A while back I picked up a very nice precise altimeter from someone on another forum. They replaced it with a new one because it failed to meet the test criteria for instrument flight. I haven't done any testing at altitude, but it's accurate on the ground and the barometric pressure adjustment works as it should. We're roughly 1100-1300 ASL here, depending on where you are. From my limited testing, I'm guessing it may have a case leak... if I put enough suction on the port to run it up to 5-6 K it will leak down several hundred feet pretty quickly, then much more slowly after that. I am using a somewhat cheesy setup that may have its own leaks, it was just a quick check.

Now, having read the test procedures and remembering what I know about how altimeters work, it seems like a leaky case would really only matter if the static pressure and the cabin pressure were significantly different. As I'm building an open cockpit wooden airplane and the instrument enclosure will be pretty well ventilated, I wonder if I even care if the case leaks. I'll have a pitot tube for the airspeed indicator, of course, but I wonder if I'll even need an external static source. It seems that a simple filter on the static line might be enough, and a leaking case on the altimeter is probably a non-issue.

Thoughts?