Hi Christy.
What I mean is that when we talk about DO as a saturation percentage, it is at a specific temperature. For example, 100% saturation at 20C is about 9ppm. However, 100% saturation at 25C is about 8ppm. So the percentage is always based on saturation at a specific temperature (and other factors. see below).
Measurements in PPM is absolute and isn't a function of temperature (although they do increase or decrease with temperature). 10ppm is simply 10ppm at any temperature. PPM is more useful as a measurement in some cases. For example, marine life require a certain ppm (different for each animal) to survive regardless of the temperature of the water. An animal that can tolerate wide temperatures but requires 6ppm to survive (for example) can survive at 60% DO at some temperature but die at 70% DO at higher temperatures because although the % saturation is higher, the absolute ppm is lower.
% saturation as a measurement is useful when you want to know how far off you are from ideal water quality. This link suggests that 100% is ideal.
http://www.fivecreeks.org/monitor/do.html
I know for someone who deals with biology all the time (like yourself), all of this goes without saying but it is still worth pointing out to people who don't measure DO regularly. % saturation is basically a function of: DO (in ppm), temperature, atmospheric pressure, and altitude. Here's a good site with info on DO % saturation vs DO in ppm. Others can check out the 3 step equation to calculate DO % based on those function parameters.
Its ugly.
http://wow.nrri.umn.edu/wow/under/pa...rs/oxygen.html
Here's the formula in Excel format:
=(($C$3*EXP(7.7117-1.31403*LN(B7+45.93)))*(1-EXP(11.8571-
(3840.7/(B7+273.15))-(216961/((B7+273.15)^2)))/$C$3)*(1-(0.000975-
(0.00001426*B7)+(0.00000006436*(B7^2)))*$C$3))/(1-EXP(11.8571-
(3840.7/(B7+273.15))-(216961/((B7+273.15)^2)))/$C$3)/(1-(0.000975-
(0.00001426*B7)+(0.00000006436*(B7^2))))
