Why is the volume of co2 production rather than o2 uptake an adequate measure of respiration for the study of respiration rate?

In order to measure the respiration rate, the scientists generally use the volume of carbon dioxide generated in spite of the volume of the oxygen that was uptake. The carbon dioxide volume is used for determining the rate of respiration as it is tough to estimate small modifications in the concentration of oxygen.  

Also because of the existence of oxygen in the atmosphere, the atmospheric oxygen can easily get administered into the sample been used, therefore, making the outcomes of the experiment invalid. The respiration rate is also tightly associated with the rate of cell metabolism, thus, the determination of respiration through the measurement of carbon dioxide also permits scientists to easily monitor the metabolic states of the tissues.