  Measuring the gas mass of protoplanetary disks, the reservoir available for
giant planet formation, has proven to be difficult. We currently lack a
far-infrared observatory capable of observing HD, and the most common gas mass
tracer, CO, suffers from a poorly constrained CO-to-H$_2$ ratio. Expanding on
previous work, we investigate if N2H+, a chemical tracer of CO poor gas, can be
used to observationally measure the CO-to-H$_2$ ratio and correct CO-based gas
masses. Using disk structures obtained from the literature, we set up
thermochemical models for three disks, TW Hya, DM Tau and GM Aur, to examine
how well the CO-to-H$_2$ ratio and gas mass can be measured from N2H+ and C18O
line fluxes. Furthermore, we compare these gas masses to independently gas
masses measured from archival HD observations. The N2H+ (3-2)/C18O (2-1) line
ratio scales with the disk CO-to-H$_2$ ratio. Using these two lines, we measure
4.6e-3 Msun < Mdisk < 1.1e-1 Msun for TW Hya, 1.5e-2 Msun < Mdisk < 9.6e-2 Msun
for GM Aur and 3.1e-2 Msun < Mdisk < 9.6e-2 Msun for DM Tau. These gas masses
agree with values obtained from HD within their respective uncertainties. The
uncertainty on the N2H+ + C18O gas mass can be reduced by observationally
constraining the cosmic ray ionization rate in disks. These results demonstrate
the potential of using the combination of N2H+ and C18O to measure gas masses
of protoplanetary disks.
