How Is Oxygen Measured?

Colorimetrically

• Measuring oxygen colorimetrically works by measuring the color of a modified substance. Chemicals are added to the substance where the oxygen is being measured. These chemicals react with the oxygen and change the substance's color. This new color identifies just how strong the oxygen saturation is within the substance. The substance will generally turn some color between yellow or orange, a result of the chemical reaction. This measurement is not a result of guesswork however as after the color change, scientists measure the free iodine left in the substance to find out the exact amount of oxygen.

Electrochemically

• This type of measurement is also called polarographic measurement. This measurement method relies on a dissolved oxygen probe attached to a meter. Two electrodes surround the dissolved oxygen within the water through a series of filters and provide a current. The reactions that occur based on this current are monitored by the meter attached to the probe. This can be an unstable measurement method as the dissolved oxygen is consumed by the measurement method itself.

Luminescence

• Measuring dissolved oxygen through luminescence means measuring it based on light. This is a more expensive method of measuring the oxygen than the other methods, but the equipment is easier to maintain between measurements. Two LEDs are submerged in the substance, one red and one blue. The red light is used solely for equipment calibration. The blue light is activated and then deactivated, beginning a process through which the light returns to its normal state. This transition is quicker the more oxygen is present in the substance. The equipment includes a meter than converts this timing into the amount of oxygen present.

Recording the Measurements

• Oxygen is measured in milligrams per liter or parts per million. The first is a metric measurement, but the latter method of recording the results refers to the molecules present in a particular sample. In water, 1 ltr. weighs about 1 kg and 1 mg per liter would be roughly equal to one part per million.