A comparison of density measurements between cylindrical Langmuir probes, resonance fluorescence scattering and microwave methods is performed in a singly ionized barium plasma of a Q-device by varying the following parameters: 1) Density n: 3 × 108 cm−3 ≤ n ≤ 3 × 1011 cm−3; 2) Plate temperature Tp: 2100°K ≤ Tp ≤ 2500°K; 3) Plate material: Re and W; 4) Magnetic field B: 2 kG ≤ B ≤ kG; 5) Mode of operation: Double and single ended operations; 6) Ion temperature Ti down to about room temperature, keeping the electron temperature Te = Tp; 7) Degree of ionization γ: 0.1% ≤ γ ≤ 100%; 8) Probe dimensions.If the density n is evaluated by equating the extrapolated ion saturation current Is at the plasma potential Vs to the random ion current, it is shown that this yields a reliable relative value of the plasma density, which is by a factor of about 2.5 higher than the most probable value.