Ah-ha, then my usual solution is a few floodgates. If you set a row or two of floodgates along the inside of the chamber from one door to another, then link them all to a lever, you can close and open them to instantly atom-smash any liquids. Bridges work as well.
-attempting to build a bridge over top of a pressure plate returns the error "Building Present" and prevents construction.
-attempting to build a pressure plate under a retracted bridge returns the error "Building Present" and prevents construction.
-attempting to build a floodgate over top of a pressure plate returns the error "Building Present" and prevents construction.
-attempting to build a pressure plate under a raised floodgate returns the error "Building Present" and prevents construction.
Currently the entire floor of the grand goblin melter, as an example, are retracting bridges. Everywhere is a bridge-floor except the pressure plates. Hence the problem.
Even if I took out the bridge floors or grate floors, the problem would still exist on the pressure plate tiles inside the hallway.
For 34.07, I've verified the following is still true:
- Screw pumps will not pump all the liquid off of a tile (down to zero), they'll just pump to 1/7.
- Non-flying creatures fall through hanging pressure plates.
I think you need to verify those things again, because the first isn't true. A pump won't pump 1/7-depth fluid, but if there's more than that, it will pump the whole block. Obviously, you simply need to arrange for the entire hallway to be filled to 2/7 depth or more, then arrange for the entire hallway to be pumped out simultaneously to prevent any 1/7-depth tiles from appearing. Your flood-system priming device should involve a line of raised drawbridges to narrow the hallway to a width of 2, fill from the side or below through grates, and empty out the top, using 2 lines of pumps pumping to opposite sides of the device with hanging floodgates or doors to block their intakes (so they always drain when the thing is open and not when it's filled). You can work the whole thing with 2 levers, 1 to prime it for use pulled whenever a siege is on-map and the other to fire.
Edit: Hmm... I can see how to make the device with levers, but with pressure-plate-driven system would require the use of a fluid-logic inverter. Simply priming the device with a lever pull when the vile force of darkness message appears is one thing, but I think you want it automated beyond that, and adding an inverter complicates things, especially as I've never used one As I've been working with mods that heavily use trap-avoiding enemies for 2 1/2+ years, I've not really developed skills for pressure plate use, instead using lever-triggered architectural traps.
-The rooms I've pumped out are filled with 7/7 (right up to the pump itself, as well) magma, and they are not cleared down to 0/7 100% of the time. Sometimes they are, sometimes they aren't. I'm looking for a technique that is reliable. You're correct that a pump will pull the whole liquid, but at the end of the pumping, adjacent 2/7 squares will flow onto the pumped tile, making a 1/7 adjacent to the 2/7. This of course can happen up to 8 times (once for every adjacent tile) on every tile being pumped, which results in it happening too often.
You're correct though, I'm looking for a technique or solution that will fit into a completely automated system. Doing this with levers is very straightforward, but still suffers from the pathing block due to magma 1/7 on the pressure plates.
since 1/7 water is easily walkable I'll assume your working with magma...
To easily clear 1/7 magma without waiting for evaporation well what about flooding the hall with WATER after you get the magma to 1/7 that should make a Obsidian floor rather than a wall which should HOPEFULLY not destroy the pressure plates but I've never tested it myself.
-This might be a good solution, except for two problems. How to ensure only a little water gets on the tiles with the pressure plates? And also, where does the water drain, compared to where does the magma drain? In the worst case, they both drain to the same place, creating obsidian walls everywhere. An option, however, may be a mist generator above the pressure plates. I don't know the interaction of mist with 1/7 magma, so I will test that. It will still be challenging to automate the temporary firing of a mist generator, but I think it could be integrated, if all it does is create obsidian floor.
Thanks for the responses so far, I appreciate the brains and eyes.