Dissolved CO₂ instruments are analytical devices that measure the amount of carbon dioxide (CO₂) dissolved in a liquid sample such as water. They typically include a submerged probe that is covered by a thin organic membrane. When the probe is submerged in the liquid sample, carbon dioxide diffuses through the membrane at a rate proportional to the partial pressure. Increasing the partial pressure increases the diffusion amount. The liquid sample is considered to be saturated when the molecular activity of the carbon dioxide equals that of the liquid. Air contains only 0.035 % carbon dioxide by volume; however, CO₂ is nearly 30 times as soluble in water as oxygen. Carbon dioxide moves across the air-water interface according to the same physical process that affect the dissolving of oxygen. Both temperature and pressure affect the diffusion rate measured by dissolved CO₂ instruments. Accuracy and diffusion range are typically measured in parts per thousand or parts per million.