Solving electrochemical gas sensor technology challenges clears the way for us to enter the era of ubiquitous sensing

With the advent of the era of ubiquitous sensing technology, countless new gas detection applications have emerged in many industries, such as automotive air quality monitoring or electronic noses. Evolving regulations and safety standards place more challenging requirements on new and existing applications than in the past. In other words, future gas detection systems must accurately measure much lower concentrations, be more selective for target gases, operate longer on battery power, and provide stable and consistent performance over longer periods of time, all while maintaining safe and reliable operation.

The popularity of electrochemical gas sensors can be attributed to their linear output, low power requirements, and good resolution. In addition, once calibrated to a known concentration of the target gas, the repeatability and accuracy of their measurements are also very good. Decades of technological development have allowed these sensors to provide very good selectivity for specific gas types.

Industrial applications, such as toxic gas detection for worker safety, have pioneered the use of electrochemical sensors due to their numerous advantages. The economical operation of these sensors has facilitated the deployment of regional toxic gas monitoring systems, ensuring safe environmental conditions for employees in industries such as mining, chemical industry, biogas plants, food production, pharmaceutical industry, etc.

While the detection technology itself is constantly improving, the basic operating principles and inherent shortcomings of electrochemical gas sensing have not changed since its inception. Typically, electrochemical sensors have a limited shelf life of six months to a year. Aging of the sensor can also have a significant impact on its long-term performance. Sensor manufacturers typically specify that sensor sensitivity can drift by up to 20% per year. In addition, while target gas selectivity has improved significantly, the sensor still has cross-sensitivity issues to other gases, resulting in interference in the measurement and an increased chance of erroneous readings. Sensor performance is also temperature dependent and must be internally temperature compensated.

The technical challenges that need to be overcome in designing advanced gas detection systems can be divided into three categories, corresponding to different stages of the system life cycle.

The first is sensor manufacturing challenges, such as manufacturing repeatability and sensor characterization and calibration. The manufacturing process itself, while highly automated, will inevitably introduce differences to each sensor. Due to these differences, sensors must be characterized and calibrated during production.

Secondly, there are technical challenges throughout the life cycle of the system. This includes system architecture optimization, such as signal chain design or power consumption considerations. In addition, there is a special focus on electromagnetic compatibility (EMC) and functional safety compliance in industrial applications, which can have a negative impact on design costs and time to market. Operating conditions also play an important role and pose challenges to maintaining the required performance and service life. Electrochemical sensors age and drift during their service life (this is the nature of this technology), resulting in the need for frequent calibration or replacement of sensors. If operating in harsh environments, the change in performance is further accelerated, as described later in this article. Maintaining the performance of the sensor while extending its service life is one of the key requirements for many applications, especially when the cost of system ownership is critical.

Third, even with the use of life extension techniques, all electrochemical sensors will eventually reach the end of their life, at which point the performance is no longer satisfactory and the sensor needs to be replaced. Effectively detecting the end-of-life condition is a challenge that, if solved, can reduce unnecessary sensor replacements and thus significantly reduce costs. Furthermore, if the ability to accurately predict when a sensor will fail can reduce the operating costs of the gas detection system even more.

The utilization of electrochemical gas sensors is increasing in all gas detection applications, which brings challenges to the logistics, commissioning and maintenance of such systems, resulting in an increase in the total cost of ownership. Therefore, dedicated analog front ends with diagnostic capabilities are used to reduce the impact of technical shortcomings (mainly the limited sensor life) and ensure that gas detection systems are sustainable and reliable in the long term.

Traditional signal chains are mostly designed with independent analog-to-digital converters, amplifiers, and other building blocks, which are quite complex and force designers to make compromises in power efficiency, measurement accuracy, or PCB area occupied by the signal chain.

An example of this design challenge is an instrument with a multi-gas configuration that can measure multiple target gases. Each sensor may require a different bias voltage to operate properly. In addition, the sensitivity of each sensor may be different, so the gain of the amplifier 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 should be able to interface with different sensors without changing the BOM or schematic. A simplified block diagram of a single measurement channel is shown in Figure 1.

Just like any other electronic system, integration is a logical step in the evolution, and integration allows the design of more efficient and powerful solutions. Integrated single-chip gas sensing signal chains simplify system design by integrating TIA (transimpedance amplifier) ​​gain resistors or using digital-to-analog converters as sensor bias voltage sources, as shown in Figure 2. Due to the signal chain integration, the measurement channel can be fully configured through software to interface with many different types of electrochemical sensors while reducing the complexity of the design. In addition, the power requirements of this integrated signal chain are significantly reduced, which is crucial for applications where battery life is a key consideration. Finally, the measurement accuracy is improved due to the reduced noise level of the signal chain and the possibility of utilizing better performing signal processing devices (such as TIA or ADC).

