A few pikkies of how building model steam boilers can go wrong! In this case I remember thinking that the fire tube material I had was a wee bit thinner than it should be, but decided that it would be OK for 60 psi (approx 4 bar). I also remember thinking just how wrong I'd been when the test pressure gauge suddenly dropped to zero without the gurgle of water!! My tutors on the pressure vessel design course would have been black affronted had they got to hear of it. My only excuse - it was a long time ago!! I'm not all that proud of the brazing either ............... If anyone doubts why cold pressure testing is necessary, imagine the result had the boiler been steaming.