A fluid separation system for separating a plasma-containing fluid into separated plasma and a concentrated fluid, the fluid separation system being configured to cooperate with a fluid flow circuit including a fluid separation chamber and a plasma outlet line associated with the fluid separation chamber for removing separated plasma from the fluid separation chamber , the fluid separation system comprising: an optical sensor assembly configured to monitor the contents of the plasma outlet line and produce an output indicative of the concentration of free plasma hemoglobin in the plasma outlet line ; and a controller programmed to calculate the free plasma hemoglobin concentration of the contents of the plasma outlet line based at least in part on the output of the optical sensor assembly , wherein the controller includes a pre-programmed reference equation having one or more constants and representing an experimentally determined relationship between the output of the optical sensor assembly and free plasma hemoglobin concentration and is further programmed to, during a calibration test, generate an equation having one or more constants and representing the actual relationship between the output of the optical sensor assembly and a known free plasma hemoglobin concentration, compare at least one of the one or more constants of the pre-programmed reference equation to the corresponding constant or constants of the generated equation, register the optical sensor assembly as having passed the calibration test if the differences between the at least one of the one or more constants of the generated equation are less than a predetermined tolerance away from the corresponding constant or constants of the pre-programmed reference equation, and register the optical sensor assembly as having failed the calibration test if the differences between the at least one of the one or more constants of the generated equation are greater than a predetermined tolerance away from the corresponding constant or constants of the pre-programmed reference equation. an optical sensor assembly configured to monitor the contents of the plasma outlet line and produce an output indicative of the concentration of free plasma hemoglobin in the plasma outlet line ; and a controller programmed to calculate the free plasma hemoglobin concentration of the contents of the plasma outlet line based at least in part on the output of the optical sensor assembly , wherein the controller includes a pre-programmed reference equation having one or more constants and representing an experimentally determined relationship between the output of the optical sensor assembly and free plasma hemoglobin concentration and is further programmed to, during a calibration test, generate an equation having one or more constants and representing the actual relationship between the output of the optical sensor assembly and a known free plasma hemoglobin concentration, compare at least one of the one or more constants of the pre-programmed reference equation to the corresponding constant or constants of the generated equation, register the optical sensor assembly as having passed the calibration test if the differences between the at least one of the one or more constants of the generated equation are less than a predetermined tolerance away from the corresponding constant or constants of the pre-programmed reference equation, and register the optical sensor assembly as having failed the calibration test if the differences between the at least one of the one or more constants of the generated equation are greater than a predetermined tolerance away from the corresponding constant or constants of the pre-programmed reference equation. <EOS>