Are NPT & NPS close enough to work?

[strantor]

I've got 2 tanks on my HPU; a diesel tank and a hydraulic fluid tank. Both have 3/4"NPT bungs on them, and I want to install tank level transmitters on them. The only affordable thing I can find are these sensors which screw into a 3/4"NPS. I know the two NP variants have the same pitch and TPI; I believe the only difference is one is tapered and one isn't. I just want to be sure; if I order these sensors, will they screw into my tanks? I figure they won't make any kind of a seal, but this isn't a pressured connection, and I don't really care about having/not having a pressure seal.

 

[JimDawson]

It's been my experience that it will work fine. The non-tapered thread will screw into the tapered fitting because the thread is the about same size as the small end of the male tapered thread.

 

[John Hasler]

They are not considered interchangeable. In practice that plastic NPS adapter should screw into a NPT hole ok. However, I see no reason you couldn't replace the adapter they supply with your own. Call them and ask them about it.

 

[12bolts]

.....the thread is the same size as the small end of the tapered thread.

I disagree Jim. The smaller diameter of a tapered fitting is marginally less than the minor diameter of a comparable sized straight thread. However i agree that the the fitting will thread together. I would use a good dollop of PTFE goop as a sealant also.

Cheers Phil

 

[JimDawson]

.......same size as the small end of the tapered thread.

I disagree Jim. The smaller diameter of a tapered fitting is marginally less than the minor diameter of a comparable sized straight thread. However i agree that the the fitting will thread together. I would use a good dollop of PTFE goop as a sealant also.

Cheers Phil

Phil, you are of course correct, I was trying to say what you said, but didn't do it very well. What I meant to say was ....about same size as the small end of the male tapered thread. I need to quit trying to do 3 things at once.

 

[mzayd3]

I'll add my two cents.  I am an electrician.  Part of my job is process instrumentation and controls.  We routinely run into this problem.  All electrical fittings with a male thread (aside from threaded pipe ends) use straight threads.  All fittings with a female thread use tapered threads.  As far as instrumentation (what you are dealing with), most level transmitters use straight threads.  They work just fine in tapered threads and even hold pressure when properly sealed.  Go with it!

 

[Falcon67]

I sell fittings that adapt the ports in a Ford C4 to -AN braided lines. The trans ports are NPS and I re-thread 1/8-27 NPT fittings to work. The seal to the trans is by o-ring and not thread. Also, threading a NPT fitting into the trans may cause the fitting boss to split. Normal line pressure is around 30 psi but I have tested a hand tightened NPS/o-ring seal to 100 PSI.

YMMV as they say, but I would not use a NPS thread in an NPT port. You may only be able to seal the connection with JB Weld. My NPS tap passed over maybe the first 2-3 threads on a fitting, then begins cutting. If there is NO pressure and some bit of sealer, it MAY hold for some period of time.

 

[12bolts]

The OP's choice of level gauge is designed for screwing into the top of a drum. He is not looking for a pressure/leak tight seal. It will screw in to the tank he has and perform its intended function.

cheers Phil