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Optical Oxygen Measurement Improves Ease of Use and Reliability

November 6, 2016  

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For many decades, oxygen measurement systems based on amperometric technology have been a reliable and easy to use solution in a wide range of applications. But the interest of the market in new solutions has grown in line with increasing requirements for reliability, user friendliness and cost efficiency in demanding biotech and brewery processes. Optical measurement technology has significant advantages over amperometric technology. This white paper explains what these advantages are and why optical measurements are replacing amperometric systems.

A natural phenomenon
Fluorescence is a natural phenomenon of many organic and inorganic substances. It is present in many situations in our daily life such as illumination for watches and security features for bank notes. Optical sensors for dissolved oxygen (DO) measurement use the principle of fluorescence quenching. This technology was first published in 1931 but it took nearly 70 years to be suitable for process analytical sensors. Over the past decade, this technology has increasingly entered the process analytical market since it offers important advantages compared to other technologies in terms of stability, user friendliness and cost efficiency.

From light to oxygen measurement
In order to measure the dissolved oxygen in a liquid, it is necessary that oxygen is in contact with the chromophore inside the sensor and influences the fluorescence properties. To achieve this, the chromophore is embedded in an oxygen permeable matrix in the sensor tip. The chromophore is illuminated by an LED inside the sensor and the fluorescence light is measured with a photo detector inside the sensor. Oxygen is able to penetrate into the sensor tip and interact with the chromophore. Without oxygen, most of the absorbed energy is released as fluorescence.

The more oxygen is present the less fluorescence light appears, hence the shorter is the fluorescence lifetime. For the calculation of the oxygen partial pressure in most process analytical sensors only the change of the fluorescence lifetime is used. The change in the intensity of the fluorescence is less accurate and thus not suitable for accurate measurement.

To measure the fluorescence lifetime it is necessary to modulate the intensity of the excitation light. As a consequence, the intensity of the fluorescence is also modulated. The detector now measures a sinus shaped curve describing the intensity change. The time between the maximum intensity of the excitation and the maximum intensity of the fluorescence is used for calculating the oxygen value. This time shift or phase shift is not linearly correlated to the oxygen concentration.

In amperometric systems the measured current is linearly correlated to the oxygen value. In optical systems the phase decreases exponentially with increasing oxygen concentration. This decrease is described by the Stern Volmer equation.

Calibration, a challenge for optical systems
Depending on the process conditions such as oxygen level, temperature or use of cleaning solutions, the readings from an oxygen sensor exhibit drift. For amperometric and optical systems the reasons drift occurs are different. Whereas in amperometric sensors the progressive degradation of the electrolyte or the membrane are the most important drift factors, in optical systems the degradation of the chromophore is the reason for the sensor drift. An important drift factor is the measurement itself. If the chromophore transfers the energy to the oxygen molecule, the oxygen molecule becomes more aggressive and is able to destroy the chromophore or the matrix. This effect is greater the higher the oxygen concentration or temperature is. Also, high temperature or treatment with cleaning agents may influence the drift of the sensing element. These factors result in a change of the fluorescence intensity and the phase of the signal over time. To ensure accurate measurement over the whole measurement range, accurate calibration is necessary. The newest systems are able to determine the degradation of the chromophore and automatically compensate for sensor drift.

Calibration in amperometric sensors is quite easy. The linear correlation between the measured current and the oxygen concentration can be described with the zero value and the slope. In most amperometric sensors the zero current results in a zero oxygen reading. A slope correction with an air calibration is sufficient to reach the required accuracy in many applications. If high accuracy at very low oxygen levels is needed, a zero point calibration is necessary.

In optical oxygen measurement the correlation of the oxygen value to the measured phase is not linear. The individual calibration curve of a sensor depends on several factors: the phase at 100 % air, the phase at zero and additional factors that describe the shape of the curve. Many determinants are sensor specific and are determined during factory calibration in the manufacturing, but some parameters change over time and have to be adjusted during a calibration. Slope correction of a nA or a mA signal from the sensor or a transmitter (as can be performed for amperometric systems) is insufficient for optical systems. The challenge is to find easy calibration routines for fast and accurate adjustment of the sensor. Wrong calibration is the main source of measurement errors with optical sensors. For the user, the handling should be nearly the same as for amperometric sensors.

Intelligent controlling of optical technology
Today’s optical sensors contain all routines needed for sensor calibration. With a transmitter all sensor functions can easily be accessed. Especially for use in continuous processes such as fermentation (fed-batch) or in a brewery filler line it is necessary to have in-line calibration routines. High-end systems offer various calibration routines to fully meet the process requirements. For example, the process calibration can be performed while the process is running. All optical sensors need tools or computer software for configuration or real calibration. A slope correction of a nA or mA signal is not sufficient for long term accuracy. Intelligent sensor systems significantly simplify all tasks and offer all necessary routines.

Reduced maintenance
The main advantage of METTLER TOLEDO’s sensors with optical technology is the easier handling due to the fact that only one consumable, the OptoCap, has to be replaced periodically. No electrolyte, no fragile membrane and no inner body has to be maintained. The OptoCap can be replaced in less than a minute and no polarization is necessary. Due to this simple construction, the risk of handling errors is minimized. The lifetime of the OptoCap in standard fermentations (37 °C, 100 % air saturation) is significantly longer than that of the membrane body of amperometric sensors.

Intelligent sensors know what to do
Wrong oxygen measurement may result in a significantly reduced growth rate during fermentation. The more complicated the measurement system is, the more important it is to find intelligent solutions for monitoring the sensor status in real time. Since the correlation between the raw data and the oxygen value is not as simple for oxygen sensors as it is for amperometric sensors, the sensing element and the changes of the quality should be monitored. To ensure a reliable measurement, intelligent routines are necessary to monitor the process conditions, calculate the sensor load or measure the changes in the sensing element. The user needs to be aware of the condition of the sensor. With this information the user can make the decision when to perform the next calibration or to replace the sensing element.
Intelligent Sensor Management (ISM), a technology developed by METTLER TOLEDO, offers significant additional advantages to the end user.

Ease of use
With ISM, pre-calibrated and pre-configured sensors make installation significantly easier than before. Sensor data such as serial number, calibration history and diagnostic data are stored in the sensor memory and are always available. No manual documentation is necessary. Errors that can happen with manual documentation are largely avoided. Enhanced diagnostics improve process safety. The Dynamic Lifetime Indicator (DLI) continuously calculates the stress on the OptoCap and translates this information into a predicted lifetime. Before a batch is started, the user has up to date status information on the quality of the sensor and the Opto- Cap. The lifetime of the OptoCap is calculated for standard conditions and during the process the sensor takes the process conditions into account for on-time calculation of the remaining lifetime of the OptoCap. The risk of sensor failure during the process is reduced to a minimum.

The Adaptive Calibration Timer (ACT) provides information on the time until a calibration of the system is necessary. As long as this timer has not expired, sensor accuracy is within the specified values. Even if the ACT has expired, the system will give an oxygen reading and the user knows that the accuracy of the measurement may be out of the specified value. The accuracy of the system is now predictable.

The diagnostics are not only used to inform the user of sensor condition, but are also used for automatic control of sensor stability.

The clear advantages in terms of easier and faster maintenance, the performance, together with ISM technology allow for highly improved process control and safety. The risk of out-of-spec production due to wrong oxygen control is significantly reduced.