Originally Posted by Hoppy
Have you figured out the relationship between the ppm in the air volume vs. ppm in the adjacent water? If the room air is at 500 ppm, and conventional wisdom isn't way off, that would correspond to around 3 ppm in the water. But, without that relationship the ppm in the air volume doesn't tell us much. I'm sorry my knowledge isn't sufficient for me to figure out the relationship.
is it as easy as this?
500ppmv = .0005 mole (500/1000000)
.0005 X 44.01 (molar weight of CO2) = .022005 / 1000000 = 2.2005 ppmw
(molar weight/1000 kg/m^3 = ppmw)
1000 kg/m^3 (density of water) = 1000000 mg/l
There are lots of assumptions of course like water density and temperature. If you want absolute accuracy, you have to take those into account.
So if this is right, the tub of tap water has 4.26ppmw of CO2
and my low tech tank this morning has 20.18 ppm of CO2