Posted by Mike Field on July 09, 2000 at 13:29:14 from 203.18.21.200:
In Reply to: Re: Semaphore & other important matters posted by Dave W on July 09, 2000 at 09:09:11:
"Parts per million" simply measures concentration --ie proportionate number of units, whatever sorts of units they might be--whether of volume, mass, area, length, density, time, or anything else, I suppose (although as far as I know it's only generally applied to the first two.) Ppm is a measure of concentration. So one wouldn't be measuring Imgredient X against Ingredient Y in that way. If one is adding two parts of one ingredient to three parts of another, it should indeed be specified "by weight" or "by volume," a la the concentrations of "active ingredients" appearing on bottles of pharmacueticals, etc ("w/v" weight per volume, etc.)
So, to answer your last sentence, Dave, neither does ppm.