It's that time of year again, and the temperature is expected to drop from 65F this afternoon to 19F at dawn tomorrow morning.
I decided to replace the pipe insulation on the 2 inches of exposed pipe in the back and 8 inches of exposed pipe in the front.
I took out my calipers and measured the pipe as almost exactly 1 inch.
But I get to the hardware store and first they hardly have any pipe insulation for 1" piple (one piece left) and it looks too big anyway.
I ask the assistant and he says a little air won't hurt, I should get the correct size for my pipe.
I ask some customer whether pipe is determined by the inside diameter or the outside diameter.
He said "I wish I could figure that. I'm just a carpenter. It makes no sense to me."
One little clue was seen in the online descriptions. A particular size was good for one size copper pipe and another size iron pipe.
How can that be, if it's determined by outside diameter?
It's because pipe people have long had "nominal" sizes, which are "determined" by the outside diameter being some predetermined difference from the measured size.
So a "1 inch pipe" would actually have an outside diameter of 1.315 inches...regardless of the inside diameter.
So when I measure an outside diameter of 1 inch, which is determinative, and it's iron pipe, that means it's a 3/4 inch iron pipe. (To be exact, 3/4 inch iron pipe has an outside diameter of 1.05 inches.)
These numbers almost look like the inside diameter, and that's how I'm going to think of it (as almost looking like the inside diameter).
Fortunately I also bought the 3/4 inch pipe insulation, which fit snugly as it is supposed to.
