Because the carbon dioxide will dissolve readily in water which in turn reduces the pH of the water. If the pH is already low, the solubility of CO2 decreases and you can more accurately measure the total biogas volume with the water displacement method. Alternatively, some researchers use a strong basic solution to completely strip the CO2 and only measure methane volume. 

to avoid the diffusion of CO2 in the liquid displacement measuring gasometers but this give false results and should not be used.

thank you..

Thank you for your valuable suggestions.How much time it may take for complete dissolution of CO2 into a strong basic solution during methane measurement in biogas (In water displacement method)?

biogas is used to power IC engines, lightning and other several applications. co2 is detrimental to the biogas powered engine. to improve the efficiency we need to upgrade the gas by removing unwanted gasses such as co2 and improve methane content to say 90% or more.

the widely applied method of biogas upgrading is water scrubbing method but bubbling it into acidified water also remove co2. so we used acidified water to measure biogas yield to reduce concentration of co2

as regard to your second question, check out this

http://madsci.org/posts/archives/2009-03/1236608128.Ch.r.htm