A simple respirometer designed to measure oxygen uptake consists of a sealed container with the living specimen together with a substance to absorb the carbon dioxide given off during respiration, such as soda lime pellets, or cotton wads soaked with potassium hydroxide. The oxygen uptake is detected by displacement of manometric fluid in a thin glass U-tube connected to the container. When the organism takes in oxygen it gives off an equal volume of carbon dioxide. As this is absorbed by the soda lime, air is sucked in from the U-tube to keep the pressure constant, displacing the liquid. The rate of change gives a direct and reasonably accurate reading for the organism's rate of respiration.
As changes in temperature or pressure can also affect the displacement of the manometric fluid, a second respirometer identical to the first except with a dead specimen (or something with the same mass as the specimen in place of the organism) is sometimes set up. Subtracting the displacement of the second respirometer from the first allows for these factors.