This post breaks down the real world differences between NPTand G-thread (BSPP), why flush diaphragm transmitters shouldn’t be NPT, and the gotchas that can wreck a perfectly good install.
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Threaded Process Connections Explained: NPT vs. G Thread
The Big Picture
- NPT is a tapered pipe thread (60°). It seals on the threads using PTFE tape or pipe dope. You tighten until the taper wedges enough to seal.
- G thread (BSPP) is a parallel thread (55°). It seals on the face or shoulder with a gasket or O ring—the threads hold, the face or shoulder seals. Two common interfaces you’ll see on instrumentation:
- EN 837 (gauge style): flat face + crush/flat gasket (copper, fiber, elastomer).
- DIN 3852 (Form A/B): shoulder stop with sealing ring (A) or O ring groove (B).
Translation: NPT = thread seal. G thread = face/shoulder seal. They are not interchangeable.
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NPT in Practice
NPT is everywhere in legacy North American piping. It’s simple and rugged, but two realities bite engineers:
- Seal quality depends on technique. Clean threads, correct tape/dope, and torque discipline matter. Get any of that wrong, and you’ll chase micro leaks.
- Flush diaphragm could be crushed. Because the diaphragm is located at the end of the process connection of flush mounted sensors the diaphragm could be crushed by the force applied to the threads to seal.
We offer 1/2 14 NPT and 1/4 18 NPT options on many non flush BD SENSORS transmitters for plants standardized on NPT. But when media or hygiene demands a flush diaphragm, NPT is not the move (see below).
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G-Thread (BSPP) in Practice
G-thread’s parallel geometry means you’re not wedging a taper to seal—you’re compressing a gasket/O-ring against a defined metal face or shoulder. That gives you:
- Serviceable reseals (swap the gasket/O-ring),
- Cleaner leak paths (face seal is easy to inspect).
In our catalog you’ll see G1/2 and G1/4 offered as either EN 837 (flat-face + gasket) or DIN 3852 (Form A/B). Functionally both are BSPP threads; the difference is where and how the seal happens.
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Why Flush-Diaphragm Transmitters Should Not Be NPT
A flush-diaphragm transmitter places the sensing element right at the tip to avoid pockets that clog with viscous or crystallizing media. That design doesn’t play well with NPT’s thread seal:
- With NPT, you tighten deeper into the mating port to wedge the taper. So the final depth is not defined and could create pockets
- The axial force concentrates through the threads, not the shoulder.
- On a flush design, that can load the diaphragm cap—worst case, crush or deform the sensing element and shift the zero.
Bottom line: Flush designs need a shoulder sealing interface like the G-thread DIN 3852 (sealing ring/O-ring). That’s why we don’t pair flush diaphragms with NPT.
