Telephones connected to a gateway can still make calls when connectivity to a central processing device is lost. An annular structure comprising a plurality of radially inwardly projecting, essentially vertical impactor plates of high-hardness material is located immediately above a vane structure such that entrained particulate matter from a milling bowl impacts one or both of the impactor plate surfaces. The selected data include data units that have been modified after a prior triggering event. An arrangement to adjust and calibrate the depth of the inlet to the draft tube is disclosed. At least first and second reference voltages are transmitted to respective ones of the comparators.