A spiral wound module assembly comprising: a pressure vessel comprising: a chamber extending along an axis between a first end and a second end, at least one feed inlet port, concentrate outlet port and permeate outlet port, and a removable end plate located at an end (38, 38') of the vessel, wherein the permeate outlet port extends axially through the removable end plate; a plurality of spiral wound modules (2, 2') axially aligned in serial arrangement within the chamber with a first module located adjacent the first end and a second module located adjacent the second end, wherein each spiral wound module (2, 2') comprises at least one membrane envelope wound about a permeate collection tube including an inner periphery and wherein the permeate collection tubes of each spiral wound module (2, 2') are in sealed fluid communication with each other; a permeate adaptor tube in sealed fluid communication with the permeate collection tube of the first spiral wound module and the permeate outlet port and a flow plate including an opposing first and second side (60, 61) positioned within the chamber between the first spiral wound module and the first end of the pressure vessel, wherein the flow plate is disposed concentrically about the permeate adaptor tube and extends radially outward within the chamber with the first side facing the first spiral wound module and the second side facing the first end, and wherein the flow plate includes a plurality of holes passing from the first side to the second side that create a pressure drop in fluid between the first spiral wound module and the closer of the feed inlet port and concentrate outlet port; and said spiral wound module assembly being characterized in that it further comprises a differential pressure sensor located within the chamber and in simultaneous communication with the first and second sides (60, 61) of the flow plate, wherein the differential pressure sensor is adapted to measure differences in pressure between fluid located on the opposing sides (60, 61) of the flow plate, and wherein the differential pressure sensor includes a diaphragm, which separates solutions contacting opposing first and second sides (60, 61) of the flow plate. a pressure vessel comprising: a chamber extending along an axis between a first end and a second end, at least one feed inlet port, concentrate outlet port and permeate outlet port, and a removable end plate located at an end (38, 38') of the vessel, wherein the permeate outlet port extends axially through the removable end plate; a plurality of spiral wound modules (2, 2') axially aligned in serial arrangement within the chamber with a first module located adjacent the first end and a second module located adjacent the second end, wherein each spiral wound module (2, 2') comprises at least one membrane envelope wound about a permeate collection tube including an inner periphery and wherein the permeate collection tubes of each spiral wound module (2, 2') are in sealed fluid communication with each other; a permeate adaptor tube in sealed fluid communication with the permeate collection tube of the first spiral wound module and the permeate outlet port and a flow plate including an opposing first and second side (60, 61) positioned within the chamber between the first spiral wound module and the first end of the pressure vessel, wherein the flow plate is disposed concentrically about the permeate adaptor tube and extends radially outward within the chamber with the first side facing the first spiral wound module and the second side facing the first end, and wherein the flow plate includes a plurality of holes passing from the first side to the second side that create a pressure drop in fluid between the first spiral wound module and the closer of the feed inlet port and concentrate outlet port; and said spiral wound module assembly being characterized in that it further comprises a differential pressure sensor located within the chamber and in simultaneous communication with the first and second sides (60, 61) of the flow plate, wherein the differential pressure sensor is adapted to measure differences in pressure between fluid located on the opposing sides (60, 61) of the flow plate, and wherein the differential pressure sensor includes a diaphragm, which separates solutions contacting opposing first and second sides (60, 61) of the flow plate.