Plumbing notes (yes, really plumbing)

This post was written by eli on June 3, 2019
Posted Under: offtopic


FPGA and Linux and all that hi-tech stuff is nice, but nothing compares to the self pride of getting a simple plumbing job done right. So this time it was about installing a pressure gauge under a bathroom sink, between the water outlet for the faucet’s cold water and the faucet itself.

Pressure gauge and tee

No need to tell me the reading is valid only if the taps are all closed. Anyhow, this is a simple task if you happen to have a Tee fitting that happens to match the stuff that is supposed to connect to it. If not, go find adapters. Or another Tee. If you’re a plumber, you probably have a large box with stuff to try around. As I’m not (the pipelines I usually deal with are digital register pipelines), the trick is to define the exact parts needed, and find them on AliExpress or Ebay. Or maybe even at the hardware store. The latter option turned out pretty difficult, as these parts are cheap, the motivation to help is accordingly, and if I can’t define what I need exactly, it’s a lost battle. So I ordered the stuff from AliExpress eventually. And I got it right.

So here are the notes to myself for the next time I’ll need to do something similar.

The standards

Spoiler: In Israel, everything follows BSP. A former British colony, after all.

Pluming is a local thing, performed by local people, depending on their local hardware stores, with a “give me that thingy” kind of communication. It’s therefore quite difficult to find exact definitions for plumbing fittings. It goes by “see if it fits”.

For threaded fittings, terms like ½” and ¾” are often used, but they refer to nothing measured on the piece of metal itself. These figures used to tell the inner diameter of the pipe itself, but that doesn’t work anymore. So if you want to measure a fitting and tell what it’s called in the market, you need to go to the standard tables.

And here comes the real fun. There are mainly two standards for pipe sizes, which detail the dimensions for the pipes and threads. It seems like the most common ones in Israel (and Europe) follow the British Standard (BSP), but there’s also American National Standard Pipe Thread standards (Often referred to as Nominal Pipe Size, NPS or National Pipe Taper / Thread, NPT).

Sometimes IPS (Iron Pipe Size) is mentioned, but it usually means NPS.

The two standards are incompatible, despite similar terminology and measures. In particular, the thread pitch doesn’t match between the two standards. But there’s also the thread form: American goes with Sellers, which has sharp edges, and British with Whitworth thread form, which is a sine wave shape. Not that I can tell the difference just by looking. So if you try to mix British with American fittings, screwing will probably be difficult, and it won’t hold pressure well, if at all.

Either way, for historical reasons, the inch number used in these standard matches none of the measured diameters of the pipe: Neither the inner or outer. The OD, outer diameter, is the easiest to measure, and should be compared with the standard.

So the main headache is BSP vs. NTP (or NPS, IPS, MIP, FIP and all other abbreviations meaning “American”).

And then we have this thing with DN sizes. For example, DN15 means ½”, and DN20 means ¾”. Even though these were coined for the American standard (and listed on Wikipedia’s page for NPS) it seems like they’re also used in context of BSP. So if a product is listed with a DN number, it probably means nothing on which BSP vs. NTP.

Actual measurements

This is what I measured on my own stuff.

  • The tap for my washing machine is a ¾” according to the manual, I measured 0.97″ outer diameter (1.05″ per standard). Apparently washing machines go BSP.
  • A typical shower head has a ½” fitting, not clear if American or British (for this size, it seems like NPS and BSP are roughly the same).
  • My supply stop valves (those wall taps for bathroom faucets) are 3/8″ (measured 0.64″ outer diameter). In Israel, these wall-mounted angle valves are called “NIL taps (ברז ניל)”, which most likely refers to the German company NIL, and therefore conforms to BSP.
  • The pressure gauge is an ¼” (measured outer diameter 0.5″).

Iron and brass

It’s pretty well known, that if brass fittings are used on iron pipes, or if these two metals are mixed in any other way, the iron will corrode rapidly (within a few years), as they work together as a battery. So the material is crucial.

In Israel, all valves, taps and faucets are made of chrome or nickel plated brass, and is therefore OK for use with brass Tees and adapters.

“Teflon” tape (or PTFE)

When there’s no rubber ring sealing, teflon tape is applied on the thread. It works better when there’s a hard end to the screwing, as the force of the end works on the thread and the teflon applied to it. But it can work well otherwise.

It’s 20 rounds around, or it won’t seal. Apply evenly on the thread with slight tension. The direction is as for the turn direction while fitting, i.e. the circular motion will tighten the “teflon” even more (and not unwind it). Don’t cover the few first threads (the end of the pipe) for easier fitting. If the fitting torque is easy all the way, it’s not going to seal.

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