Volume Force

Hello everyone,

I am trying to model free convection in air and water using the Boussinesq approximation.
I use a volume force but I hesitate in the right value of this term.
I have seen a lot of example with two cases : -g_const*(spf.rho-rho_ref) ou only -g_const*spf.rho.
What is the good value ?

Thank you for your advice.

Regards