A fluid processing system configured to cooperate with a fluid flow circuit, wherein the fluid processing system is configured as a fluid separation system for separating a plasma-containing fluid into separated plasma and a concentrated fluid, the fluid processing system comprising: an optical sensor assembly configured to receive a portion of the fluid flow circuit, wherein said portion of the fluid flow circuit comprises a portion of a plasma outlet line associated with a fluid separation chamber of the fluid flow circuit, and comprising a light source configured to emit a light (68, 70), and a light detector configured to receive a portion of the light (68, 70) emitted by the light source and to produce an output indicative of the amount of received light (68, 70); and a controller configured to receive the output from the light detector, wherein the controller is programmed to determine whether said portion of the fluid flow circuit is properly received by the optical sensor assembly by comparing the output from the light detector to a baseline value that is indicative of the amount of light (68, 70) from the light source that is received by the light detector in the absence of said portion of the fluid flow circuit, and determining that said portion of the fluid flow circuit is properly received by the optical sensor assembly if the output from the light detector is equal to or less than a selected percentage of the baseline value, and determine the concentration of free plasma hemoglobin in the fluid flowing through said portion of the fluid flow circuit using the same light (68, 70). an optical sensor assembly configured to receive a portion of the fluid flow circuit, wherein said portion of the fluid flow circuit comprises a portion of a plasma outlet line associated with a fluid separation chamber of the fluid flow circuit, and comprising a light source configured to emit a light (68, 70), and a light detector configured to receive a portion of the light (68, 70) emitted by the light source and to produce an output indicative of the amount of received light (68, 70); and a light source configured to emit a light (68, 70), and a light detector configured to receive a portion of the light (68, 70) emitted by the light source and to produce an output indicative of the amount of received light (68, 70); and a controller configured to receive the output from the light detector, wherein the controller is programmed to determine whether said portion of the fluid flow circuit is properly received by the optical sensor assembly by comparing the output from the light detector to a baseline value that is indicative of the amount of light (68, 70) from the light source that is received by the light detector in the absence of said portion of the fluid flow circuit, and determining that said portion of the fluid flow circuit is properly received by the optical sensor assembly if the output from the light detector is equal to or less than a selected percentage of the baseline value, and determine the concentration of free plasma hemoglobin in the fluid flowing through said portion of the fluid flow circuit using the same light (68, 70). determine whether said portion of the fluid flow circuit is properly received by the optical sensor assembly by comparing the output from the light detector to a baseline value that is indicative of the amount of light (68, 70) from the light source that is received by the light detector in the absence of said portion of the fluid flow circuit, and determining that said portion of the fluid flow circuit is properly received by the optical sensor assembly if the output from the light detector is equal to or less than a selected percentage of the baseline value, and comparing the output from the light detector to a baseline value that is indicative of the amount of light (68, 70) from the light source that is received by the light detector in the absence of said portion of the fluid flow circuit, and determining that said portion of the fluid flow circuit is properly received by the optical sensor assembly if the output from the light detector is equal to or less than a selected percentage of the baseline value, and determine the concentration of free plasma hemoglobin in the fluid flowing through said portion of the fluid flow circuit using the same light (68, 70).