A disposable assembly (200; 500; 600; 1000) comprising: a body having a lid portion (222; 522; 622; 1022) and a base portion (220; 520; 620; 1020), the base portion of the body forming at least a portion of a fluid flow passage and forming an inlet (206; 506; 606; 1006) and an outlet (218; 518; 618; 1018), the lid portion (222; 522; 622; 1022) having a first opening (224; 524; 624; 1024) and a second opening (226; 526; 626; 1026) and being secured to the base portion (220; 520; 620: 1020); a flow restricting element (212; 512; 612; 1012) positioned in the fluid flow passage at a location between the inlet (206; 506; 606; 1006) and the outlet (218; 518; 618; 1018) and between the first opening (224; 524; 624; 1024) and the second opening (226; 526; 626; 1026), the flow restricting element (212; 512; 612; 1012) having an orifice (201; 501; 601; 1001) formed therethrough and being placed within the base portion (220; 520; 620 1020), characterised in that the disposable assembly (200; 500; 600; 1000) is configured for releasable coupling engagement with a reusable assembly to define a differential pressure based flow sensor assembly, the disposable assembly (200; 500; 600; 1000) further comprising, a fluid pressure membrane (210; 510; 610; 1010) disposed along the fluid flow passage between the inlet (206; 506; 606; 1006) and the outlet (218; 518; 618; 1018), the fluid pressure membrane (210; 510; 610; 1010) being captured between the lid portion (222; 522; 622; 1022) and the base portion (220; 520; 620; 1020) of the body and defining a flexible diaphragm type membrane that fluid pressure within the fluid flow passage displaces upwardly in or towards the first opening (224; 524; 624; 1024) and the second opening (226; 526; 626; 1026) of the lid portion (222; 522; 622; 1022) to press against respective first and second displacement sensitive pressure sensors (120, 122) of the reusable assembly. a body having a lid portion (222; 522; 622; 1022) and a base portion (220; 520; 620; 1020), the base portion of the body forming at least a portion of a fluid flow passage and forming an inlet (206; 506; 606; 1006) and an outlet (218; 518; 618; 1018), the lid portion (222; 522; 622; 1022) having a first opening (224; 524; 624; 1024) and a second opening (226; 526; 626; 1026) and being secured to the base portion (220; 520; 620: 1020); a flow restricting element (212; 512; 612; 1012) positioned in the fluid flow passage at a location between the inlet (206; 506; 606; 1006) and the outlet (218; 518; 618; 1018) and between the first opening (224; 524; 624; 1024) and the second opening (226; 526; 626; 1026), the flow restricting element (212; 512; 612; 1012) having an orifice (201; 501; 601; 1001) formed therethrough and being placed within the base portion (220; 520; 620 1020), characterised in that the disposable assembly (200; 500; 600; 1000) is configured for releasable coupling engagement with a reusable assembly to define a differential pressure based flow sensor assembly, the disposable assembly (200; 500; 600; 1000) further comprising, a fluid pressure membrane (210; 510; 610; 1010) disposed along the fluid flow passage between the inlet (206; 506; 606; 1006) and the outlet (218; 518; 618; 1018), the fluid pressure membrane (210; 510; 610; 1010) being captured between the lid portion (222; 522; 622; 1022) and the base portion (220; 520; 620; 1020) of the body and defining a flexible diaphragm type membrane that fluid pressure within the fluid flow passage displaces upwardly in or towards the first opening (224; 524; 624; 1024) and the second opening (226; 526; 626; 1026) of the lid portion (222; 522; 622; 1022) to press against respective first and second displacement sensitive pressure sensors (120, 122) of the reusable assembly.