A floor cleaning machine for cleaning a floor surface, wherein the floor cleaning machine is designed to be moved over a floor surface to be cleaned and preferably has a chassis, having a cleaning device for engaging with the floor surface to be cleaned, having a receiving device for receiving liquid from the floor surface to be cleaned, having with a dirty water tank for receiving liquid collected by the receiving device, wherein the dirty water tank is connected to the receiving device via a connecting line and wherein the connecting line opens at an inlet into the dirty water tank, which is arranged above a maximum level for liquid in the dirty water tank, having a negative pressure device, which is designed to generate a negative pressure in the dirty water tank above the maximum level, so that liquid is conveyed from the receiving device to the inlet and into the dirty water tank, having a first pressure detection device, which is arranged in such a way that it is suitable for detecting the pressure in the dirty water tank above the maximum level, having a second pressure detection device, which is connected to the interior of the dirty water tank via a conduit, having a device for determining the pressure difference between the pressures detected by the first device and the second device, wherein the conduit has an opening directed towards the dirty water tank, characterized in that the conduit has a first portion which, when the floor surface to be cleaned extends horizontally and the floor cleaning machine is arranged on this floor surface, extends vertically, and wherein the conduit has a second portion between the first portion and the opening, which second portion is designed in such a way that when the floor surface to be cleaned extends horizontally and the floor cleaning machine is arranged on said floor surface, the second portion extends horizontally or rises as viewed from the opening, wherein the tangents to the second portion between a starting point and an end point (43") of the second portion are inclined by less than 25° to the horizontal. having a cleaning device for engaging with the floor surface to be cleaned, having a receiving device for receiving liquid from the floor surface to be cleaned, having with a dirty water tank for receiving liquid collected by the receiving device, wherein the dirty water tank is connected to the receiving device via a connecting line and wherein the connecting line opens at an inlet into the dirty water tank, which is arranged above a maximum level for liquid in the dirty water tank, having a negative pressure device, which is designed to generate a negative pressure in the dirty water tank above the maximum level, so that liquid is conveyed from the receiving device to the inlet and into the dirty water tank, having a first pressure detection device, which is arranged in such a way that it is suitable for detecting the pressure in the dirty water tank above the maximum level, having a second pressure detection device, which is connected to the interior of the dirty water tank via a conduit, having a device for determining the pressure difference between the pressures detected by the first device and the second device, wherein the conduit has an opening directed towards the dirty water tank, characterized in that the conduit has a first portion which, when the floor surface to be cleaned extends horizontally and the floor cleaning machine is arranged on this floor surface, extends vertically, and wherein the conduit has a second portion between the first portion and the opening, which second portion is designed in such a way that when the floor surface to be cleaned extends horizontally and the floor cleaning machine is arranged on said floor surface, the second portion extends horizontally or rises as viewed from the opening, wherein the tangents to the second portion between a starting point and an end point (43") of the second portion are inclined by less than 25° to the horizontal.