These Agilent Varian VHS-4 extended cold caps are OEM Agilent Varian replacement parts for the VHS-4 diffusion vacuum pumps. The extended cold cap is sometimes referred to as a "Mexican Hat" due to its shape. There are two types of cold caps, standard and extended. The type of cold cap used will influence factors, such as, oil backstreaming and final pumping speed of the diffusion pump. For example, the Extended Cold Cap is an option that fits inside the VHS-4, VHS-6, and VHS-10 pumps in place of the standard cold cap. It stops backstreaming as effectively as an optically dense baffle, yet it retains 80% of the pump’s speed. Residual backstreaming is so low that it cannot be measured by the American Vacuum Society’s standard collection method. For removal installation procedures of cold cap see page 37 of instruction manual PDF below. These extended cold caps are OEM original Agilent Varian replacement parts with part number F6898301, see item #16 on (diagram exploded view & parts illustration) below.