Anyway,I just got my hands on this German Lufft pre-wwii pocket altimeter, the previous owner told me that this particular one was made back in the 1920s.
The instrument case is of chromed steel.and the dial is in metric(elevation index on the bezel ring is in meters and the pressure index on the fixed dial is in cmHg) The bezel along with the elevation marks is rotatable for calibration to QNH or a known local elevation.The overall design is essentially the same with British or American ones. The bezel ring has somehow developed several brown stain marks on it. I have seen these same rust marks on many vintage barometers, say the Victorian era ones. Dunno why barometers in particular have these rust marks.
Then come the problems:1.The altimeter works fine when first tested in a plastic bag. I then set the pressure reading correctly by taking reference to the built-in barometer on my phone. In the following days the altimeter reading starts to fluctuate in both direction. The hand always showns a reading that is either one or two notches off, plus or minus. Recalibrating it would only set it accurate for a time and fluctuation would start all over again after a while. Although marked "COMPENS." on the dial(which I interpret as "Compensated" just like the markings on British ones) I cant figure out just why such unneglectable errors would still appear. So Is such reading normal for a aneroid barometer?
I tried to disassemble the altimeter to inspect the internal movement for friction or faulty parts but it appears that there's no obvious way to get it open apart from prying the bezel open. I didnt dare to do so for fear of damaging the bezel,hand or craking the crystal. So is there anyone here could offer some correct ways to remove the bezel while doing minimal damage?