The absolute measurement of low-density gases, especially water, is of interest to space experiments. Water measurements have been advanced by the development of a primary standard for low-density water vapor. This standard, which uses arrays of laser-drilled holes as a Knudsen-effusion water source, was used to calibrate and characterize the performance of several vacuum instruments, including transfer standards used to calibrate the MSX flight mass spectrometer. This mass spectrometer and a total pressure sensor will be used to measure the absolute densities of molecules outgassing from the spacecraft during orbit as well as gases present in the ambient atmosphere. Results for the performance of these two flight instruments over a large range of partial pressures of H2O, H2, He, N2, O2 and Ar are presented, as well as test results for the reference standards.