I will review the results of the  analysis of the correlations between the frequency of quasi-periodic  oscillations (QPOs) and power law photon index in nine Galactic black hole  sources. We investigate the implications of the scaling of the index-QPO correlation patterns in the frequency domain for the possibility of measuring masses of central BHs. We firmly establish that for sources having independent information on mass from dynamical methods and which exhibit self-similar correlation patterns, the observed index-QPO scaling coefficient is consistent with being the ratio of BH masses. This fact strongly support our theoretically motivated idea that low frequency QPO for a particular BH state is inversely proportional to BH mass.Based on  our scaling technique, we determine BH masses  in   H1743-322, GX 339-4, XTE J1650-500, XTE J1859-226, 4U 1543-47, 4U 1630-47, using independently known masses of XTE J1550-564, GRO J1655-40 and GRS 1915+105 as references. Particularly notable results is obtained for XTE J1650-500, where, according to the scaling to XTE J1550-564 data, BH mass is $3.8\pm 0.5$. This is close to the theoretical mass limit for neutron stars, i. e. 3 solar masses.