A device for testing substrates includes magnetic elements, preferably printed products printed with magnetic printing ink. The device has a powered suction belt travelling around it, which transports the substrates under a measuring device and is assigned to the top of at least one extractable air space. At least one suction fan is assigned to the suction belt and is connected to the air space. At least the suction belt is made of a non-ferromagnetic material. Each suction fan assigned to the suction belt takes the form of a motorless fan, which is connected to a remote power source by way of a transmission.