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The bottled or canned beer is brought to 20 °C and mechanically shaken, thereby achieving equilibrium between the oxygen dissolved in the beer and the oxygen present in the headspace (Henry’s and Dalton’s laws). By directly measuring either the oxygen in the beer or in the headspace, the total oxygen can be calculated through referencing a table of values, which includes the headspace volume as a percentage of the fill volume.

The measurement process using a Digox Analyzer works according to the principle of the potentiostatic three-electrode measurement system developed by TÖDT and TESKE and does not require a membrane.

The measuring electrode consists of solid silver, while the counter electrode is made of stainless steel. The reference electrode is composed of silver/silver chloride.

After generating a defined “polarization voltage”, an electrochemical reaction occurs at the measuring electrode, inducing a reduction of the oxygen molecules in the sample.

Measuring electrode (silver):

O₂ + 2 H₂O + 4 e⁻ → 4 OH⁻ (cathodic process)

Counter electrode (stainless steel):

4 OH⁻ → O₂ + 2 H₂O + 4 e⁻ (anodic process)

The flow of current as a result of this reaction is directly proportional to the amount of dissolved oxygen in the sample, if the polarization voltage is fixed as close to the level of the diffusion threshold current as possible.

In this case, the relationship can be represented as follows:

I = K × C_O2, whereupon K = n × F × A × 1/d

I = sensor current

C_O2 = oxygen concentration

F = Faraday constant

n = number of electrons per molecule

A = cathode surface

d = thickness of the “undisturbed layer” along the wall

The thickness of the undisturbed layer along the wall is determined by the hydrodynamic relationships at the measurement electrode and the transportation of oxygen molecules across the boundary layer brought about by temperature-dependent diffusion processes. Both of these clearly defined factors are precisely measured and compensated.

In order to adjust the polarization voltage between the two electrodes, a third electrode, the reference electrode, is employed in Digox measurement devices. This reference electrode remains in contact with the surface of the measuring electrode over a diaphragm in order to prevent mass transfer [1, 2, 3].

Active calibration:

In-line calibration is integrated into the device and is initiated by pressing a button. Taking advantage of Faraday’s Law, an exactly defined amount of oxygen is produced through the electrolysis of water.

I × t = m × F

I = current required for electrolysis

t = time

m = mass, g/mol

F = Faraday constant

The oxygen dissolves in the medium as it flows through and is detected at the measuring cell. The hydrogen liberated during the electrolysis reaction is not relevant for the measurement. The microprocessor monitors the calibration values and carries out any necessary corrective measures. The electrolysis enables the calibration of the device to be carried out under the same conditions and in the same medium as the analysis. Measurement operations are not disrupted during the calibration process [3].

The following applies to Digox 6.1 and all later models: In order to precisely determine the necessary potential for the measurement system, the Digox Analyzer possesses a scanner, which records the product-specific behavior of the medium subject to analysis. This can establish, whether the medium – due to the additives or supplements – must be measured at another potential. In this way, all types of beer-based beverages, non-alcoholic beverages and wine can be analyzed. Moreover, oxygen-reducing substances which can cause the calibration to be inaccurate may also be detected using the calibration scanning process. The device automatically implements the necessary compensative measures with the factors it has determined.

The analytical determination of oxygen using amperometric electrodes is achieved through measurement of the electrical current. The electrodes consist of a cathode and an anode, which are connected conductively through an electrolyte solution (KCl/KOH). Precious metals, such as platinum and gold are chosen for the cathode, and silver, for the anode. The gas-permeable membrane separates the two electrodes from the solution being measured. An appropriate polarization voltage causes diffusion of the oxygen across the membrane into the measurement cell, where it reaches the surface of the cathode and is reduced, producing hydroxide ions.

Reaction at the cathode:     O₂ + 4e^– + 2 H₂O      → 4 OH⁻

Reaction at the anode:        4 Ag + 4 Cl⁻               → 4 AgCI + 4e⁻

This chemical reaction creates an electrical current that is proportional to the partial pressure p_O2 of the oxygen. Oxygen must be steadily liberated from the solution being measured for the oxygen electrode to receive a constant supply. The concentration of oxygen in the medium can be determined using HENRY’s law and the solubility coefficient of oxygen [1]. Three different variations on the types of the equipment required for performing this analysis are present below.

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.