These Agilent Varian VHS-400 diffusion pump standard cold caps are a replacement part that can be installed at the customer's location. 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-400 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 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 41 of instruction manual PDF below. These standard cold caps are OEM original Agilent Varian replacement parts with part number K4823301.