constant fig2 is a pneumatic diagram in which the injector 1 with its entraining air connection 3 and dosing air connection 8 is represented as a block the entraining air and dosing air are supplied from a single compressed air source 30 to which a regulating valve 15 which delivers a constant air flow at its outlet line 15 39 is connected by a line 24 the air flow can be preset by an electrical control signal received via the control line 23 from a control unit not shown the valve 15 will then automatically maintain a constant air flow such a valve which may also be referred to as a flow controller may for example by a type f 201 c flow controller supplied by the company bronkhorst of the netherlands a switchable valve 16 may be provided as a main valve downstream of the flow controller 15 so that the air supply to the injector can be switched on and off the airflow discharged by the flow controller 15 then passes a via the line 17 and non return valve 18 to the entraining air connection of the injector 1 and b via lines 19 and 21 and non return valve 22 to the dosing air connection 8 an adjustable flow controller 20 with.