A model describing growth of an outdoor algal (Spir‐ulina platensis)culture was developed. The model can simulate biomass production, pH, growth rate, oxygen evolution, and CO2 fixation rate. It was calibrated and validated against experimental data obtained by a novel automatic data logger/controller instrumentation which can number most vital parameters of the culture including on line estimation of oxygen production rate (OPR). The importance of understanding light distribution through the pond and its effects on the photosynthesis and respiration processes are emphasized. A maximum yield of about 38g day−1 m−2 under optimal conditions is predicted. The present model can also be a useful tool for optimization of algal mass production sites.