A cuvette positioning system comprising: a rotatable carousel assembly having a plurality of cuvettes (30; 30a; 30b) around a motorized central shaft; the plurality of cuvettes (30; 30a; 30b) positioned in a cuvette holder plate such that a plurality of cuvette walls of each of the plurality of cuvettes (30; 30a; 30b) contact the cuvette holder plate, the cuvette holder plate connected to the motorized central shaft, thereby allowing the cuvette holder plate to rotate each of the cuvettes of the plurality of cuvettes (30; 30a; 30b) around the motorized central shaft and into a testing position; a cuvette positioning arm assembly having a cuvette positioning arm connected to a cuvette positioning arm post located adjacent the testing position, the cuvette positioning arm aligning an individual cuvette of the plurality of cuvettes (30; 30a; 30b) in a particular measurement position that is repeatable for each individual cuvette in the plurality of cuvettes (30; 30a; 30b) when the each individual cuvette (30; 30a; 30b) is rotated into the testing position; the system being characterized in that it comprises: a lip spring and a lip spring holder plate connected to the cuvette positioning arm, the lid spring being held in place by the holder plate; and a cuvette positioning lip connected to the cuvette positioning arm, the lip spring allowing the positioning lip to pivot away from and then back towards the carousel when the cuvette in the carousel slides in front of the positioning lip on its way to being locked in the testing position, wherein the cuvette positioning lip has a plurality of tongues, the tongues aligning an individual cuvette (30; 30a; 30b) when the cuvette (30; 30a; 30b) is being held by the cuvette positioning arm assembly; wherein the lip spring presses the tongues of the cuvette positioning lip against the cuvette (30; 30a; 30b) positioned by the cuvette positioning arm assembly, thereby causing the cuvette (30; 30a; 30b) to be cornered against the tongues. a rotatable carousel assembly having a plurality of cuvettes (30; 30a; 30b) around a motorized central shaft; the plurality of cuvettes (30; 30a; 30b) positioned in a cuvette holder plate such that a plurality of cuvette walls of each of the plurality of cuvettes (30; 30a; 30b) contact the cuvette holder plate, the cuvette holder plate connected to the motorized central shaft, thereby allowing the cuvette holder plate to rotate each of the cuvettes of the plurality of cuvettes (30; 30a; 30b) around the motorized central shaft and into a testing position; a cuvette positioning arm assembly having a cuvette positioning arm connected to a cuvette positioning arm post located adjacent the testing position, the cuvette positioning arm aligning an individual cuvette of the plurality of cuvettes (30; 30a; 30b) in a particular measurement position that is repeatable for each individual cuvette in the plurality of cuvettes (30; 30a; 30b) when the each individual cuvette (30; 30a; 30b) is rotated into the testing position; the system being characterized in that it comprises: a lip spring and a lip spring holder plate connected to the cuvette positioning arm, the lid spring being held in place by the holder plate; and a cuvette positioning lip connected to the cuvette positioning arm, the lip spring allowing the positioning lip to pivot away from and then back towards the carousel when the cuvette in the carousel slides in front of the positioning lip on its way to being locked in the testing position, wherein the cuvette positioning lip has a plurality of tongues, the tongues aligning an individual cuvette (30; 30a; 30b) when the cuvette (30; 30a; 30b) is being held by the cuvette positioning arm assembly; a lip spring and a lip spring holder plate connected to the cuvette positioning arm, the lid spring being held in place by the holder plate; and a cuvette positioning lip connected to the cuvette positioning arm, the lip spring allowing the positioning lip to pivot away from and then back towards the carousel when the cuvette in the carousel slides in front of the positioning lip on its way to being locked in the testing position, wherein the cuvette positioning lip has a plurality of tongues, the tongues aligning an individual cuvette (30; 30a; 30b) when the cuvette (30; 30a; 30b) is being held by the cuvette positioning arm assembly;