Intermediate Pressure

[JohnnyQuest]

I'm a little confused about intermediate pressure. Why do manufacturers specify an intermediate pressure at a certain tank pressure? For example, my Sherwood Magnums are supposed to have an intermediate pressure of about 145 psi at 2700 psi tank pressure. Shouldn't the intermediate pressure be independent of tank pressure by virtue of being a balanced regulator? And if the IP does change with tank pressure is there any chart that shows normal readings for various tank pressures?

 

[Lemonade]

Actually it does change. The better the reg designed and built the lesser the IP deviation. For most balanced ones the deviation is so negligible it can be ignored for all practical purposes, given the tank pressure is significantly higher than the desired IP. The closer tank pressure gets to the set IP the greater effects of tank pressure change are.

 

[redacted]

As you know, some piston 1st's are not balanced and IP may swing 10 to 15 psi as a function of tank pressure. Even balanced 1st's may not be perfectly balanced or may, by design, be intentionally left slightly unbalanced. I believe this is done with some balanced diaphrams, but I can't remember why. Guess I'll have to go back and read Harlow again.

 

[Rick Murchison]

The IP should be checked and set with basically a full tank to insure there's no problem with the high pressure seat. That and the slight deviation in IP over the reg's operational range make it wise to set a pressure near a full tank.
Rick