The basic idea here is pretty simple.  An existing gas pressure regulator has the valve removed and the back tapped for 1/4" NPT.  We re-use the diapragm and adjustment bits, spring, etc and the diapragm acts on a reach rod, which lifts a ball off the seat several inches above.  This keeps the heat of the steam away from the diapragm, since the steam simply condenses in the 6" length of 1/4" pipe.  I used a 3/8" ball end-mill to cut the seat in the stainless plumbing bits for the ball.  The only difficulty I have with this is that it sometimes buzzes a bit; this is probably because there's water in the steam.