The existence of the first drying period is usually demonstrated by the observation of a constant drying rate stage. However, with highly deformable materials, the overall drying rate is misleading due to the substantial decrease of the exchange surface. The drying flux is a more useful quantity to consider, but it requires both the exchange surface area and the overall drying rate to be determined during the entire drying process. Another proof of evidence of a first drying stage lies in the sample temperature, which should stay equal to the wet bulb temperature during the first drying period. In this paper, we proposed a new experimental set-up that allows the existence of a first drying stage to be proved by two independent experimental data. Using a climatic chamber, a microbalance and two contactless methods (two laser beam micrometers and an infrared thermometer) the sample dimensions, the drying rate and the surface temperature can be collected simultaneously versus time. The results obtained from both methods were consistent and confirmed the existence of a constant drying flux period for potato. The effect of relative humidity and sample dimension was also analyzed.