Looking back at the example of multi-gas instrumentation, signal chain integration enables it to:

• Enables fully configurable measurement channels while reducing signal chain complexity, allowing easy reuse of a single signal chain design

• Reduce the PCB area occupied by the signal chain

• Reduce power consumption

• Improve measurement accuracy

While signal chain integration is an important step forward, it does not by itself address the fundamental drawback of electrochemical gas sensors, which is that their performance degrades over time. Understandably, this is a result of the sensor’s operating principle and construction. Operating conditions also contribute to performance degradation and accelerate sensor aging. Sensor accuracy degrades until it becomes unreliable and no longer fit for purpose. In this case, the usual approach is to take the instrument offline and manually inspect the sensor, which is both time-consuming and expensive. Depending on its condition, the sensor can then be recalibrated and used again, or it may need to be replaced. This incurs considerable maintenance costs. By utilizing electrochemical diagnostics, the health of the sensor can be analyzed and performance changes effectively compensated for.

Common factors that cause performance degradation include excessive temperature, humidity, and gas concentrations or electrode poisoning. Short-term exposure to higher temperatures (above 50°C) is generally acceptable. However, repeatedly subjecting the sensor to high temperatures can cause the electrolyte to evaporate and cause irreversible damage to the sensor, such as causing a shift in baseline readings or a slow response time. On the other hand, very low temperatures (below –30°C) can greatly reduce the sensitivity and responsiveness of the sensor.

Humidity is the factor that has the greatest impact on sensor life. The ideal operating conditions for electrochemical gas sensors are 20°C and 60% relative humidity. Ambient humidity below 60% can cause the electrolyte inside the sensor to dry out, affecting the response time. On the other hand, humidity above 60% can cause water in the air to be absorbed by the sensor, diluting the electrolyte and affecting the sensor's characteristics. Absorbed moisture can also cause sensor leakage and may cause pin corrosion.

The magnitude of the above degradation mechanisms will affect the sensor, even if it is not very large. In other words, things like electrolyte depletion are natural occurrences that cause sensor aging. Regardless of the operating conditions, the aging process will limit the lifetime of the sensor, although some EC Sense gas sensors can operate for more than 10 years.

The sensor can be analyzed using techniques such as electrochemical impedance spectroscopy (EIS) or chronoamperometry (applying a bias voltage pulse while observing the sensor output).

EIS is a frequency domain analysis measurement performed by exciting an electrochemical system with a sinusoidal signal (usually a voltage). At each frequency, the current flowing through the electrochemical cell is recorded and used to calculate the impedance of the cell. The data is then usually displayed in the form of Nyquist and Bode plots. The Nyquist plot shows the complex impedance data, with each frequency point plotted by the real part on the x-axis and the imaginary part on the y-axis. The main disadvantage of this data representation is that the frequency information is lost. The Bode plot shows the impedance magnitude and phase angle as a function of frequency.

The experimental measurements show a strong correlation between the drop in sensor sensitivity and the change in EIS test results. The example in Figure 3 shows the results of an accelerated life test where an electrochemical gas sensor was placed in a low humidity (10%RH) and high temperature (40°C) environment. Throughout the experiment, the sensor was periodically removed from the environmental chamber and left for an hour, followed by a baseline sensitivity test and an EIS test at a known target gas concentration. The test results clearly show the correlation between sensor sensitivity and impedance. The disadvantage of this measurement is that it is quite time-consuming, as it is very time-consuming to obtain measurement results at very low sub-hertz frequencies.

Chronoamperometry (pulse testing) is another technique that can help analyze the health of a sensor. The measurement is made by superimposing a voltage pulse on the sensor bias voltage while observing the current flowing through the electrochemical cell. The pulse amplitude is typically very low (e.g., 1 mV) and short (e.g., 200 ms) so it does not disturb the sensor itself. This allows the test to be performed fairly frequently while the gas detection instrument remains operational. Chronoamperometry can be used to check that the sensor is physically inserted into the device before performing a more time-consuming EIS measurement and can also indicate changes in sensor performance. An example of a sensor's response to a voltage pulse is shown in Figure 4.

Previous sensor probing techniques have been used in the electrochemical field for decades. However, the equipment required for these measurements is typically expensive and bulky. Testing the large number of gas sensors deployed in the field using such equipment is simply not practical and financially feasible. To enable remote, in-line sensor health analysis, diagnostic features must be integrated directly as part of the signal chain.

