- By Kevin Stultz
- September 04, 2025
- Emerson
- Feature
Summary
Sensor advancements help optimize process efficiency, safety and quality with reliable measurement, while reducing maintenance requirements and operational costs.
Analytical sensors play an indispensable role in ensuring process safety, product quality and efficient production in many industries. Depending on the application, dissolved oxygen (DO) sensors are often used to optimize aeration efficiency, monitor microbial conditions, ensure regulatory compliance and/or keep equipment corrosion to a minimum.
As DO instrument technology evolves, particularly with the combination of optical sensing and digital communication protocols, there are increasing opportunities for plants to operate optimally, with improved process control and reduced costs. Let's discuss these advancements in DO instrumentation and compare modern capabilities to conventional measurement and maintenance methods.
Conventional sensing limitations
Historically, amperometric sensors were used for the bulk of industrial DO measurements. Although tried-and-true, these conventional methods introduced inherent challenges related to maintenance, calibration, accuracy and data integration.
Amperometric sensing is driven by an electrochemical reaction, where oxygen diffuses across a permeable membrane for reduction at a cathode. This reduction generates an electrical current proportional to the partial pressure of oxygen in the sample (Figure 1).
While effective under ideal conditions, amperometric sensors present several operational challenges:
- High maintenance requirements: Amperometric sensors rely on an internal electrolyte solution and a permeable membrane. The electrolyte depletes over time, requiring periodic replenishment, while the membrane is susceptible to both fouling and physical damage, demanding frequent cleaning and eventual replacement. These tasks consume valuable personnel time, introduce the risk of improper disassembly and reassembly, and increase a sensor's lifetime cost.
- Frequent and complex calibration: Sensor drift is common with amperometric technology, requiring frequent calibration to maintain accuracy. Calibration typically calls for sensor immersion in a zero-oxygen solution, followed immediately by exposure to a solution of known oxygen concentration. The time requirements to conduct multi-point wet calibration add up over the course of many cycles, and each procedure includes potential for manual error.
- Flow dependency: Most amperometric sensors consume oxygen while measuring, requiring a minimum process flow to maintain a constant supply of oxygen across the membrane surface and prevent localized depletion. This makes accurate measurement challenging in low-flow or static conditions, such as in tanks with minimal agitation, or during intermittent flow cycles.
- Signal instability and warm-up time: Amperometric sensors often require a significant polarization period after power-up or maintenance before providing stable readings. Signal drift can also occur due to changes in membrane permeability or electrolyte concentration.
- Analog signal limitations: Traditional amperometric instruments typically output an analog signal. While widely compatible with numerous host controllers, analog signals are susceptible to electromagnetic interference (EMI) and signal degradation over long cable runs. Furthermore, this analog signal can only transmit one data point.
These challenges place substantial demands on maintenance resources, requiring costly upkeep and limiting viability in challenging process environments.
Modern enhancements improve process control
Addressing these and other challenges, luminescence quenching optical sensors provide more accurate, timely, and versatile DO measurement, with decreased maintenance and operational cost requirements. This technology uses a light-emitting diode (LED) and luminescent dye to determine DO content in process media (Figure 2).
To measure DO, the sensor’s LED emits light of a known wavelength that excites the dye molecules. Oxygen molecules in the process media interact with the excited dye molecules, “quenching” some of the luminescence, which directly influences the resulting emitted wavelength and time for the dye molecules to return to their ground state. The degree of quenching is directly proportional to the partial pressure of oxygen in the process media. A photodiode then measures the characteristics of the emitted light, and internal sensor electronics calculate the dissolved oxygen concentration.
Optical technology (Figure 3) addresses the most prominent drawbacks of amperometric sensors, eliminating oxygen consumption during measurement. This makes these sensors ideal for use in low-flow, no-flow and other flow conditions, and they are accurate even at low DO concentrations.
Additionally, optical sensors do not require electrolyte refills or traditional membrane replacements, significantly reducing routine maintenance needs. The sensor cap with luminescent dye is the primary consumable, providing a lifespan of up to two years, compared to amperometric membrane and electrolyte changes every few months.
These sensors also exhibit significantly lower drift than their conventional counterparts, potentially extending calibration intervals from weeks to many months, depending on regulatory requirements. Leading optical sensors can be conveniently air calibrated without the need for media immersion, simplifying the procedure considerably compared to wet chemical calibration.
Furthermore, today’s leading optical DO instruments transmit measurement information as a digital instead of an analog signal. This advancement enhances information integrity because digital communication is not subject to electromagnetic noise, or signal degradation over long distances. This improves the accuracy of the measurement value received by a host, typically a control or an asset management system. Supported digital protocols—such as Modbus RTU—allow multi-drop configurations to connect multiple sensors on a single loop with a shared transmitter, such as the Rosemount 1058 digital process transmitter.
