<Dd> ρ substance = S G × ρ H 2 O . (\ displaystyle (\ rho _ (\ text (substance))) = SG \ times \ rho _ (\ rm (H_ (2) O)).) </Dd> <P> Occasionally a reference substance other than water is specified (for example, air), in which case specific gravity means density relative to that reference . </P> <P> Specific gravity can be measured in a number of value ways . The following illustration involving the use of the pycnometer is instructive . A pycnometer is simply a bottle which can be precisely filled to a specific, but not necessarily accurately known volume, V (\ displaystyle V). Placed upon a balance of some sort it will exert a force . </P> <Dl> <Dd> F b = g (m b − ρ a m b ρ b) (\ displaystyle F_ (b) = g (m_ (b) - \ rho _ (a) (m_ (b) \ over \ rho _ (b)))) </Dd> </Dl>

Formula of pressure in terms of specific gravity