Originally Posted by
pilotmike
Really? How does 'per million' (divide by 1e6) "add an extra step to a calculation", where 'per cent' (divide by 1e2) does not?
Indeed, in such a high dilution ratio as this (1: 0.0001, or 0.001% or 100PPM), it is arguable that PPM is the clearest way to express it, as it uses only whole numbers. Also, being a multiple of 1e3 (1,000), it works very well with standard x1,000 multiples {eg. (k)g, (m)l etc.}, where % is x100, which prevents such easy multiples and the relatively simple use of k(ilo) m(illi) prefixes.
Both PPM and % are valid ways for engineers / scientists to express dilution ratios. Neither should be more or less likely to be prone to error, and certainly not by "add[ing] an extra step to a calculation", as you claim. Competent engineers should be expected to be able to work accurately with either.
Scientists don't really use PPM because it's far too ambiguous. We generally use molar concentrations for liquids. Makes it far easier.