How is primary length measured?

[rbogh901]

Guys, hey, can someone explain a basic concept I've been hunting: How is primary length measured? I assume it would be down the middle of the tube and be an average of the shortest and longest distances within a primary.


Does it end at the merge or at the mid confluence of the gases?

In other words, when primary length is specified how is it measured?
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Maybe it's been addressed here but I haven't found it. Some great discussions overall but this one eludes me.

[rbogh901]

delete.