NPT vs. ID
 

just wondering...
say there is a barb 5/8"IDx3/8"NPT.

does the 3/8"NPT size mean that the ID is actually only 3/8"!?

if not, then what is the relationship between ID of a hose barb and the NPT size.

thanks

Heheheh, I do not know how many times I have seen this post.

Defined NPT: "National Pipe Thread." A specification for tapered pipe threads from ANSI (American National Standards Institute). Actually taken from American National Standard Taper Pipe Threads. When listed as NPT-F, indicates female ends.

As the definition states the inside Inside Diameter taperes, therefore not a constant Inside Diameter. Also depending on what is on the other side of that NPT fitting, your inside diameter might be either larger or smaller.

_________________________________________________
Pipe Thread Size | Threads Per Inch | Theoretical I.D. of Ftg.
||||||||||||| 1/16'' |||||||||||||||||||||||| 27 ||||||||||||||||||||||||| 1/16'' ||||||||||||||||
|||||||||||||| 1/8'' ||||||||||||||||||||||||| 27 |||||||||||||||||||||||||| 1/8'' ||||||||||||||||
|||||||||||||| 1/4'' ||||||||||||||||||||||||| 18 |||||||||||||||||||||||||| 1/4'' ||||||||||||||||
|||||||||||||| 3/8'' ||||||||||||||||||||||||| 18 ||||||||||||||||||||||||| 3/8'' |||||||||||||||||
|||||||||||||| 1/2'' ||||||||||||||||||||||||| 14 ||||||||||||||||||||||||| 1/2'' |||||||||||||||||
|||||||||||||| 3/4'' ||||||||||||||||||||||||| 14 ||||||||||||||||||||||||| 3/4'' |||||||||||||||||
|||||||||||||||| 1'' |||||||||||||||||||||||| 11-1/2 ||||||||||||||||||||||| 1'' ||||||||||||||||||
||||||||||||| 1-1/4'' |||||||||||||||||||| 11-1/2 ||||||||||||||||||| 1-1/4'' |||||||||||||||
||||||||||||| 1-1/2'' |||||||||||||||||||| 11-1/2 ||||||||||||||||||| 1-1/2'' |||||||||||||||
||||||||||||||| 2'' ||||||||||||||||||||||||| 11-1/2 |||||||||||||||||||||||| 2'' |||||||||||||||||
_________________________________________________

To answer your question:
As the table indicated the "Theoretical ID" is indeed 3/8". There is no relationship as they are both independent. Whichever is a smaller measurement will determine your max throughput through the fitting.

PS: Html code would of been nice to make a table. Maybe vB code has a table option but the help on the bottom didn't show any.

3/8NPT is about 5/8 wide actually. The 3/8 just references the threading, not the width of the pipe (though the two are proportional).

From another search i found this site: http://www.russellperformance.com/

A quote from their site:

"While on the subject of pipe threads, we need to pass along that the nomenclature used for specifying pipe thread sizes can be quite confusing to those not familiar with pipe terminology. When a pipe thread size is given, such as 3/8" NPT (National Pipe Thread), the 3/8" refers to the inside diameter of the pipe."

I have also measured various fittings and have found the above to be true.

The ouside diameter on the threaded area is indeed 5/8", but that wasn't what he was asking.

Brian W

I don't agree Brian.

All my 1/2" fittings are 1/4" NPT Thread yet the internal diameter is well over 1/4".

Issues such as this are always dificult to convey what you mean, as often its up to the reader as to the exact meaning.

My barbs are usually referred to (By the vendors) as 1/4" NPT thread to 1/2" barb.

what exactly IS the internal diameter? in that case?
thanks.

BrianW - is that true for male NPT sizes too?

The Male NPT internal diameter will be the Outside diameter of the female NPT fitting. However it doesn't really matter because the smallest diameter will be the Inside Diameter of the female piece.

Brian W

Edit: Update:

Hehe I just realized that you asked if that applied to male NPT, and the whole time we were talking about male NPT. Hahahaha Any way if you reverse the Male and the Female in what I have said above, it will be correct. Hahahah

Here I will try again: The Female NPT Inside diameter will be the Outside diameter of the Male NPT fitting. However it doesn't really matter because the smallest diameter will be the Inside Diameter of the Male piece.

Thanks, and I apologize for my dumbness.

Brian W

Thats only true if you use the female piece, which no one does. All barbs are just screwed into a threaded hole.

And that Female threaded hole is the same diameter as the outside threads on the male NPT. That is that same thing. Heheh.

