To measure the rate of organismal respiration, a 10 mL microrespirometer was constructed. These measure relative volume as oxygen is consumed by respiring organisms. However, as oxygen gas is consumed during aerobic respiration, it is replaced by $CO_2$ gas at a ratio of one molecule of $CO_2$ for each molecule of $O_2$.
As such, in the following protocol, the $CO_2$ produced is removed by a small piece of cotton treated with potassium hydroxide ($KOH$). The $KOH$ reacts with $CO_2$ to form the solid potassium carbonate ($K_2CO_3$), thereby negating the contribution of $CO_2$ to the resulting volume.
A control microrespirometer was filled with 5 mL of glass beads in one setup, while 5 mL of germinating pea seeds were placed in another. $KOH$ plugs were inserted into both respirometers. Over the course of 20 minutes, changes in the volume were measured by following the distance of a red soap bubble placed on the end of a 40 uL capillary tube at time = 0.
Which of the following explains why the control microrespirometer is important to have?