Sample variance associated to line measurements. The centroid shift (left) and broadening (right) in apertures of growing sizes for clusters presenting identical emissivity models are shown for a spherical β-model with core radius of 107 kpc. These curves are predicted analytically by Eqs. (9) and (11). They have been normalised to the value of the 3D turbulent velocity dispersion σ_turb. The different shapes of the curves as the aperture radius grows can be used as a diagnostic to discriminate between various injection scales.