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Oxygen saturation is a relative measure of the amount of oxygen that is dissolved or carried in a given medium. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water.

In medicine, oxygen saturation (S_O2) measures the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. At low partial pressures of oxygen, most hemoglobin is deoxygenated. At around 90% (the value varies according to the clinical context) oxygen saturation increases according to an oxygen-haemoglobin dissociation curve and approaches 100% at partial oxygen pressures of >10 kPa. A pulse oximeter relies on the light absorption characteristics of saturated hemoglobin to give an indication of oxygen saturation. An Sa_O2 (arterial oxygen saturation) value below 90% is termed hypoxemia. This may be due to various medical conditions.

The Sv_O2 (venous oxygen saturation) is measured to see how much oxygen the body consumes. Under clinical treatment, a Sv_O2 below 60%, indicates that the body is in lack of oxygen, and ischemic diseases occur. This measurement is often used under treatment with a heart-lung machine (Extra Corporal Circulation), and can give the perfusionist an idea of how much flow the patient needs to stay healthy.

Tissue oxygen saturation (St_O2) can be measured by near infrared spectroscopy. Although the measurements are still widely discussed, they give an idea of tissue oxygenation in various conditions.

Saturation of Peripheral Oxygen (Sp_O2) is an estimation of the oxygen saturation level usually measured with a pulse oximeter device.

Annual mean sea surface dissolved oxygen for the World Ocean. Data from the World Ocean Atlas 2001.

In aquatic environments, oxygen saturation is a relative measure of the amount of oxygen (O₂) dissolved in the water. Supersaturation can sometimes be harmful for organisms and cause decompression sickness. Dissolved oxygen (DO) is measured in standard solution units such as millimoles O₂ per liter (mmol/L), milligrams O₂

• 20 °C, normal pressure, freshwater: 9.1 mg/L = 100% saturation

Solubility tables (based upon temperature) and corrections for different salinities and pressures can be found at the USGS web site. Tables such as these of DO in milliliters per liter (ml/L) are based upon empirical equations that have been worked out and tested (e.g.,Weiss, 1970):

  ln(DO) = A1 + A2 * 100 / T + A3 * ln(T / 100) + A4 * T / 100 + S * [B1 + B2 * T / 100 + B3 * (T / 100)2]
   
  Where:
  A1 =  − 173.4292
  A2 = 249.6339
  A3 = 143.3483
  A4 =  − 21.8492
  B1 =  − 0.033096
  B2 = 0.014259
  B3 =  − 0.001700
  T = temperature in kelvins
  S = salinity in g/kg.
  DO = dissolved oxygen in ml/L.  
       ln is the natural log.  Multiply DO by 1.4276 to obtain mg/L.

• Apparent oxygen utilisation
• Wastewater quality indicators discusses both BOD and COD as indicators of wastewater quality.

• Weiss, R. (1970). "The solubility of nitrogen, oxygen, and argon in water and seawater". Deep-Sea Res. 17: 721-35. 
• Interactive haemoglobin saturation graph