With integrated diagnostics, gas sensors can be tested automatically without human intervention. If gas sensors are characterized in production, the data obtained from the sensors can be compared with these characterization data sets, providing insight into the current condition of the sensor, and then using smart algorithms to compensate for the loss of sensor sensitivity. In addition, the history of the sensor can support predictions of when its lifetime will end and alert the user when the sensor needs to be replaced. Built-in diagnostics ultimately reduce the need for maintenance of gas detection systems and extend the service life of sensors.

Safety and reliability are of paramount importance, especially in industrial settings. When operating in harsh industrial environments, such as chemical plants, there are strict regulations to ensure that gas detection systems meet these requirements and remain reliable and fully functional.

Electromagnetic compatibility (EMC) refers to the ability of different electronic devices to operate normally in a common electromagnetic environment without interfering with each other. EMC involves tests such as electromagnetic radiation emission or radiation immunity. Radiated emission testing studies the harmful radiation of the system to help reduce it, while radiation immunity testing checks the system's ability to maintain its function in the presence of interference from other systems.

The structure of the EC gas sensor itself has a negative impact on EMC performance. The sensor electrodes act like antennas and can pick up interference from nearby electronic systems. This effect is more pronounced for wirelessly connected gas detection devices, such as portable worker safety instruments.

EMC testing is often a very time-consuming process and may require multiple iterations of the system design before the requirements are finally met. This testing has a significant impact on the cost and time invested in product development. Using an integrated signal chain solution that has been pre-tested to meet EMC requirements can reduce time and cost expenditures.

Functional safety is another area that is taken into serious consideration and is also a technical challenge. By definition, functional safety means that when a potentially dangerous situation is detected, a protective or corrective mechanism is activated to prevent any dangerous event from occurring. The relative degree of risk reduction provided by this safety function is defined as the Safety Integrity Level (SIL). Functional safety requirements are of course already included in industrial standards.

In industrial gas detection applications, the importance of functional safety mainly relates to safe operating environments, where explosive or flammable gases may be present. Chemical plants or mining facilities are good examples of such applications. In order to comply with functional safety standards, the system must be certified for functional safety to a satisfactory safety integrity level.

To address the above challenges and enable customers to design smarter, more accurate, and more competitive gas detection systems, Analog Devices has introduced the ADuCM355, a single-chip electrochemical measurement system for gas detection and water analysis applications.

The ADuCM355 integrates two electrochemical measurement channels, an impedance measurement engine for sensor diagnostics, and an ultra-low power mixed-signal ARM® Cortex®-M3 microcontroller for running user applications and sensor diagnostic compensation algorithms. Figure 5 shows a simplified functional block diagram of the ADuCM355.

Understanding market trends and customer needs has helped ADI design highly integrated on-chip measurement systems, including:

• One 16-bit 400 kSPS ADC

• Two dual-output DACs for generating bias voltages for electrochemical cells

• Two ultra-low power, low noise potentiostats with TIA amplifiers

• A high-speed 12-bit DAC with a high-speed TIA

• Analog hardware accelerators (waveform generator, digital Fourier transform block, and digital filters) to support diagnostic measurements

• Internal temperature sensor

• 26 MHz ARM Cortex-M3 microcontroller

The ADuCM355 provides the means to overcome the technical challenges of electrochemical gas sensing. The two measurement channels support not only the most common 3-electrode gas sensors, but also 4-electrode sensor configurations. The fourth electrode can be used for diagnostic purposes or as a working electrode for the second target gas in a dual gas sensor. Either potentiostat can also be configured in sleep mode to reduce power consumption while maintaining the sensor bias voltage, thereby reducing the stabilization time that may be required before the sensor can operate normally. Analog hardware accelerator modules support sensor diagnostic measurements such as electrochemical impedance spectroscopy and chronoamperometry. An integrated microcontroller can be used to run compensation algorithms, store calibration parameters, and run user applications. The ADuCM355 is also designed with EMC requirements in mind and is pre-tested to comply with EN 50270 standards. If the application does not require an integrated microcontroller, a front-end-only version, the AD5940, can be used.

Thanks to technological innovation, we now have all the knowledge and tools necessary to effectively address the technical challenges of electrochemical gas sensors and clear the way for us to enter an era of ubiquitous sensing. From low-cost wireless air quality monitors to process control and worker safety applications, signal chain integration and built-in diagnostic features will enable the widespread use of these sensors while reducing maintenance requirements, improving accuracy, extending sensor life, and reducing costs.