Tube Fitting Installation
The following concerns the use of polymer tubing and brass/stainless steel fittings in our lab for air, N2, and vacuum lines. In sum, please be mindful of the fitting type you are using when deciding to use or to not use Teflon tape. This sounds minor but WILL prevent damage to our fittings and devices over time. I hope this helps and isn't too elementary for anyone.
Compression fittings (also known by Swagelok)
These are the brass/stainless steel threaded fittings used to connect flexible tubing to other fittings (T-fittings, reducers, compression-to-NPT, etc.) and to equipment (spin coater, etc.). They consist of a threaded nut, a cone-shaped front ferrule, and a back ferrule with a step feature. The two ferrules fit over the tube and the threaded nut then "compresses" the ferrules into the tube when tightened onto the receiving fitting.
It is very important that, when using a compression fitting, teflon tape should not be used. If we take a cross-section of the compression fitting in place, you would see that the air-tight seal is made when the cone-shaped ferrule is compressed into the receiving fitting, NOT by the threaded nut. In short, Teflon tape is used when the threads of a nut are meant to provide the seal. This is not the case here and, in fact, the Teflon tape could create gaps in the threads and actually loosen the seal causing a leak. The leak would be slow and likely undetected but would cause the loss of vacuum pressure, nitrogen, etc. over time. Also, compression fittings should never be over-tightened. As a general rule, make the connection finger-tight, then tighten with a wrench no more than 3/4 of a turn. Remember, the threaded nut is not forming the seal, the ferrule is. Over-tightening can deform the cone-shaped ferrule and cause leaks! This rule is just as important as the "no Teflon tape" rule.
NPT (National Pipe Thread)
These are fittings are brass/stainless steel threaded fittings used to connect rigid pipe and other brass/stainless steel parts together. While not specifically NPT, you can think of such things as connecting a shower head to the water pipe, connecting the end of a hose onto a hose bib, or even screwing a lid onto a jar of peanut butter as good examples. In these cases, the threads actually serve to make the water- or air-tight seal.
It is very important that, when using NPT or similar fittings, teflon tape should be used. In these cases, Teflon tape serves to fill the small gaps formed between the male and female threads of the fittings. In short, unlike compression fittings, NPT-type fittings are tapered, and the Teflon tape (or pipe sealant, etc.) plays an important role. With these fittings, over-tightening can still occur. There is not a general rule about how many turns to apply, just do NOT use your superhuman strength to tighten them. A few turns beyond finger-tight will usually do the job.