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1. 07:18 PM - Re: Re: Plumbing Instruments (Robert L. Nuckolls, III)
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| Subject: | Re: Plumbing Instruments |
At 09:47 PM 5/4/2021, you wrote:
>
>Hi Group After a nice job of plumbing I tried a leak test pulling
>altimeter to 1,000 from Zero. Leaks about 108 feet per minute.
>Shoot. Every one of my nothing installed Nylon fittings were
>leaking. A day job to pull them out (all barb fittings, tubing would
>need replacing). I tried some thin no stink silicone #3140 while
>pulling a 1,500 foot vacuum. Worked great. After 5 minutes I dropped
>pressure to 1,000 for half an hour, then let it go static and cure
>overnight. The fittings are from McMaster Carr and don't have a seam
>I can see from molding. I tightened them smartly. Ron P.
Good data!
Did a search of literature and manufactured offerings
on our favorite plumbing technologies . . . only
spent a couple hours on it but a simple and time-
honored understanding of the NTP threaded parts
was confirmed.
Making a leak proof joining of parts in fluids
is not unlike joining conductors of current.
The term "gas tight' is equally applicable
in both instances.
Studies of threaded fluid fastenings
reveals a potential for two, spiral leak paths
where thread tips and roots in the mated joint
do not come into solid physical contact.
99.99+ percent of such joints use
some form of malleable sealant material
that exploits the wedging action of a
tapered thread to compress and extrude
a sealant into small spiral voids that
exists between the 'imperfectly' mated
threads.
Unlike nuts and bolts with thread engagement
requirements that speak to strength (screw
should break before threads strip), tapered
threads on fluid fittings strive for
an interference or interruption, fluid-tight
fit of mated, wedge shaped parts.
Leak-free pitot static systems assembled
with 'dry fitted' parts depend on physical
deformation of the mated parts to achieve
zero clearance fits of the spiral leak
paths.
Here is one of several articles I found
that discussed technology of tapered threads.
http://www.aeroelectric.com/articles/Pipe_Threads.pdf
I did find some claims for plastic parts
that would achieve gas tightness without
additional sealants. The quality of such
joints assumes a knowledge of thread forms
for BOTH the parts along with confidence
in the long term dimensional stability
of the parts (did anyone say 'plastic').
I found only anecdotal claims of sealant-free
joining where one or both parts were plastic
of some variety. One writer said the seal
was not permanent; leaking developed with
time in service.
No doubt there are some plastic parts with
characteristics that will dry seal on assembly.
But unless specifically called out by the
manufacturer and proven in qualification testing,
the integrity of such joints is problematic.
If I were assembling my own panel today, I'm
pretty sure I would find confident comfort
in application of the technique suggested in
the 1978 Cessna service manual that calls out
Teflon tape covering all but the first two
threads of a male fitting.
Sometimes the best way to drive a nail is
with a hammer . . .
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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