# [APD] Re: ppm and mg/l

I am in trouble.
ppm = mg/l as long as we talk about solutions. But if we gas CO2 into water what about
then with the equation? Is 20ppm CO2 still 20mg/l CO2? Silly question?

Thank you very much for your input.

Yes, it is the same. The post Scott referenced is correct. Remember that gases have weight too -- even the "lighter than air" things like helium and hydrogen. All that is different is the volume taken up by a given weight of a given substance.
ppm really does mean just "parts per million", for weight, count, or whatever. If you had 1 red ball and 1000 blue balls, all in a bucket, then the concentration of red balls would be 1000 ppm. The actual unit of measurement doesn't matter for ppm concentrations -- ppm is just a measure of the ratio of one thing to some other thing in a group of both things. Even timing precision of oscillators is specified this way, with ppm meaning how many hertz of deviation is allowed (100 ppm on a 1 megahertz oscillator would allow for + or - 100 hertz deviation).

All you need to remember is that ppm (and ppb, ppt, etc.), are measures of CONCENTRATION, which is a RATIO in this case. The units in the ratio don't matter for the "ppm type" number as long as they are the same, for instance you wouldn't have 10 ppm if you had 10 grams of somethign in 50 tons of something else -- the units of measure don't match on both sides of the ratio.

I hope this helps clarify things a bit.

-Bill

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