Which Volume Force is correct?

Hi all,

So far I have seen two different definitions for Volume Force for free air flow. First one is g_const*(rho_ref-nitf.rho) and second is g_const*(comp1.mat1.def.rho(p_atm,T0)-nitf.rho).
where,
mat1 is material for air.
p_atm=1 atm, T0=initial temperature (also ambient temperature) and rho_ref=density of air at T0.
Is there any difference between these two definitions and which one is correct? I ran simulations for both definitions and got different velocity fields.
Thanks!

Akim