The colorimeter absorbance scales come with two different sensitivities. These are given as 0 - 1 and 0 - 2. 1 and 2 would be called the Full Scale Deflection (FSD) because that is the reading when the needle is deflected to the furthest end of the scale. It's easy on the 0 - 2 scale because the numbers are written onto the scale at the bottom.
So the picture above onto the 0 - 2 setting would be 1.83. Absorbance is a logarithm of a ratio so has no units. The scale divisions are 0.05 apart so the way of giving the resolution as half a scale division means we say 1.83 +- 0.025. I'd say by inspection that we can do better than +-0.025. Surely by stating it as 1.83 I really mean +-0.01.
There are two ways of dealing with the 0 - 1 scale. I have relabeled the scale to show how. I like the top green numbers best. I'm going to say the reading is 0.92 +- 0.01 using the half scale division resolution. But that's the same resolution I was claiming for the top scale. So maybe it's really 0.92 +- 0.005.
