In an oxygen consumption calorimeter system for measuring heat release rate, the CO/CO₂ analyzer and the O₂ analyzer for chemical species analysis of the sampling gas are the main factors that increase the construction cost of the system. In this experimental study, a low-cost device with performance similar to that of the existing high-cost device was developed. To evaluate the contribution of the CO/CO₂ analyzer to the measurement of the heat release rate, the performance of unsteady and steady-state conditions depending on whether CO/CO₂ was measured for diffusion flames with various fire growth rates was examined. As a result, it was confirmed that when only the O₂ concentration was considered, the heat release rate could be measured accurately. Based on these results, the measurement performance of a low-cost O₂ analyzer manufactured using an oxygen sensor for healthcare was verified. As a result of the verification of the low-cost O₂ analyzer for diffusion flames with various fire growth rates, it was confirmed that the relative error was within 10% for unsteady and steady-state fire sources and can be used to measure the heat release rate.