On 2/24/2014 6:55 PM, Wayne.B wrote:
On Mon, 24 Feb 2014 18:05:10 -0500, "Mr. Luddite"
wrote:

On 2/24/2014 5:06 PM, Wayne.B wrote:
On Mon, 24 Feb 2014 16:46:48 -0500, wrote:

I am starting to prefer the teflon paste.

===

Yes. I just finished a fairly complex plumbing job on the boat (new
distribution manifold for 4 zones of A/C cooling water). It has more
than 15 individual pipe joints and is driven by a 1 hp pool pump so
there is lots of pressure and lots of opportunity for leaks. Knock
on wood, everything worked fine with no leaks first time it was
powered up. I've always used teflon tape previously but I've had my
share of failed joints with that.


Teflon tape is tricky to use properly. It is often used in the high
vacuum industry for all the feedthrough fittings that need to seal
against a vacuum equivalent to 200 miles in space to atmospheric
pressure. Too little tape, it leaks. To much it leaks. One secret is
to wrap it in the direction of the thread, so when you are tightening
the connection fitting, the tape is not being stretched back against itself.

We couldn't use Teflon paste because it never completely cures and would
outgas into the vacuum.


===

I had some experience with high vacuum work many years ago near the
beginning of my adult life. It was a high energy synchrotron
particle accelerator at Cornell University and had a magnet ring 1/2
mile in circumference. There was a vacuum chamber running through the
middle of the magnets where the actual particle acceleration took
place.

http://www.chess.cornell.edu/Outreac...sMacCHESS.html

http://www.cornell.edu/outreach/prog...programid=1801


The vacuum requirements in a system like that precludes use of o-rings
and teflon tape. They will seal, but they are permeable meaning the
smaller gas molecules will work their way through the teflon or o-ring
material. In systems like that, all metal seals are typically used.
They are usually copper rings used in a feedthrough called a "Con-Flat".
The fitting is stainless and has machined knife edges that, when
tightened, compress into the copper ring forming the seal.

High vacuum levels are hard to visualize. The best way to describe it
is "mean free path" which is the distance a gas molecule will travel
before crashing into another gas molecule. At atmospheric pressure,
that distance is so short to be almost immeasurable. At high vacuum
levels (like that of space) the mean free path distance is more like 3
feet or more. In ultra-high vacuum systems the distance is even greater.