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Anybody Here Familiar With Pipe Thread Standards?

Visitor

Well-Known Member
This question is kind of electronics-related, since it's about the threads on Chinese flow sensors, and I know there's an abundance of experience here....

I'm looking at using a flow sensor in a project, probably one with connections G3/4" (or possibly G1/2"). Since I'm in the US and we have to do everything our own way, we use NPT fittings. Looking at Amazon and ebay, adapters are available, for about $15 each (so $30 of fittings to connect a $10 flow meter).

It's confusing trying to figure out the exact details of a 'G3/4" ' thread when comparing specs, particularly about tapered vs. straight threads. I suspect G3/4" use a straight thread and an o-ring to seal?

I am in Mexico for a couple more weeks and I've been looking at when threads are in use here. It seems to be a mix of NPT and G threads, so adapters might be common here. One option I see is to get G3/4" female copper sweat fittings and make adapters.

Any good ideas?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
G sizes are parallel, the same as "BSP" parallel threads. The stated size is approximately the pipe bore they were originally used with.

Sealing would be either something like a copper or fibre washer, or a "Dowty seal" which is like a washer with a lipped seal bonded to the internal edge. These have the advantage they do not have to be extremely tight to seal but they can handle extreme pressures.

 

rjenkinsgb

Well-Known Member
Most Helpful Member
From a quick look, G1/2 / G3/4 and 1/2 or 3/4 NPT have the same pitch and nearly the same diameter. Depending which way the adaption is, you may get away with a direct fit plus some loctite or similar thread sealer.

 

Les Jones

Well-Known Member
Most Helpful Member
I have attached some information on BSP threads. I think it should give yoy all the information you need on 3/4" BSP dimensions.

Les.
 

Attachments

Visitor

Well-Known Member
Thanks Gentlemen, that helps my education along. The seal washers are something I've never seen and wouldn't have thought to get if I purchase other fittings.

Here's a picture of the type of flow meter I'm looking at. These things appear to have pretty good engineering for the $10 price. The impeller spins a magnet, which is picked up by an external Hall effect sensor, so no seal to worry about.

flow meter.jpg
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks Gentlemen, that helps my education along. The seal washers are something I've never seen and wouldn't have thought to get if I purchase other fittings.

Here's a picture of the type of flow meter I'm looking at. These things appear to have pretty good engineering for the $10 price. The impeller spins a magnet, which is picked up by an external Hall effect sensor, so no seal to worry about.

View attachment 130704
Yes, I've played with that exact sensor - it simply connects with the old UK screw-on hose fittings - but everything now seems to be Hoselock, and 3/4" to Hoselock fittings are commonplace.

Here are the ones we use:

https://cpc.farnell.com/hozelock/2175/outdoor-tap-garden-hose-connector/dp/SI15639 there's a rubber seal in the adaptor, so no extra one required.

We don't use the white part, but I can't bring myself to throw them away, so I've got a fair pile of them now. Just don't lose the rubber seal when you unscrew the white part (and not notice) as it WILL leak with out the rubber seal.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Searching Amazon.com for something to fit the flow meter produces adapters for 3/4 "hose thread" - I think Les has found the compatible fittings, apparently garden hose connections in the US use 3/4" BSP the same as in the UK?

You may also find that a 3/4 NPT barrel connector fits straight on the plastic thread, though?

3/4" is also the standard for bath tap plumbing fittings in the UK - I have no idea what is used in the USA; are those also teh same?
eg.

ps. Those dowty seal washers are intended to go outside the thread of a fitting. Internal washer fittings generally use either rubber or fibre seals.
 

Les Jones

Well-Known Member
Most Helpful Member
Another source of these connections is the water feed hose to washing machines.

Les.
 

Visitor

Well-Known Member
I was just looking at garden hose fittings. Again, the US has to do things its own way.

From Wikipedia:

" In other countries, a British Standard Pipe (BSP) thread is used, which is 3⁄4 inch (19 mm) and 14 TPI (male part outside diameter is 26.441 mm or 1.04 in). The GHT and BSP standards are not compatible, and attempting to connect a GHT hose to a BSP fitting, or vice versa, will damage the threads. "
 
Last edited:

Visitor

Well-Known Member
G3/4 is a US garden hose connector.
Not so according to Wikipedia, as I posted in post #10.

" In other countries, a British Standard Pipe (BSP) thread is used, which is 3⁄4 inch (19 mm) and 14 TPI (male part outside diameter is 26.441 mm or 1.04 in). The GHT and BSP standards are not compatible, and attempting to connect a GHT hose to a BSP fitting, or vice versa, will damage the threads. "
 

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