A method for determining a control and/or regulating variable for changing the volume flow of a gaseous fluid through a device for separating a substance from a gaseous fluid, wherein the fluid is sucked in through at least one inlet opening of the device by at least one suction device having a drive element and is sucked through at least one filter element in a flow direction, wherein the filter element is arranged upstream of the suction device in the flow direction, comprising the steps of: a.) determining a master curve of the suction device, which represents the dependence of the volume flow V M of gaseous fluid on the fluid pressure p M in the suction device; b.) measuring the actual value of the total pressure loss p I of the device up to the suction device with a pressure sensor; c.) detecting a control voltage output by a controller of the suction device at the time of measurement of the total pressure loss of the device; d.) calculating an actual rotational speed n I of the drive element of the suction device using the formula n I = k × control voltage − z, wherein k is the slope of the characteristic curve and z is a displacement constant, e.) calculating the pressure loss of the master curve p M associated with the measured actual value of the total pressure loss p I of the device at the control voltage output by the controller of the suction device at the time of measurement of the total pressure loss of the device using the formula p M = n M / n l 2 × p I wherein n M is the rotational speed of the drive element at a control voltage corresponding to the master curve, calculated using the formula given under point d. and/or stored as a numerical value in an electronic memory element of the device for separating a substance from a gaseous fluid; f.) calculating the volume flow Vm of the master curve at the pressure loss of the master curve p M; g.) calculating the actual value of the volumetric flow V I at the control voltage output by the controller using the formula V I = n I / n M × V M h.) comparing the actual value of the volumetric flow V I with a target value of the volumetric flow at the control voltage output by the controller; i.) optionally changing the control voltage to change the rotational speed of the suction device of the device in order to adjust the actual value to the target value of the volume flow. a.) determining a master curve of the suction device, which represents the dependence of the volume flow V M of gaseous fluid on the fluid pressure p M in the suction device; b.) measuring the actual value of the total pressure loss p I of the device up to the suction device with a pressure sensor; c.) detecting a control voltage output by a controller of the suction device at the time of measurement of the total pressure loss of the device; d.) calculating an actual rotational speed n I of the drive element of the suction device using the formula n I = k × control voltage − z, wherein k is the slope of the characteristic curve and z is a displacement constant, e.) calculating the pressure loss of the master curve p M associated with the measured actual value of the total pressure loss p I of the device at the control voltage output by the controller of the suction device at the time of measurement of the total pressure loss of the device using the formula p M = n M / n l 2 × p I wherein n M is the rotational speed of the drive element at a control voltage corresponding to the master curve, calculated using the formula given under point d. and/or stored as a numerical value in an electronic memory element of the device for separating a substance from a gaseous fluid; f.) calculating the volume flow Vm of the master curve at the pressure loss of the master curve p M; g.) calculating the actual value of the volumetric flow V I at the control voltage output by the controller using the formula V I = n I / n M × V M h.) comparing the actual value of the volumetric flow V I with a target value of the volumetric flow at the control voltage output by the controller; i.) optionally changing the control voltage to change the rotational speed of the suction device of the device in order to adjust the actual value to the target value of the volume flow.