The “cluster-tube” diffusion batteries have been developed in a portable and less costly form. A series of commercially available “Collimated Holes Structures” (CHS) are mounted so that the aerosol penetration can be measured at a number of outlets along the series. The actual length of the appartus is about 2 feet but the equivalent length is 3.25 miles. The calculated curves of penetration versus particle size are used to evaluate size distribution and show that the “equivalent size” frequently reported from one measurement with a rectangular diffusion battery is practically meaningless. The value depends as much on the characteristics and mode of the operation of the diffusion battery as on the aerosol; the longer the battery and the lower the air flow, the greater the equivalent size will appear to be.