Optical DO Whitepaper: Technology

Size: px

Start display at page:

Download "Optical DO Whitepaper: Technology"

Daisy Atkinson
6 years ago
Views:

INGOLD Leading Process Analytics Optical Oxygen Measurement Enlight your process control Optical oxygen measurement improves ease of use and reliability.

1 INGOLD Leading Process Analytics Optical Oxygen Measurement Enlight your process control Optical oxygen measurement improves ease of use and reliability. Amperometric oxygen measurement systems offer reliable and easy to use solutions for many applications. But the interest of the market for optical solutions increases since optical technology offers significant advantages in demanding applications like fermentation in biotech and brewery processes. The optical technology shows significant advantages towards amperometric measurement which need to be fully understood in order to maximize the benefits of chemo-optical measurement techniques Fluorescence, a natural phenomenon Optical sensors for dissolved oxygen (DO) measurement use the principle of fluorescence quenching for oxygen measurement. This technology was first published in 1931 but it took nearly 80 years to be suitable for process analytical sensors. Optical oxygen measurement with fluorescence is in contrast to absorption not a direct oxygen measurement. With fluorescence the interaction of oxygen with another substance is measured. Some molecules which are able to absorb light of a specific wavelength can release the absorbed energy after a short delay time as light. This mechanism is called Luminescence. Fluorescence is a special form of luminescence where the lifetime of light has different properties. Luminescence is present in many situations. Illumination of watches, security features for bank notes, are a few examples of daily life.

2 From light to oxygen measurement In order to use fluorescence to measure the dissolved oxygen in a liquid, it is necessary that oxygen gets in contact with the fluorescing substance and influence the release of the fluorescence. In optical oxygen sensors, oxygen is able to penetrate a permeable layer and diffuse to a matrix where the fluorescing molecule (dye) is embedded. When no oxygen is present, most of the absorbed energy is released as fluorescence. When oxygen is present, it can get in contact with the dye and it is able to take over the energy and no fluorescence appears. 1. Excitation with light 2. Energy Transfer Energy 3. Relaxation through energy transfer Figure 1: Fluorescence quenching As a result of the presence of oxygen, not only the intensity of the fluorescence light is reduced, but also the time under which the fluorescence is taking place. The reason is that the longer the dye is in its excited state, the higher is the probability to get in contact with oxygen. This results into a higher quenching for the molecules with longer lifetime of the excited state. For oxygen measurement only the time between the excitation and the release of the fluorescence is used. The measurement of the change in the intensity of the fluorescence is less accurate. To measure this time, it is necessary to modulate the intensity of the excitation light. As a consequence, also the intensity of the fluorescence shows a modulation. 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 now the raw measurement used for calculating of the oxygen value. This time shift or phase is in contrast to amperometric systems not linearly correlated to the oxygen concentration. See Fig. 2. Natural florescence in minerals 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. In modern optical DO systems, the Stern Volmer plot is integrated. Calibration, a challenge for optical systems Depending on the process conditions like oxygen level, temperature or use of cleaning solutions, the reading of an oxygen sensor shows a drift. For amperometric and optical systems the reasons are different. Whereas at amperometric sensors the progressive degradation of the electrolyte or the membrane is the most important factor, in optical systems other changes can be observed. Depending on the oxygen level, we find production of very aggressive singlet oxygen which is a meta-stable state of oxygen with higher energy. These oxygen radicals can react with the dye or the matrix where it is embedded and destroy it. 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 range, accurate calibration is necessary. Calibration in amperometric sensors is quite easy. The linear correlation can be described with the zero value and the slope. In most amperometric sensors the zero current results in zero oxygen reading. The slope of the current is constant over a wide measurement range. Only a slope correction with an air calibration is sufficient to reach the required accuracy. In optical oxygen measurement the correlation of the oxygen value to the measured phase is much more challenging. The individual Reference Low oxygen High oxygen Sensor Signal 0.1 µs Figure 2: Fluorescence signals measured in the sensor. 2 METTLER TOLEDO Time % Air Figure 3: Correlation between the measured signal and the oxygen concentration.

Intelligent controlling of optical technology Todays optical sensors contain all routines needed for sensor calibration. With a transmitter all sensor functions can easily be accessed.

We offer a procedure for process calibration that can be performed while the process is running. For controlling the sensor, different systems are avail- able in the market.

