Problems with Charles' Law

We are fortunate to have a set of apparatus for performing the Charles' Law experiment. The problem is trying to get the concentrated sulfuric acid plugs into the capillary tube. Concentrated sulfuric acid is sued because it absorbs water vapour from the air column making sure that it is just dry air pressure being measured. We dry the empty tubes in the oven before up-ending them into the acid. The idea is that the air then cools and contracts, pulling the acid down the tube. It doesn't go down far enough so that the air column is covered by the water bath. It doesn't seem to affect the results but is still not very satisfying.