The difference between the water level in the eudiometer tube and the water level in the beaker must be measured because we have to put into consideration, the pressure of the gases in the eudiometer tube. This said pressure of gas in the eudiometer must equal the atmospheric pressure. If or by chance, the water levels happens not to be at the same height, then this is not the case. And then, as a result, in order to account for the difference between both, while also being able to get accurate results, you have to find the difference or subtract the water levels and then go ahead in converting them to mmHg.
