Hi Bong; in it's day (early '70s ?) that densitometer would have been second to none. Readings should (assuming the instrument is in good condition) only vary by about 0.01; that is, if you read a density of 0.50 one time, it would not be unususal to get a second reading of 0.49 or 0.51. If the second reading were 0.51, all subsequent readings should continue to be either 0.50 or 0.51.(posted 8334 days ago)Here are a couple of things you should be aware of: 1) these instruments needed a good warm up to get stable; say 30 minutes. 2) Like Eric Blevins pointed out, you MIGHT be influenced by outside light. There should be a black (rubbery material) ring around the probe to seal out extraneous light; if this is missing, room light might be affecting you.
Another obvious (?) thing is the lamp alignment. The filament should be projected squarely into the measuring aperture. If it's off-center, jiggling, etc. might make your readings vary. To check this, lay a small piece of tracing paper (a post-it note or cigarette paper, etc will do fine) over the measuring aperture. You should see an image of the lamp filament here; fuzzy is ok, but it should be close to centered.
Anything else, I probably can't help much, but feel free to email with questions. PS; in taking readings, just push down the handle that lowers the probe; as soon as the reading comes on, it should be stable.