These Agilent Varian HS-20 diffusion pump standard cold cap are a replacement part that can be installed at the customer's location. There are two types of cold caps, standard and Halo Baffle. The type of cold cap used will influence factors, such as, oil backstreaming and final pumping speed of the diffusion pump. For example, the standard cold cap is an option that fits inside the HS-20 pumps in place of the standard cold cap. The cold cap stops backstreaming as effectively as an optically dense baffle, yet it retains 60% 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 location of the cold cap please see item #1 on diagram exploded view & parts illustration shown below. For removal installation procedures of cold cap see page 38 of instruction manual PDF below. These cold caps are OEM original Agilent Varian replacement parts with part number K1855320.