3 Intelligent controlling of optical technology Todays 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 like fermentation (feed-batch) or in a filler line of a brewery it is necessary to have in-line calibration routines. We offer a procedure for process calibration that can be performed while the process is running. For controlling the sensor, different systems are avail- able in the market. Some systems need additional tools or computer software for configuration or real calibration and offer only a na or ma output for connecting the sensor to a installed transmitter or control system. The disadvantage is that no inline controlling of the sensor is possible. Optical sensors can further be improved with intelligent control systems. Reduced maintenance The main advantage of optical technology is the easier handling due to the fact that only one consumable, the OptoCap has to be replaced from time to time. No electrolyte, no fragile membrane and no inner body as we have in amperometric systems have to be maintained. The OptoCap can be replaced within seconds 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, 0 % air saturation) is at least 3 times longer compared to the membrane body of amperometric sensor. Intelligent sensors know what to do Wrong oxygen measurement may result in significantly reduced yield during fermentation. The more complicated the measurement system is, the more it is important 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 as it is for amperometric calibration curve of a sensor depends on several factors, the phase at 0 % air, the phase at zero and additionally many factors that describe the shape of the curve. Many factors are sensor specific and determined during factory calibration in the manufacturing, but some parameters change over time and have to be determined or calculated during a calibration. Only slope correction of a na or a ma signal from the sensor or a transmitter as we can perform for amperometric systems is insufficient for optical systems. Fig. 4 shows calibration curves for a new sensor and for sensors after several fermentations. 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 % Air Figure 4: Calibration curves for INGOLD`s InPro 6880i sensor (brown: new sensor, pink: sensor after fermentations, yellow sensor after fermentations.) Dotted lines represent the curves after slope correction of a na or ma signal (orange: after fermentations, blue after fermentations. The arrows represent the errors of the reading after slope correction. METTLER TOLEDO 3

4 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 real changes of the sensing element. The user needs to be informed about the situation of the sensor. With this information the user can make the decision when to perform the next calibration or to replace the sensing element. Benefits The clear advantages in terms of easier and faster maintenance the performance together with the ISM technology allow for highly improved process control and safety. The risk of out of spec production due to wrong oxygen controlling is highly reduced. ISM (Intelligent Sensor Management), a technology developement by METTLER TOLEDO, offers significant additional advantages to the end user. Ease-of-use Pre-calibrated and pre-configured sensors make the installation easier as ever before. Sensor data like 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 improves the process safety. The DLI (Dynamic Lifetime Indicator) permanently calculates the stress on the OptoCap and translates this information into a predicted lifetime. Before a batch is started the user has all up to date information about the quality of the sensor and the OptoCap. The lifetime of the OptoCap is calculated for standard conditions and during the process the sensor takes into account the process conditions for on-time calculation of the remaining lifetime of the OptoCap. The risk of sensor failure during the process is reduced to a minimum. Features overview Short response time Low detection limit No stop of flow Plug and measure Enhanced Diagnostics Dynamic Lifetime Indicator Adaptive Calibration Timer Sensor history CIP / SIP counter Calibration data Hygienic design The ACT (Adaptive Calibration Timer) gives information about the time until a calibration of the system is necessary. As long as this timer is not expired, the sensor accuracy is within the specified values. Even if the ACT is 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. OptoCap Replacement... OptoCap

At a Glance The optical measurement principle The heart of the optical sensor is an oxygen sensitive layer containing immobilized marker molecules.

The fluorescence depends on the amount of oxygen that is present in the environment of the marker molecules. This effect allows determination of the oxygen concentration in the sample media.

5 At a Glance The optical measurement principle The heart of the optical sensor is an oxygen sensitive layer containing immobilized marker molecules. They absorb light from a light emitting diode and are able to release this energy as light at a different wavelength (fluorescence). The fluorescence depends on the amount of oxygen that is present in the environment of the marker molecules. This effect allows determination of the oxygen concentration in the sample media. LED Sensor tip sensitive layer InPro 6880 i InPro 68 i Detector Emitted fluorescence light An oxygen-sensitive layer containing immobilized marker molecules is the heart of the optical sensor. M0 for high versatility and advanced process control For more information: i i Mettler-Toledo AG Process Analytics Im Hackacker 15 CH-8902 Urdorf Switzerland Visit for more information

Oxygen Theory Guide. A Guide to Oxygen Measurement Theory and Practice of Oxygen Applications. Mettler-Toledo GmbH Process Analytics

Oxygen Theory Guide. A Guide to Oxygen Measurement Theory and Practice of Oxygen Applications. Mettler-Toledo GmbH Process Analytics Mettler-Toledo GmbH Process Analytics Industry Environment A Guide to Oxygen Measurement Theory and Practice of Oxygen Applications Copyright 2016 by Mettler-Toledo GmbH CH-8902 Urdorf/Switzerland A Guide