Digital optical DO sensor use cases
Optical sensors provide pointed benefits in certain applications, as detailed below.
Aerobic aeration in wastewater treatment
In most municipal wastewater treatment plants, process control is divided among several PLCs distributed around the plant footprint near key treatment equipment. Following primary treatment, which removes larger solids from wastewater, the product is pumped into aeration basins to begin separating organic and suspended solids from the wastewater to prepare it for environmental discharge (Figure 4).
At this stage, air is introduced into the aeration basins to maintain DO levels, enabling aerobic microorganisms to convert organic waste into inorganic byproducts. These byproducts are separated from the product water streams during secondary clarification following aeration.
If the DO content in the basin is too low, the microorganisms die, causing sludge to build up. If this occurs, expensive procedures are required to remove biomass and reintroduce bacterial organisms to react with the wastewater. To avoid this risk, facilities often overdose DO in their processes, but this can create undesired excess microorganisms. Accurate and responsive DO measurement is therefore required to hold concentration at optimal levels.
In the past, traditional analog amperometric DO sensors required separately wired loops from a PLC or remote-I/O cabinet to each aeration basin’s DO transmitter. Long wire runs sometimes compromised measurement signal integrity before receipt by the host system due to the significant distances involved, and degradation due to EMI was often an issue.
Additionally, amperometric sensors required frequent maintenance to maintain accuracy. With some large facilities containing dozens of basins, the calibration procedure could take hours or even days. Each sensor disassembly, calibration and reassembly increased the likelihood of compromised measurement.
Digital optical DO sensors, such as the Rosemount 490A, dramatically decrease calibration and maintenance requirements, translating into lower ongoing operational costs. The compact sensor design includes standard NPT threads on both ends, providing versatile mounting options that simplify retrofitting and installation in tight spaces, whether in-line, in tanks, or in open basins. Robust construction materials and an IP-68 rating ensure durability in these demanding chemical or wet environments.
Furthermore, because optical sensors do not consume oxygen, localized oxygen depletion in the low-flow environments of aeration basins is not a concern. Additionally, process control derived from these more reliable measurements provides cost savings due to optimal oxygen dosing, alleviating DO overshoot.
Optical sensors can extend the calibration interval in aeration basin applications from biweekly to just once annually, or even more infrequently, such as only when the sensor cap with luminescent dye is replaced. Digital instrumentation also enables daisy-chaining multiple sensors together with a shared transmitter, eliminating the need for individual home-run wire loops to the host system from every sensor.
Steam generation optimization
Nearly all utility management applications require measuring and controlling water quality parameters, including DO, to assure process quality, prevent corrosion, and ensure the integrity of equipment components.
The high temperature and pressure requirements for utility steam generation significantly accelerates the rate of corrosion and pitting of metal components, and DO can even cause oxygen tubercles over pitted areas. This condition allows corrosion to continue in the damaged areas, even when systems are properly maintained.
When used in boilers, the Rosemount 490A Dissolved Oxygen Sensor provides reliable DO measurement in these difficult environments, enabling quick transitions from wet to dry measurement. Flow changes are also common in these applications, and optical DO sensing can compensate quickly for variable flow with rapid response time, while conventional sensors cannot adjust as quickly to changing process conditions.
Innovation propels industry
Optical sensing technology, coupled with digital communication adoption, improves industrial DO measurement. Modern sensors provide a compelling value proposition in many industries, empowering plant personnel to overcome chronic maintenance burdens, calibration complexities, and inherent limitations of older amperometric and analog systems. These new instruments provide more reliable, accurate, and stable measurements, with significantly reduced maintenance requirements.
The ability to operate reliably in challenging conditions improves process efficiency, enhances product quality and provides regulatory compliance assurance, while reducing the propensity for human error, operational expenditures and total cost of ownership. As industry continues its pursuit of greater automation and efficiency, the addition of digital optical DO sensor technology provides another step forward for the operation and maintenance of process instrumentation.
All figures courtesy of Emerson
This feature was published in the August/September issue of Automation.com Monthly.
About The Author
Kevin Stultz is a product management leader with over 14 years of experience at Emerson. Currently serving as senior global product manager, Kevin leads a team overseeing both the Liquid and Combustion Analysis portfolios. Holding an MBA from the University of Minnesota's Carlson School of Management and a bachelor’s in chemical engineering from Iowa State University, Stultz combines technical expertise with strong business acumen.
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