Overcoming the Technical Challenges of Electrochemical Gas Sensing: Part 1
December 31, 2019
Electrochemical gas sensors are a proven technology that dates back to the 1950s, when they were developed for oxygen monitoring.
Electrochemical gas sensors are a proven technology that dates back to the 1950s, when they were developed for oxygen monitoring. One of the first applications of this technology was a glucose biosensor, where it was used to measure the depletion of oxygen in glucose. Over the following decades, the technology has advanced, allowing the sensors to be miniaturized and to detect a wide variety of target gases.
With the advent of the world of ubiquitous sensing, countless new gas sensing applications have emerged across many industries, for example, automotive air quality monitoring or electronic noses. Evolving regulations and safety standards led to requirements that are much more challenging than in the past for both new and existing applications. In other words, gas sensing systems of the future must accurately measure much lower concentrations, be more selective toward the target gases, operate for longer durations from battery power, and provide consistent performance over longer periods of time while always maintaining safe and reliable operation.
Pros and Cons of Electrochemical Gas Sensors
The popularity of electrochemical gas sensors can be attributed to the linearity of their output, low power requirements, and good resolution. Moreover, once calibrated to a known concentration of the target gas, the repeatability and accuracy of measurement is also excellent. Thanks to the evolution of the technology over the decades, these sensors can offer very good selectivity to a particular gas type.
Industrial applications, for example, toxic gas detection for worker safety, were the first to utilize electrochemical sensors thanks to their many advantages. Economical operation of these sensors enabled deployment of area toxic gas monitoring systems, ensuring safe environmental conditions for employees in industries such as mining, chemical industries, biogas plants, food production, pharmaceutical industries, and many others.
While the sensing technology itself is constantly advancing, its basic operating principle, with the disadvantages that come with it, have not changed since the earliest days of electrochemical gas sensing. Typically, electrochemical sensors have a limited shelf life, usually six months to one year. Aging of the sensor has a major impact on its long-term performance, too. Sensor manufacturers often specify that the sensor sensitivity can drift by up to 20% per year. Furthermore, even though the target gas selectivity has improved significantly, the sensors still suffer from cross-sensitivity to other gases, resulting in an increased chance of interference in measurement and erroneous readings. They are also temperature dependent and have to be internally temperature compensated.
The technical challenges that need to be overcome while designing an advanced gas sensing system can be split into three groups corresponding to different life stages of the system.
Firstly, there are sensor manufacturing challenges such as manufacturing repeatability, and sensor characterization and calibration. The manufacturing process itself, while highly automated, inevitably introduces variability to every sensor. Due to these variances, the sensors must be characterized and calibrated in production.
Secondly, technical challenges exist throughout the system’s life. These include system architecture optimization; for example, signal chain design or power consumption consideration. Primarily in industrial applications, a large emphasis on electromagnetic compatibility (EMC) and functional safety compliance negatively impacts design cost and time to market. Operating conditions also play a significant role and pose challenges to maintain the required performance and lifetime. It is the nature of this technology that the electrochemical sensors age and drift during their life, resulting in frequent calibration or sensor replacement. This change in performance is further accelerated if operating in harsh environments, as covered later in this article. Prolonging the sensor’s life while maintaining its performance is one of the key requirements for many applications, especially in cases where the cost of ownership of the system is critical.
Thirdly, even after employing techniques prolonging their operation, all electrochemical sensors ultimately reach their end of life, when the performance no longer meets the requirements and the sensor needs to be replaced. Effectively detecting the end-of-life condition is a challenge that, when overcome, can substantially decrease cost by reducing unnecessary sensor replacements. By taking a step further, and predicting when exactly the sensor will fail, the cost of operating a gas sensing system can be reduced even more.
The utilization of electrochemical gas sensors is increasing in all gas sensing applications, and this creates challenges in terms of logistics, commissioning, and maintenance of these systems, which results in increased total cost of ownership. Therefore, application specific analog front ends with diagnostic capabilities are employed to reduce the impact of disadvantages of the technology, mainly the limited sensor life, to ensure long-term sustainability and reliability of the gas sensing systems.
Signal Chain Integration Reduces Design Complexity
The complexity of traditional signal chains, which are in most cases designed with standalone analog-to-digital converters, amplifiers, and other building blocks, forces designers to compromise on power efficiency, measurement precision, or PCB area consumed by the signal chain.
An example of such a design challenge is an instrument with a multigas configuration, which measures several target gases. Each sensor might require a different bias voltage for its proper operation. Moreover, each sensor’s sensitivity might be different—thus the amplifiers’ gains must be adjusted to maximize the signal chain performance. For the designer, these two factors alone increase the design complexity of a configurable measurement channel that would be able to interface with different sensors without BOM or schematic changes. A simplified block diagram of a single measurement channel is shown in Figure 1.
Just like in any other electronics system, integration is a logical step in evolution, enabling the design of more efficient and more powerful solutions. Integrated, single-chip gas sensing signal chains simplify the system design by, for example, integrating the TIA (transimpedance amplifier) gain resistors or employing a digital-to-analog converter as a sensor bias voltage source (as seen in Figure 2). Thanks to the signal chain integration, the measurement channel can be fully configurable through software to interface with many different electrochemical sensor types while reducing the complexity of the design. Furthermore, the power requirements of such an integrated signal chain are also notably lower, which is crucial for applications where battery lifetime is a key consideration. Finally, the measurement precision is improved as a result of decreasing the noise level of the signal chain and, potentially, utilizing signal processing components, such as a TIA or ADC with better performance.
Looking back at the example of a multigas instrument, thanks to signal chain integration it is possible to:
- Enable fully configurable measurement channels while reducing the signal chain complexity, thus easily reusing a single signal chain design
- Reduce the PCB area consumed by the signal chain
- Decrease power consumption
- Improve measurement accuracy
In part two of this series, we’ll address the fundamental disadvantage of electrochemical gas sensors—deteriorating performance over their lifetime—as well as EMC system-level issues.
About the Author
Michal Raninec is a systems applications engineer in the Industrial Systems Group within the Automation and Energy Business Unit at Analog Devices. His areas of expertise include electrochemical gas sensing and wireless sensor networks. Michal received his M.Eng. degree in electronic engineering from Brno University of Technology, Czech Republic. He can be reached at [email protected]