Mechanical solution

grabcad

EDIT: I just noticed there is already a similar solution, with only a few differences. not sure if I should keep this entry...This solution uses a small "syringe like" piston, moving in a plastic tube. The piston is pre tensioned with a screw against a spring, and tightening, or loosening the screw allows to adjust the preload. If the pressure on the piston surpasses the adjusted load, the piston moves downwards, pressing a simple tact switch. This solution is only suitable to measure whether a set pressure is achieved, or not. In return, they can operate in super harsh environments, with well chosen switches even above 200°C, and they can be really tough.