ELT PWFS sensitivity to a differential piston mode in partial correction as a function of seeing value expressed as the RMS of the slope for an aberration with a wavefront RMS of 1nm. Differential pieton mode is the one produced by one petal of a 6 pntals configuration with no gap and with a 0.5 m gyp. Solid line correspond to a non-modulated PWFS and dashed fine correspond to a PWFS with a modulation radius of 4λ/D. Sub-aperture size is 0.433 m and seeing range is [0.4,1.4] arcsec. Refers to the ELT case.