Old or abused scales can have the following problems.
- Corroded or dulled knife edges. These are little triangular pieces of metal that protrude from the beam assembly. They rest in plastic "Bearings".
- The bearings are easily replacable and cheap. The bearings go bad.
- Worn weights. This happens when the scale has seen a lot of use in dirty abrasive conditions. The 10 gm and hundreds weights no longer sit in their holes. They can be replaced or exhnged with another scale, but I doubt its wort the effort as when this has happend so usually has #4
- Worn beams. Same reason and symptoms as #3.
- Counterwieght hanging point out of balance.
- Clean your scale. Weigh your 500 gm. counterweight and compare to the "actual weight " listed on the counterweight. it should match.
- Put 500 grams of material on the scale. weigh it with the sliding weights.
- Now weigh it with the counter weight. If the counter weight side is low loosen the locknut on the couterweight hanger and and screw the hanger into the scale body. If it is high, screw it out. Lock the lock nut and recheck.
With good bearings and no wind your scale should be able to accurately measure 1 gram within 5%. Its accuracy below 1 gram is much less.
Ohaus does sell scales for weighing smaller amounts. These are good for small glaze tests and medium size tests with powerful colorants like cobalt.