Another piece of equipment in planted tank is drop checker. Drop checker will tell you about carbon dioxide level in the water, which is essential information if you've got live stock. The right optimal level which is good for plants and still acceptable by fish is 30 ppm (parts per milion) (actually the level might be slightly different from 20-40 or even 45 but 30 is fair enough). The drop checker is basically pH level indicator.
How its working? Between indicator liquid (which is dkH 4 - water plus few drops of pH indicator) and water in tank is trapped air bubble (see the second picture). The colour of the indicator solution is changing when CO2 is evaporating from tank water and dissolving into solution water, making it more acidic.
So initialy the color of the solution is blue (not enough CO2) and is changing to green when is ~30ppm, changing to green-yelowish if the level is too high.
The shape of drop checker might be different, but idea is always the same.
Step 1: Parts and assembling
For my drop checker I've used bits and pieces lying around at home which was:
small black cup (don't remember exactly where it came from, shampoo or something?)
small vial (my is made from plastic)
small sucker (unnecessary from electronic thermometer , black will be better)
piece of neoprene (my was from mouse pad)
some glue or double sided tape
scisors, scalpel, etc
First of all I've adjusted the black cup height. Then lenght of the transparent vial has been adjusted to slightly longer than black cup, to make the indocator inside visible. (see schematic cross section)
Next stage was stick the neoprene blocks from both sides of the transparent vial. The blocks was used to keep vial in place inside the black cup. (still has to be a gap so the air inside has got contact with indicator and water inside tank)
Than hole for sucker has been drilled, and that basically was end of the story.