Differential Pressure (DP) transmitters measure the refinement between two pressure. They use a reference point called the low-side pressure and balance it with the high-side pressure. Ports in the instrument are stepped high-side and low-side. The DP scrutinizing can be either negative or positive depending upon whether the low-side or high-side is the greater regard. A DP transmitter can be used as a check pressure transmitter if the low-side is left open to the air.