Why does Duotight use two o-rings when competitors use one?

[kiwipen]

Instead of asking in the Duotight leak thread I'll just make a new thread.

Why Why does Duotight use two o-rings when competitors use one? Do they actually seal better than John Guest fittings? Are there other fittings with two o-rings? How about 3 o-rings?

 

[duncan.brown]

John Guest makes a "super seal" variant that has two o-rings and a screw thread that clamps them down. They recommend these for hard metal (e.g stainless steel) but the regular ones are fine with soft metals (e.g. copper) or plastic. FWIW, I'm 100% John Guest (regular on plastic, super seal on stainless) and have never had any leak issues.

Here's a picture of a super seal fitting. From top to bottom, there's a screw thread, a plastic ferrule, and two o-rings. The top screws into the body of the fitting.

 

[kiwipen]

Interesting. Are two o-ring needed when sealing around a stainless pipe?

 

[duncan.brown]

Are two o-ring needed when sealing around a stainless pipe?

For John Guest, yes. The web site says that the super seal fitting "provides extra grip on stainless steel and other hard polished metal tubing."

 

[day_trippr]

It would have been called "Monotight" if it only had one O-ring...

Cheers!

 

[Bobby_M]

Interesting. Are two o-ring needed when sealing around a stainless pipe?

It's not a simple answer. Both JG and Duotite have no problem sealing on a stainless tube as the orings have about the same contact patch no matter what the material. The difference is in the sharp metal pawls that lock the tubing into the fitting. They can't easily bite into stainless tubing and depending on the pressure can just blow the fittings right off. This can be solved by putting a radial score in the tubing at about 1/4" in from the end of the tubing so that the metal pawls just fall into that score line/groove. The JG superseal is a minor variation on a compression fitting rather than a push to connect.

 

[BBBF]

They can't easily bite into stainless tubing and depending on the pressure can just blow the fittings right off. This can be solved by putting a radial score in the tubing at about 1/4" in from the end of the tubing so that the metal pawls just fall into that score line/groove.

Dang... I wish I knew this six months ago. Could have saved me a lot of time and money that I spent on figuring out the best combination of compression fittings and NPT adapters. I'll have to keep it in mind for future projects.