A more likely measure, which I recall seeing years ago, is to measure the impedance of the enclosure of the thing you want to protect. If someone tampers with it, you would be alerted. It works at many scales, from a protective metal mesh over your IC to a PC case.
replyAnd you'd need to have a way to know what the pressure inside is beforehand.
replyYou can find the pressure through destructive trial-and-error if money is no object - which it isn't for governments when the target hardware is juicy.
replyWhy would you make each device have the same pressure inside it? That's a bit like hardcoding the same password in each one. Any attacker is only going to have one shot on the actual device they care about no matter how much money they spend.
replyReally sounds very very easy for any lab that can measure molecular levels of gas.
replyEven a very sensitive pressure checker in a temp controlled sealed box would do it.