Some oxygen is dissolved in every liquid, and physics dictates that every liquid absorbs oxygen until the oxygen partial pressure in the liquid and the air or gas phase in contact with it is in equilibrium. The oxygen concentration depends upon a number of factors, such as temperature and ambient pressure.
At 20 °C and an ambient pressure of 1013 mbar, water contains about 9 mg/l in its saturated state, while in ethanol it can increase to 40 mg/l of oxygen.
Determination of dissolved oxygen concentration using electrochemical oxygen sensors with an optochemical sensor
This analysis is suitable for measuring oxygen in beer, wort, beer-based beverages, drinking water, carbonated and non-carbonated beverages. This method can be used to measure high concentrations of dissolved oxygen (10 to 50 mg/l in wort) or to detect low oxygen concentrations (0.01 to 1.0 mg/l in beer).
Compounds, such as sulfur dioxide, hydrogen sulfide, chlorine, and formaldehyde can disrupt this analysis. The effects of these compounds vary according to which sensor is employed; however, on the whole, these substances can be considered disruptive to this analysis.
The basis for these O2 measurements is the detection of photoluminescence produced by an oxygen-sensitive layer. The change in photoluminescence depends on the partial pressure of the oxygen. Given the values for the partial pressure of the oxygen and the temperature, the amount of oxygen gas dissolved in the liquid can be calculated. The oxygen sensor determines the O2 content of the liquid by means of optical detection through a photoluminescent process, in which an oxygen-sensitive layer is exposed to blue light. In doing so, the molecules in this layer become excited and reach a higher energy state. In the absence of oxygen, the molecules emit a red-colored light. If oxygen is present, it collides with the molecules in the oxygen-sensitive layer. The molecules in the oxygen-sensitive layer, which have collided with oxygen, cease to emit light (refer to figure 1). For this reason, a relationship exists between the oxygen concentration and the intensity of the emitted light as well as the intensity and the rapidity with which the intensity of the light diminishes. The intensity of the light is reduced at higher oxygen concentrations, although the rate at which it does so increases. The temperature of the product and the time interval between the light signal and the emission of light (phase shift) are both measured and used to calculate the oxygen content.
The device’s construction enables the state of the blue LED to be monitored using a photodiode. Another photodiode – with a red filter – measures the oxygen-dependent red light (refer to figure 2). This light is emitted by the luminophores due to photoluminescence (fluorescence) after they reach an excited state through exposure to the blue light. As a result of this exposure, the electrons of the luminophores are elevated to a higher energy level. As they return to their original energy level, they emit a red light.