A DP cell is a device that measures the differential pressure between two inputs. Example: To measure the pressure difference between a container (or vessel) and the surrounding atmosphere, you may connect 'Hi' port of the DP-cell to a fitting that enters the vessel, using suitable tubing. The 'Lo' port, you leave open to the atmosphere (open air, or possibly through a buffer or desiccant chamber). The DP-cell will indicate the relative difference between the pressure of the vessel (container) and the atmospheric pressure.