All the projects of the Extremely Large Telescopes (ELTs) are based on segmented primary mirrors. This idea, solving the problem of producing single mirrors having diameter larger than 8m, introduces the needs for an accurate co-phasing of the various segments. The paper presents some laboratory measurements aimed to investigate the use of the pyramid sensor as co-phasing sensor. The pyramid sensor behavior in measuring a differential piston of the incoming wavefront is analyzed in lab using interferometric measurements as reference. The obtained results show that the sensor can measure the differential piston with an accuracy better than 16nm on the whole measurement range 0-2π. Finally the sensor sensitivity i.e. the ratio between sensor signal and differential piston amplitude as a function of the tilt modulation is measured. Using the measured sensitivity values we estimate that the residual differential piston wavefront error due to pure photon noise, considering 2m2 area segments and a 15 R mag star is 7.4nm.