CO2 and ph Drop Test

[tackful]

I have a drop checker and am familiar with the ph/kh/co2 charts, but there's something I don't yet understand about the ph drop test. According to Rex Grigg, if there is a 1 degree difference in ph between tank water and a sample left out overnight, then the tank has 30 ppm co2. How is this correspondence determined? Just curious. Thanks.

 

[Mgamer20o0]

general guideline. not absolute truth.

 

[Canuck]

But as close as anything.

 

[DoctaQ]

when the sample gets left out over night all the co2 diffuses out and it reaches background levels again, which i think is about 3ppm in a normal environment but it depends on the kh of the water