Three respirometers ready to go into a water bath. The one on the left contains germinating peas, the middle one consists of the same number of peas but they are not germinating. The third vial contains glass beads to serve as a control. When submerged in water the rate of cell respiration will be measured by oxygen consumption in the vials. Since the data is measured by volume changes there are a number of variables kept constant to keep the experiment controlled. For instance, there are 25 peas used in each of the first two vials. In the middle vial glass beads were added to keep the volume constant between it and the vial of larger, germinating peas. In vial 3 the glass beads are of the same volume as the first two. Any change in the glass bead vial will be due to pressure or temperature changes in the room, so that can then be used to correct the readings in the pea vials.