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I am designing a flange to be use to maintain a pressure of 10-2 Torr for at least 3 months. I am using an o-ring seal from my (device) to the product weldment. I am looking for feedback on using NPT fittings in a vacuum system. I have heard it is unwise. However what I find readily available online is only NPT fitted gauges. I was thinking of using Teflon tape or Loctite. I should add there is a fitting I am using to adapt from NPT to conflat which will be welded.

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Are you going to continue pumping or have intermittent pumping ? As a engineering professor of sealing once told me "everything leaks, it is a matter of how much". Without continued pumping you will not hold that vacuum. For serious leak tight construction I have mostly seen Swageloc connections. NPT has no metal to metal seal , I think metal/metal will be necessary depending on pump.For a start use as many welds as possible to minimize the threaded connections. American Petroleum Institute (API) pipe dope has metal powders in it to seal threads like NPT for moderate pressures so it does not depend on grease-like stuff ( regular pipe dope) to stop leaks; it may not be easy for you to find.

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TLDR: NPT and vacuum - no problem - use correct type. NPT and o-ring - possible (likely) problem landing o-ring if you use a tapered thread style.

Please post a sketch so we know the size of the thing and how the o-ring relates to your flange.

NPT can be one of a dozen or so thread types. Some make sense and some don't.

The most common NPT type is a tapered thread that has an interference fit on the crest/valley of the mating threads, but has a spiral leak path around the threads. It is also not produced accurate enough to land an o-ring due to differing thread depths cut into different parts. If you are making both mating pieces, this isn't really a problem - you can jig it up so you are consistent. But if you only have control over one of the pieces in the joint, no, go with a non-tapered thread style.

You can use any of several "dry seal" NPT styles intended for fuels which don't have the spiral leak path.

You can use any straight thread type plus your o-ring to accomplish the seal.

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