Observed depolarization DP = p_n, 20/p_n, 6 between λ20.5 cm and λ6.2 cm in 1–kpc wide rings in the galaxy plane at 3′ resolution and predicted depolarization DP = DP₂₀/DP₆ by internal Faraday dispersion, using the observed variation in isotropic turbulent fields B_turb and thermal electron densities ⟨n_e⟩. The statistical uncertainty in B_turb of ≃10% leads to an uncertainty of the predicted DP of ≃7%. The systematical uncertainty in ⟨n_e⟩ of ≃20% causes a systematical uncertainty in DP of ≃14%.