Oxygen saturation

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.

Medical science
In medicine, oxygen saturation (SO 2 ) 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 SaO 2 (arterial oxygen saturation) value below 90% is termed hypoxemia. This may be due to various medical conditions.

The SvO 2 (venous oxygen saturation) is measured to see how much oxygen the body consumes. Under clinical treatment, a SvO 2 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 (StO 2 ) 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 (SpO 2 ) is an estimation of the oxygen saturation level usually measured with a pulse oximeter device.

Environmental sciences
In aquatic environments, oxygen saturation is a relative measure of the amount of oxygen (O2) dissolved in the water. 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):

$$ln(DO) = A1 + A2*100/T + A3*ln(T/100) + A4*T/100 + S*[B1 + B2*T/100 + B3*(T/100)^2]$$

$$A1 = -173.4292$$ $$A2 = 249.6339$$ $$A3 = 143.3483$$ $$A4 = -21.8492$$ $$B1 = -0.033096$$ $$B2 = 0.014259$$ $$B3 = -0.001700$$ T = temperature in kelvins, and 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.

Supersaturation can sometimes be harmful for organisms and cause gas bubble disease.  DO - at least 4 - 6 mg/L (ppm), but more is better