Sensitivity, of an open LR115 type II strippable detector for radon and its progeny determination, depends on the layer removed by etching and this is theoretically studied in this paper. Based on the results obtained by this study, three new approaches to the long term measurement of alpha emitting radon progeny in air are proposed. Track density on an open LR115 track detector grows with increasing removed layer (or with etching time). This growth is dependent on the equilibrium factor between radon and its short lived progeny. The first method, which determines the equilibrium factor, is based on this fact, and assumes multi step etching of the detector. The second method determines the concentration of 214Po in air. The method is based on the different behaviour of 222Rn, 218Po and 214Po partial sensitivities with increasing removed layer. The third method called the "obstacle" method enables the determination of 218Po and 214Po in air. This method requires an obstacle in front of an open LR115 detector. The partial sensitivities of 222Rn, 218Po and 214Po in such obstacle exposure geometry, behave differently to allow their determination. A similar method can be developed for other SSNTD possessing an upper energy limit for alpha detection.