This replacement brown cap is what goes on top of the Tropf Blumat sensor and is essential for the function of the Blumat carrot. This small brown cap allows you to adjust the flow rate of the Blumat, but more importantly also serves as a critical function in making sure to stop the water when the carrot senses that it is wet enough. If you lose or break this small essential piece, your Bluamt sensor will no longer serve its function so make sure to replace it with this replacement piece.