced in 1959 as a versatile and simplistic family of self-acting pressure regulators with control pilots and interchangeable main valves for most steam applications. The interchangeable springs allow quick and easy change of downstream pressure range eliminating the need to isolate the reducing valve or break the steam line.  The internal components are made of stainless steel for excellent durability and resistance to corrosion.  The fatigue tested diaphragm is designed without a piston which prevents danger of sticking.
This move outlines the relationship between the spring compression, steam pressure, diaphragm chambers and pilot/main valve positions under various conditions.  While watching the movie, observe the pilot and main valve motions and spring compresses.  Notice the steam pressure reduces from red to orange, green, violet, and blue.

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