Precisely, what I ask is: I pump magma up to z1, then let it flow through a diagonal channel, then let it down a channel on z0. Later, will it overflow to z1 or not?
That arrangement will flood with any fluid. The diagonal channel must be on z0.
And yes, diagonals depressurize magma just like water, but you have to do it right.
I don't think this is correct. Magma has pressure coming from a pump. If you pump magma to z1, and it goes through a diagonal, it is no longer under pressure and should not flow back to z1 again.
I don't have a setup that tests this, but I do have a setup where I breach my magma pipe on the bottom z level, the magma flows 3 squares on the bottom level, and then I use a pump to pressurize it on bottom+1. The pipe does not overflow up to my dwarf powered pumps because magma isn't pressured and so the 15z of magma above my channeled hole in to it doesn't matter.
Also, check out Kandrak's thread on hydrodynamics, it's a very good post on how the pressure works:
http://www.bay12games.com/forum/index.php?topic=32453.0With what was described by petersohn I see like this:
z1
XXXXX
XX.XX
XXX+X
X?XpX
XXXpX
XXX.X
XXXXX
z0
XXXXX
X+++X
X+XXX
X+XXX
X+XXX
XXX7X
XXX7X
X's are walls, p = pump, + = floor, . = channel , 7 =magma source, ?= channeled out square to test "pressure"
the pump will move mama from the south to the north, it will flow diagonal to the channel and fall if there is room below it. You'll have 2 squares of magma on that level... the output of the pump and the channeled square diagonal from it.
On the level below, everything connected to the channeled out square will fill to 7/7 magma, but there will not be pressure to push magma in to the ? square above. Once the system is "full" it would look like this (note that the ? tile is just open space over 7/7 magma)
z1
XXXXX
XX7XX
XXX7X
X.XpX
XXXpX
XXX.X
XXXXX
z0
XXXXX
X777X
X7XXX
X7XXX
X7XXX
XXX7X
XXX7X