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Re: [TCML] 8" Secondary Woes

On 9/3/13 10:09 AM, AusTesla wrote:
I found the same thing.  That is why I am doing a 10" coil as mentioned just
previously, because this was the next size available up in PVC stormwater
from 6" here in Australia.  I fell into the same trap - the pipe is marked
as 225mm but I don't know for the life of me where they take that
measurement from - the inside diameter is greater than that at just under
240mm, and the outside diameter (5.5mm thick PVC) is very close to

So you can't even trust the markings on the pipe itself!  Luckily for me I
haven't wound my primary yet.

Well, you see, it's all about "trade" and "nominal" sizes. Somewhere back in the depths of time, they were making lead or clay pipe to some standard diameter (like curved roof tiles, the size of the tile maker's thigh). and they called that 8" because it happened to measure 8" on whatever ruler they had around.

Then King John standardized the inch as the length of three barley corns laid end to end. (I guess barley corn was more standardized, or it's a statistical thing.. beer making requires consistency, and barley is used make beer, etc).

The pipe actually measured 25 barley corns across, rather than the expected 24, but since they already had an investment in the tooling and fittings to make the stuff, they just called it "8 inch" and promulgated the standard, etc.

Not really, but that *is* how these things work. One size gets picked, then fittings are made for it, then someone else makes a pipe/tube with different wall thickness, so the OD has to be the same to match the fittings, etc.

There's a fascinating article out there about the history of the 2x4 and its dimensions (it has NEVER been 2 by 4 inches, apparently). Today, in the U.S., it's 1.5x3.5 inches, but it used to be 1 5/8x3 5/8 or some such. And it's nominally depending on the saw kerf in a sawmill of some sort, alomg with the planer to make smooth surfaces. (rough sawn lumber is closer to nominal size, e.g. a rough 4x12 is pretty close to 4x12 inches)

Let's not get started on metal gauges, screw threads, and lead shot sizes.

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