Inline and Co Planar Pressure

The only thing that comes to my mind is the difference in Rosemount brand differential pressure (DP) transmitter designs, from the legacy "in-line" Rosemount model 1151 transmitter to the newer "coplanar" Rosemount 3051 transmitter. I cannot be sure this is what you are referring to without more information.

In the "in-line" 1151 design, the two isolating diaphragms are parallel to the sensing diaphragm. One of the problems of this design is that changes in bolt stress at the isolating diaphragm flanges resulting from temperature swings will influence the sensing diaphgram's ability to sense pressure. With the "coplanar" 3051 design, the isolating diaphrams and associated flanges lie in a plane perpendicular to the sensing diaphragm, thus helping to de-couple the sensing diaphragm from those bolt stresses.

The new 3051S "supercell" coplanar design is even better, with the sensing diaphragm located inside of a capsule detached from the isolation diaphragm assembly entirely by two capillary tubes. This is one of the reasons why the 3051S can be sold with its calibration warranteed against drift for 10 years (versus 5 years for the regular 3051).