The pressure limit is 256. After simulating every frame, the pressure will be reset to between -256 and 256. This is a fairly arbitrary limit, and doesn't have any translation to IRL pressure units.

During the simulation of a frame, pressure is uncapped, with effectively no limit. It will then get reset between frames to the real cap of -256 to 256.

@Supercrafter (View Post)
Yeah, signs can capture the uncapped amount, based on the timing of when pressure is reset. It will still be reset between every frame, but various things may operate on the larger pressure, including particles being thrown off with very high velocity from the pressure during the frame.