---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Thu 05/06/21: 1 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 07:18 PM - Re: Re: Plumbing Instruments (Robert L. Nuckolls, III) ________________________________ Message 1 _____________________________________ Time: 07:18:39 PM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: 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?" ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message aeroelectric-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/AeroElectric-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/aeroelectric-list Browse Digests http://www.matronics.com/digest/aeroelectric-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.