The measuring appts. has at least one conduction device such as a hopper (11) which is electrically connected to earth. An outflow of product (12) from the hopper orifice hits an insulated detection electrode (13) before collection in a receptacle (14). The electrode is polarised at a different potential from the hopper by a continuous floating voltage source (15). Electric current flowing from the electrode is amplified in an amplifier (18) having a negative feedback resistor (10) connected between its terminals and which is protected by a series mounted resistor (16) and two limiting diodes (17). The amplified current is fed to a measuring circuit having three main branches (7,8,9). The first branch (7) has an amplifier-integrator (19) associated with a low-pass filter (6), a gain adjusting resistor (3) and a continuous current measuring device (20). The second branch (8) has an alternating signal amplifier (22) with a capacitor (21) at its input and a rectifier bridge (23) with continuous current measuring device (24) at its output and a negative feedback resistor (2). The third branch (9) has a threshold amplifier-comparator (25) and a pulse counter (26). The measuring circuit thus produces a signal representing the flow of a product of known grain size structure, or the grain size structure of a product of known volumetric or mass flow.
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