A pump forces a continuous air stream trough a screen-filter arrangement during a defined time interval. The probe volume will be calculated from the duration of the interval and the flow rate (6 min * 2.5l/min = 15l in case of the RPM/EQF).

The air contents both, unattached (free atoms) and attached (to aerosols) Radon daughter products. Because of the higher aerosol mass, their thermal movement is much slower than (constant thermal energy distribution) that of the unattached atoms.

Therefore, a  collision while passing the screen is more likely for the unattached progeny. The possibility of a screen attachment decreases with increasing particle size. These dependence is shown at the graphic below.

The particle size with an attachment rate of 50% is called 'cut off'. The 'cut off' may be manipulated by variation of the flow rate or the mesh grid. SARAD's progeny monitors (EQF3x20) realise a 'cut off' value of 5 nm. The remaining attached particles are collected at the surface of the filter, following the screen.

After turning off the pump, the alpha activity concentration of the progeny at screen and filter will be analysed by two independent semiconductor detectors. The analysis time is set to 114 minutes - long enough for decay of the major part of the short living Radon-daughters.

This features the devices to take a new sample every two hours without replacing screen and filter. The result is a quasi-continuous measurement of Radon progeny with 120 minutes time resolution. At the end of each cycle the low remaining activity will be determined to compensate the background of the following sample.