An apparatus for measuring a composition of a fluid, comprising: a fluid conduit having a wall which defines a fluid flow path (8,108,208); a transmitting antenna (10, 110, 210) and an electrical signal generator connected electrically to the transmitting antenna (10, 110, 210); a receiving antenna (12, 112, 212) and an electrical signal detector connected electrically to the receiving antenna (12, 112, 212); and a waveguide member (20,120,220) configured for electromagnetic coupling with the transmitting antenna (10,110,210) and the receiving antenna (12,112,212) and for guiding an electromagnetic field from the transmitting antenna (10,110,210) to the receiving antenna (12,112,212) along a waveguide path (22,122,222), wherein the waveguide member (20,120,220) is separated from the fluid flow path (8,108,208) by an electrically insulating inner region of the wall and the waveguide member (20,120,220) defines an opening (32,132,232) which is disposed towards the fluid flow path (8,108,208) to permit a portion of the electromagnetic field to extend through the opening (32,132,232) and the inner region of the wall into the fluid flow path (8,108,208), characterised in that the waveguide member (20,120,220) defines a groove (30,130,230) disposed towards the fluid flow path (8,108,208) and the groove (30,130,230) defines the opening, wherein the waveguide member (20,120,220) comprises an electrically conductive layer or coating, wherein the fluid conduit comprises a base member which defines the electrically insulating inner region of the wall, wherein the waveguide path (22,122,222) extends along and/or around the base member, wherein the waveguide path (22,122,222) encircles or circumscribes the base member one or more times and/or wherein the waveguide path (22,122,222) extends helically or generally helically around the base member, wherein the opening (32,132,232) is disposed towards the base member, wherein the fluid conduit comprises an electrically insulating waveguide filler member and wherein the waveguide filler member extends from an outer surface of the base member to an inner surface of the waveguide member (20,120,220), wherein the apparatus comprises Z a controller configured to measure an input electrical signal provided by the electrical signal generator to the transmitting antenna (10,110,210) and an output electrical signal received by the electrical signal detector from the receiving antenna (12,112,212), the controller being configured to determine an electromagnetic loss from the measured input electrical signal and the measured output electrical signal and to determine the composition of the fluid from the determined electromagnetic loss and a known relationship between the composition of the fluid and the electromagnetic loss. a fluid conduit having a wall which defines a fluid flow path (8,108,208); a transmitting antenna (10, 110, 210) and an electrical signal generator connected electrically to the transmitting antenna (10, 110, 210); a receiving antenna (12, 112, 212) and an electrical signal detector connected electrically to the receiving antenna (12, 112, 212); and a waveguide member (20,120,220) configured for electromagnetic coupling with the transmitting antenna (10,110,210) and the receiving antenna (12,112,212) and for guiding an electromagnetic field from the transmitting antenna (10,110,210) to the receiving antenna (12,112,212) along a waveguide path (22,122,222), wherein the waveguide member (20,120,220) is separated from the fluid flow path (8,108,208) by an electrically insulating inner region of the wall and the waveguide member (20,120,220) defines an opening (32,132,232) which is disposed towards the fluid flow path (8,108,208) to permit a portion of the electromagnetic field to extend through the opening (32,132,232) and the inner region of the wall into the fluid flow path (8,108,208), characterised in that the waveguide member (20,120,220) defines a groove (30,130,230) disposed towards the fluid flow path (8,108,208) and the groove (30,130,230) defines the opening, wherein the waveguide member (20,120,220) comprises an electrically conductive layer or coating, wherein the fluid conduit comprises a base member which defines the electrically insulating inner region of the wall, wherein the waveguide path (22,122,222) extends along and/or around the base member, wherein the waveguide path (22,122,222) encircles or circumscribes the base member one or more times and/or wherein the waveguide path (22,122,222) extends helically or generally helically around the base member, wherein the opening (32,132,232) is disposed towards the base member, wherein the fluid conduit comprises an electrically insulating waveguide filler member and wherein the waveguide filler member extends from an outer surface of the base member to an inner surface of the waveguide member (20,120,220), wherein the apparatus comprises Z a controller configured to measure an input electrical signal provided by the electrical signal generator to the transmitting antenna (10,110,210) and an output electrical signal received by the electrical signal detector from the receiving antenna (12,112,212), the controller being configured to determine an electromagnetic loss from the measured input electrical signal and the measured output electrical signal and to determine the composition of the fluid from the determined electromagnetic loss and a known relationship between the composition of the fluid and the electromagnetic loss.