A rate analysis instrumentation system including a sample cell and detector for producing an electrical cell signal proportional to a parameter of a sample reacting within the cell. In a preferred embodiment the enzyme activity of a sample is measured by detecting the light absorbance characteristics of the sample. A differentiator is utilized to obtain a rate signal from the light absorbance characteristic, and the rate signal is displayed. A constant rate detection circuit is used to measure the rate signal only during periods when the reaction rate and the cell signal are linear. The constant rate detection circuit differentiates the rate signal whereby a second derivative of the cell signal is provided. The second derivative signal is compared in a comparator to a threshold value to determine when the second derivative signal approaches zero. An overrange protection circuit is provided for excluding false rate signals, such as signals having characteristics that fail to meet predetermined acceptable limits.