The continuous monitoring of radon concentrations is a useful method to study the effects of ventilation on radon concentration levels. The determination of average radon concentration is preferable than the active-type. For this purpose a model chamber, of cubic shape, made of plexy glass with a volume of 0.65 m3, is constructed For this purpose, two devices RDA - 200 and TRI - MET 372 are used for active-type. Devices RGM l/l and CRM- 1027 are used for continuous monitoring. The experiments are carried out at two different environmental conditions, one by using high temperature (40oC) and relative humidity (100%), the other by using low temperature (16oC) and low relative humidity (50%). Radon calibration chamber is a good tool in testing for radon measurements to different devices and techniques, a 65m3 plexy glass chamber is constructed to compare different detecting devices with the change of environmental conditions (temperature â€“ relative humidity â€“ samples concentration) as well as the use of devices inside or outside the chamber. This chamber resemble to a great extend the mines atmosphere which help in using it as a pilot plant. The obtained results show that: Lower stability of measurements with the change in environmental conditions, more stability for the RDA - 200 and Tri - met devices at high temperature and high humidity. From the other hand a more stability for both RGM and CRM devices clearly appeared at low temperature and low humidity conditions. This may be attributed to that the measurements depend mainly on an electronic circuit which have a higher performance at these environmental conditions.