It would be nice to create recipes for the Pressure Chamber in CraftTweaker that only craft under vacuum pressure (for example a replacement for the vanilla chorus fruit -> popped chorus recipe). I think the best way to implement this in the syntax without invalidating existing scripts would be to add an optional/default 5.0 fourth term which defines the maximum pressure for a recipe. That would also allow modpack makers to require say 3-3.5 bar for a recipe if they wanted to require a narrow pressure range but avoid the risk of explosion with pressure requirements close to the maximum.

Actually, if the pressure supplied in your CraftTweaker recipe is negative, it's already automatically treated as a maximum. E.g. a pressure of -0.5 will only allow crafting if the pressure chamber's pressure is -0.5 or lower. That probably needs to be documented in the CraftTweaker docs...

Requiring a particular pressure range is something that could be added, but I'm not convinced yet about the usefulness of that...

Ah, I was confused by the fact the recipe in JEI still shows the required pressure as (for example) -0.5-5 bar (for a recipe requiring -0.5 to craft). I presume that's a bug. The recipe works fine when I actually try it, although I did run into an oddity that you may be interested in:
Since pressure chamber valves have a minimum pressure they obviously won't work in a chamber with a vacuum which makes it fiddlier to automate. That's not necessarily an issue that needs to be addressed (in fact, I'd probably leave it as-is if I were you and let modpack makers implementing negative pressure recipes to come up with ways to automate it if necessary) but I thought it was worth mentioning in case you wanted to implement a toggle that makes chamber interfaces run off of vacuum pressure or have them run in reverse or something.

Yeah, sounds the JEI support needs looking at.

When you say "valves", do you mean the interfaces? It's true they need pressure, and yeah I quite like the idea of reversing their direction when the pressure is negative (although it may confuse people a bit, it would need to be made clear somehow). I'll have a think about that anyway.

I do mean interfaces, sorry. On further testing having the interfaces reverse seems like a good idea, because teleporting items inside using the Botania spectranthemum (which was the alternative I had in mind) doesn't work since the items inside the pressure chamber aren't supposed to be actual item entities.

OK, you'll find the gauge renders a bit better for negative pressure recipes now.

As for the interface, there's no problem here. Interfaces do run with negative pressure, although their pressure will rise (instead of fall) toward 0 when items are inserted/extracted.

Ah, turns out the 'insufficient pressure' error was because the one bar of negative pressure isn't enough to transfer a whole stack. I'll have to remember to make a not of that in the relevant quests so players aren't confused.

Going to mark this as closed now, I think we're in good shape here?