On 4 Nov 2020 at 20:42:39 GMT, "Peter Burke" wrote:

"NY" wrote in :


I never understood why the change from imperial to metric was
accompanied by a change in measuring internal versus external diameter
of pipe. Or at least, why imperial denoted the inside diameter of the
pipe.


Old school emphasis was on the carrying capacity of the pipe/tube whether
it be lead, copper or iron, the inner dia, new age thinking is the
compatibility of the fittings, the outside dia. We now have multiple 15mm
pipes/tubes in materials with different carrying capacities, plastic with
thickers walls and restrictions of internal stiffeners at joints will not
carry the same volumes for a given pressure drop as copper.


I didn't realise that solder and compression fittings for 1/2"
(nominal OD 15.9 mm) and 3/4" (nominal OD 22.2 mm) would sometimes
accept 15 and 22 mm pipe respectively. A difference of 0.9 mm for 15
mm pipe is quite a large amount for an olive or solder joint to
bridge.


Your imperial pipe dims don't seem right, an imperial 3/4" olive is
thicker than a 22mm olive to compensate for the _smaller_ o/d of a 3/4"
pipe.

A quick search shows:

3/4" imperial, 0.846" o/d == 21.49mm, too much diff for a 22mm olive
hence the availability of 3/4" olives but perhaps ok on 22mm solder but
prob only on end feed with extra solder.

1/2" imperial, 0.596" o/d == 15.14 mm so basically compatible with 15mm
fittings, this is readily accepted as the case

IME it took a *lot* of work with steel wool on 15mm end feed fittings to get
the 1/2" pipe to go in at all. (Only applicable at the weekend when convertor
ones weren't handy.)
--
Roger Hayter