A bottle cap adapter for supplying liquids, particularly viscous liquids, comprising: a body part; and a vent tube comprising a first opening and a second opening, the first opening being configured to be put inside a bottle in the vicinity of the bottom of the bottle, wherein the body part further comprises a first surface and a second surface, the first surface being perpendicular to the vent tube and the second surface being parallel to or tilted to the first surface, e.g., by an angle of 0° to 40°, wherein the body part further comprises a channel from the first surface to the second surface, wherein at least one part of the channel comprises a connector extending to the second surface, the connector being complementary to a connector of the bottle, wherein the body part is further configured to be coupled to a bottle supply system, and wherein the vent tube is at least partially located in the channel of the body part. Also a bottle supply system for supplying liquids, particularly viscous liquids, comprising: a top part configured to house a bottle cap adapter; a bottom part comprising a reservoir configured to receive an amount of liquid from a bottle inserted in the bottle cap adapter and comprising a pressure sensor configured to measure a fluid level in the bottle; and an output connected to the reservoir.