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The objectives of this study are to (1) improve a previously developed optical density (OD) sensor for the measurement of biomass concentration in algae cultures, and (2) test the performance of the improved sensor. The sensor was improved in the following several aspects. First, the sensor housing was redesign to accommodate new optical measurement configuration and a reference cell. Second, a constant current LED driver circuit was built and included. Third, a feedback temperature control mechanism (including thermistors and thermo-electrical cooling modules) was built to control the temperature of LEDs. Finally, a logarithmic IC chip was used for processing of raw outputs from photodiodes. The new sensor was tested with a 40 L algae cultivation raceway for seven days. The sensor showed high accuracy (R2 = 0.98 and root mean squared error = 0.034 absorbance unit between sensor-predicted and spectroscopically-determined OD). The sensor responded to the long-term growth of algae (i.e., the alternate growth patterns in light-on and light-off cycles) very well. The sensor also responded to cultivation events including water and media addition and culture transfer very well. The temperature dependency of the new sensor was 0.0033 and 0.0039 V/C for the NIR and red channel, respectively, compared to 0.0137 and 0.08 V/C for the old sensor.