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Newbie making a slow start with pump & gauge

Chris Thorne on Wed October 17, 2007 9:02 PM User is offline


It's cooling off here (which means that I can start to experiment with
auto AC DIY and not be under the gun from the wife to FIX IT NOW if
something doesn't go right).

I am running a test here with a Robinair 14830A gauge connected
directly to the inlet of a Robinair 15400 pump. After warming up both
pump and gauge, I opened the iso-valve to let the pump draw from
the hose to the gauge.

The display on the gauge ticked down within 15 seconds from the
highest (25000 micron) to the second-highest (2500 micron). After
another minute it ticked down again to 1300 micron. And then quit
dropping, sticking hard on 1300.

Leak? I tightened the cap on the unused part of the pump inlet.
Bingo! Display dropped pretty quickly to the 200 micron mark, and
it stayed there.

Upon closing the iso-valve between pump and gauge, the gauge
reading instantly returned to the highest 25000 micron level.

Hmmmm. I would parse that as being a leak in the connecting
hose or fittings. Am I on target with that?

My immediate instinct with any kind of threaded fitting which
holds against a pressure differential is to put Teflon tape on
the threads before installation. I also know that this is strictly
verboten in some applications which don't tolerate fragments
of tape. Would it be correct or incorrect to use it here?

Thanks in advance for illuminating my neophyte brain!

CT

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NickD on Thu October 18, 2007 7:08 AM User is offline

Try a half a drop of Nylog on the rubber, it binds when dry not giving a good seal, can the Teflon tape idea, you need a good seal between the rubber, not the threads. Threads are there to provide pressure.

HECAT on Thu October 18, 2007 9:04 AM User is offline

Teflon tape and pipe "dope" are for tapered pipe threads (NPT, BSPT) only! Technically, the teflon in these products acts more as a lubricant to achieve the necessary torque to provide for the proper metal to metal seal in the threads, than it does as being a sealer. Most any other type of liquid or gas thread connection will rely on a seal or sealing surface and straight threads to torque and maintain that seal. Using tape on these types of threads will have more of a potential to interfere with this seal, than it will to provide a solution for the leak. Follow Nick's advice and replace if necessary and lubricate the o-rings.

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