Brian W

Edit due to ignorance in above mentioned post.... I am sorry I wasn't reading correctly.

so now, just to clarify: the NPT size of a normal (male) hose barb refers to the internal diameter of the barb?

Not of the barb part, but of the NPT part. You see there are two different parts of a Male threaded NPT Hose Barb. Both parts can have independednt sizes. For instance you can have a 1/4" NPT 3/8" Barb. What that is is a fitting with 1/4" Male NPT threads on one side and a 3/8" Barb to accept Hose on the other. The ID of the Threaded part will be around 1/4"+. The barb side will accept 3/8" ID hose, and its internal diameter will be slightly less than 3/8".

Brian W

ok...
well basically my goal is to not have restrictive hose barbs.

so when you say
what exactly does that mean?
the id of the threaded part is the most restrictive part of the barb in the case of 1/4NPT. thats what im talking about when i say "ID of the hose barb"

Yes you are correct. Whichever is of the two ID's is smallest will be your restriction.

What are you screwing these into? It would help to know what context you will be using them.

Brian W

Additional resource on NPT: http://www.tflx.com/pdf/nptsizes.pdf

Brian W

i dont really have any barbs right now. its not a matter of screwing them in... im just concerned that the whole idea of using 1/2" ID tube will be defeated when the barb ID is only 3/8" or 1/4", if you see what i mean.
at this point, i understand that the NPT size of a hose barb for use with 1/2" ID tubing should be 1/2"NPT. now the problem is, how to get parts that will FIT 1/2"NPT barbs. right now, this looks pretty difficult because the OD of a 1/2"NPT barb is a whopping .84", almost 7/8". but, with a poweful pump like the md30rz, i dont want restrict the system farther than 1/2" (except at the nozzle plate for the WW)

You cannot change the Female threads on the pump. Just buy your pump, take it with you to the hardware store and play with fittings until you find what will work best for your particular setup.

I use 1/2" ID 3/4" OD silicon tubing throughout. I have a Eheim 1250 and I use 1/2" PVC and 1/2 " Copper tubing for my connections. I have a Maze 2 which uses 1/4" NPT 1/2" Barb. In my system the block is most restrictive, ie fastest part of loop. That is ideal. Keep the rest of your system wide and keep your block restrictive.

Brian W

Approx 7.5mm to 8mm.

The inside of the barb does not narrow or get bigger at either end, it is a constant size.

I was sure that the 1/4" referred to the thread size, not the inside of the barb. You could get barbs with big variance in wall thickness, therefore 1/4" cant really apply to the inside of the barb, only the constant size of the thread.

wel there can be different barbs sizes with different NPT sizes.
for example:

1/2" x 1/4 NPT
1/2" x 3/8 NPT
1/2" x 1/2 NPT

the ID of the THREADED section of the barb varies with the NPT size (according to BrianW's sources).
but since the water has to flow through that section anyways, a small NPT size means more flow restriction.

Okay, I'm using SUN brand fittings with a 1/2" barb to 3/8" BSP threads.

Not sure how BSP differs from NPT.

I can very much assure you that the barbed section of the fittings have an inner diameter of 10.5mm (roughly 13/32")

The threaded section of the fittings have an inner diameter of just over 13mm (about 13.1mm or roughly 33/64")

Then again, I've also seen 1/2" barb to 3/8" BSP fittings with just 9mm ID on the barbs (3/8") and 11mm ID on the inner thread diameter (7/16").

I have yet to see any 3/8" BSP thread fitting have an inner diameter of the threaded section less than about 11mm.

It seems to me that barb manufactures don't completely follow the rules. 1/4 npt fittings have a .504" tappered outside diameter. Since its the outside diameter thats critical for the fit, they use the 1/4 npt label even though the I.D. is 3/8". 3/8NPT barbs have a .675 outer threaded diameter while the I.D. may be 1/2" or whatever the manufacture chooses.

While this is a complete guess, it's the only thing that makes sense to me.

yah its quite confusing.
i guess the only real way to find out is to actually measure a barb (like Cathar did)

Actually, Cathar just reminded me, Im using BSP as NPT is very hard to get (My tap is NPT I think). However Its only 1 thread per inch or somthing difference.

Cathar are you still going to use 1/2" to 3/8" for your smaller blocks?

No, 1/2" to 1/4" BSP.

Yeah, NPT has a different thread pitch to BSP, just to make things a little more complicated.

Australia, being a British Commonwealth nation, uses the BSP standard.

It's one of those right side/left side of the road driving things.

I've seen substantial variations in the ID of 1/2" Barb X 1/4" NPT parts.

Unless the restriction is likely to be of some value, I just run a 3/8" drill through them.