HOW TO DO O2? Measurement of dissolved oxygen In-line measurements of the most important parameters are well established at breweries. To evaluate the quality and the shelf life of beer, measurement of dissolved oxygen is very common. In this article the two most common principles, amperometric and optical, are compared when installed in the filling process at the Krombacher Brewery in Germany.
The principlesOxygen can enter at different stages of the brewing process and influence the taste and the shelf life in a negative way, so the content of dissolved oxygen is monitored continuously at different stages of the process.
Oxygen content can be measured in different ways. The most common principles are amperometric and optical measurement. In the classical amperometric (electrochemical) procedure as described by Clark, the oxygen induces after diffusion through a membrane, and a chemical reaction with the electrolyte of the sensor creates a voltage differential. The induced voltage is the measurement signal which indicates the amount of oxygen present in the system. Essential components of this sensor type are an intrinsic cathode and anode as well as the membrane body filled with electrolyte.
The optical measurement principle is based on fluorescent quenching of a luminophore when oxygen is present. In this case the sensor contains a light source that excites the luminophore at a specific wavelength. Once excited, the luminophore emits a characteristic light with a longer wavelength. Its intensity and phase shift depends on the amount of oxygen present. Using specific filters, only the light emitted by the luminophore will be trapped and measured.
In-line measurements must fulfil high requirements. Besides accuracy, repeatability, response time, and detection limit, an easy installation, calibration, and maintenance are also important. Filling lines are the definitive test for sensors because they must withstand heavy pressure and temperature changes caused by the hammering effect of valves, as well as frequently occurring CIP cycles.
Comparison of different sensor types
In order to compare the two different measurement principles under harsh conditions, three optical dissolved oxygen sensors (VisiFerm DO) from Hamilton were installed and tested in parallel to the previously installed amperometric sensor for several months in a filling line of the Krombacher Brewery. Figure 1 shows the installation situation at the filling line. Particular attention was paid to treating all sensors the same to ensure that they withstood the same requirements.
During the testing period significant differences in the behaviour of the sensors was observed. The three VisiFerm DO sensors showed congruent results at any time whereas the amperometric sensor, depending on the situation, showed a completely different dissolved oxygen reading. The differences, all important for the daily operations, are explained in the following.
When a filling line stops, closing valves cause a pressure hammer and in mid-term an increasing temperature of the bound liquid. This pressure hammer is transferred to the electrode through the membrane to the electrolyte of the amperometric sensor. This leads to a massive disturbance of the oxygen content equilibrium on both sides of the membrane.
Hamilton optical oxygen sensors are resistant to pressure hammers because there is no liquid inside the membrane cap. The optics and electronics behind the membrane are protected by a sapphire glass. Figure 2 shows the differences between both measurement principles where the differences can easily be recognised.
During the stagnation of the system, an increase of the voltage generated by the perometric sensor can be observed. In the present case, the value exceeded the determined alarm level several times. This resulted in a further delay of the re-start of the filling line because the filler is controlled amongst others by the value of the dissolved oxygen. The measurements of the three VisiFerm DO showed that the dissolved oxygen content maintained similar values during the stagnation. The VisiFerm DO sensors showed no stop-of-flow effects and the filling could be continued at any time.
When changing to another product, after the replacement of components or sensors, or after defined intervals, a CIP procedure is necessary. During this time very harsh conditions regarding the temperature and pH-value dominate, and the different sensors will be switched off or their measurement values are out of their calibration range. After a CIP procedure the piping system and the filler are rinsed with product and the eluate will be dismissed.
At this point it is of critical importance that the sensors become operational very fast. Figure 3 shows that optical dissolved oxygen sensors are ready much faster for operation than amperometric sensors are. The difference can be several minutes where, depending on the capacity of the filler, 5, 10, or even 20 hectolitres of beer must be dismissed and have to be treated as waste water. At this stage money is poured through the gully and the filling performance gets weak.
High plant availability is an important economic factor. Thus it’s important that sensors are not susceptible to faults and maintenance, and calibration take only little time. In this respect optical sensors have great advantages compared to amperometric ones. The only consumable part of the VisiFerm DO is the sensor cap that has to be replaced, depending on the treatment and the number of CIP cycles, only every four to eight months. This procedure takes only a few minutes including the calibration with the new cap. Polarisation, replacement of the mechanical sensitive parts cathode and anode, as well as the refilling of the membrane with electrolyte are obsolete. Maintenance with the VisiFerm is easier and faster.
The optical measurement of dissolved oxygen has grown up and is suitable for controlling sophisticated processes. The advantages compared to amperometric measurement of dissolved oxygen are not only the higher plant availability, the robustness, and easy maintenance, but also the missing hyposensitivity to CO2 that can be found for some classical sensors. The VisiFerm DO can thus be used for measurement of the purity of CO2 in the gaseous phase.
The potential abandonment of an external transmitter, the self-diagnosis, and the storage of the calibration data in the sensor head when using the VisiFerm DO is not discussed here. When the advantages are summarised, the return on investment of the change-over to optical measurement of dissolved oxygen can quickly be recognised. In the long term perspective, the VisiFerm DO can contribute to easier operating procedures and better results.