Whew!! That's a lot of change for a little turned down bolt with a knurled head and locking ring. The welch unit does have a nice nickel plated hex body and an extension pipe. But even so...
I used to have a direct drive pump which had a solenoid operated ballast valve. That was quite convenient. I think it was not easily adjustable.. probably had a small drilled port. So that suggests one alternative here... use a venting type socket head set screw. It would take a little hex wrench to set it.. but unless you are doing the ballasting every day...probably would be worth it.
I am working on a simplified version of the Welch design which would just drill and tap the existing cap screw for a 10-32, 1/4-28 or M6x1.0 screw, about 3/4 of the way down the cap screw axis. The remaining distance would be a 1 mm hole (or smaller).
A small O ring would go at the bottom of the threaded hole. The valve itself is made from a machine screw with a turned down end and needle point (about 20 - 30 deg cone angle. The valve screw has an axial 1 mm hole that stops at a similarly sized cross drilled hole at the top of the needle cone. The turned down end is sized to just cut off the root of the screw threads. This should give adequate passage for the gas bleed.
I will make one and put a dwg up (if it works well enough) in case anyone else might be interested. You really don't even need a lathe... a drill press and a file will work tolerably well. This design would allow you to set the ballasting rate with your fingers just as the normal Welch hardware allows.
Dave Cooper
Created on Sunday, April 08, 2001 9:18 PM EDT by David Cooper