The purpose of the drop checker is to give a good indication that the tank CO2 levels are running at 30ppm. It is done by adding a few drops of Bromo Blue to the bulb on the dropper ( I actually use the drops from my Nutrafin pH test kit). Then you need to add a few drops of reference solution at a carbonate hardness of 4dKH, which is basically RO water with some BiCarb added until 4dKH is reached. The 4dKH reference solution is pure, without any unwanted influence on its pH, unlike using tank water, so adding the test drops to the solution we should see a dark blue colour (mine went beyond the upper colour scale of pH 7.6 on my Nutrafin chart).
When the dropper is fitted below the water level in the tank, an air gap is trapped between the tank water surface and the reference solution. What happens now is that the amount of CO2 in the tank water will, over the period of an hour or two, equalise with the CO2 in the dropper air gap, and thus affecting the reference solution. The colour of the reference solution should change from blue to green, with green corresponding to a pH of 6.6 on the Nutrafin chart. Correlating a carbonate hardness of 4dKH against a pH of 6.6 and we get a CO2 level of 30ppm. If the solution remains blue then CO2 levels are too low, and if it goes beyond green and turns yellow then CO2 levels are too high.
Dave.
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