ANSI/ASHRAE Standard 41.6-2014 defines absolute humidity as follows: in a mixture of water vapor and dry air, the mass of water vapor in a specific volume of the mixture.
ρ = (ma + mw) / V
sv = V / ma
dv = mw / V
dv = ρ – 1/sv = (ma / V + mw / V ) – ma / V = mw / V