This paper describes the validations of thermohydraulic models that predict the geysering instability observed at the riser section of a short-tube natural circulation evaporator (STNCE) at low heat flux and low pressure. The validations have been made by measuring the void fraction in the steam-water system at the riser section of a STNCE with the help of an impedance void-needle probe. The calibration of the probe shows that in the range of low void fractions the probe indicates lower values while in the range of high void fractions it indicates higher values compared to those measured by the manometer. Measured data of void fraction have been analyzed by thermohydraulic models. Both measured data and simulated data of temporal variation of void fractions during geysering have been observed to be periodic pulses of high amplitudes. Effects of riser height and loop resistance on the geysering frequency have been demonstrated in detail. The paper also discusses both the measurement uncertainties and the model uncertainties.