Originally Posted by a1lawng
It seems like these numbers are all over the place. I've found several different resistance values and I've been calculating based on the numbers listed here
Shouldn't the resistance be calculated based on the ID instead of OD since there are different wall thicknesses on different products?
I touched on this in an earlier reply, but there are a couple reasons the resistance figures are all over the place. One is that the actual resistance of a line will vary quite a bit between different brands/manufacturers, and sometimes even between different production runs at the same factory. Many of those charts just list a rough average, and some actually show a range for each type of line.
The second reason is that line resistance is not a fixed figure like those charts, calculators, and line balancing articles would lead you to believe. It's actually highly variable based on the fluid velocity. The slower the fluid velocity, the lower the resistance will be. That's why doubling the line length will only add a couple seconds to the pour rather than result in a tiny trickle from the faucet.
The problem with using those resistance figures, calculators, and equations is that they all figure a pour speed of ~1 gal/min. That works ok for commercial systems where the beer is kept extremely cold and the carb levels under ~2.9 vol, but many of us homebrewers like to serve our beer warmer, and some like to use higher carb levels for some beers. If you try to serve 45° beer or a beer carbed to 3+ vol that fast you'll end with a glass of foam. If you need to slow the flow down below 1 gal/min for any reason, all of those figures and equations become useless.
And the line resistance is absolutely calculated using the ID of the line, but they list the stainless lines by OD since that's how they're commonly referenced.