Author: Michal raninec, ADI System Application Engineer

Electrochemical gas sensor is a proven technology, its history can be traced back to the 1950s, when the electrochemical sensor for oxygen monitoring was developed. One of the first applications of this technology is glucose biosensor, which is used to measure glucose hypoxia. In the next few decades, the technology has been developed, the sensor has become miniaturized and can detect a variety of target gases.

With the advent of the era of ubiquitous sensing technology, numerous new gas detection applications have emerged in many industries, such as automotive air quality monitoring or electronic nose. The continuous development of regulations and safety standards put forward more challenging requirements for new and existing applications than in the past. In other words, the future gas detection system must be able to accurately measure much lower concentrations, be more selective to the target gas, rely on battery power for a longer time, and provide stable and consistent performance for a longer time, while always maintaining safe and reliable operation.

Advantages and disadvantages of electrochemical gas sensors

The popularity of electrochemical gas sensors can be attributed to its linear output, low power consumption and good resolution. In addition, once calibrated according to the known concentration of the target gas, the repeatability and accuracy of the measurement are also very good. With the development of technology for decades, these sensors can provide very good selectivity for specific gas types.

Because of its many advantages, electrochemical sensors are first used in industrial applications, such as toxic gas detection to protect workers. The operation economy of these sensors promotes the deployment of regional toxic gas monitoring system and ensures the safety and environmental conditions of employees in mining, chemical industry, biogas plant, food production, pharmaceutical industry and other industries.

Although the detection technology itself is constantly improving, its basic working principle and inherent shortcomings have not changed since the emergence of electrochemical gas detection. In general, the shelf life of electrochemical sensors is limited, generally from six months to one year. The aging of the sensor will also have a significant impact on its long-term performance. Sensor manufacturers usually specify a sensor sensitivity drift of up to 20% per year. In addition, although the target gas selectivity has been significantly improved, the sensor still has the problem of cross sensitivity to other gases, which increases the probability of measurement interference and reading error. The performance of the sensor is also related to temperature, so it must be compensated internally.

Technical challenges

The technical challenges to design advanced gas detection system 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. Although the manufacturing process itself has been highly automated, it will inevitably bring differences to each sensor. Because of these differences, sensors must be characterized and calibrated in the production process.

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, industrial applications pay special attention to electromagnetic compatibility (EMC) and functional safety compliance, which will have a negative impact on design cost and time to market. Working conditions also play an important role and pose challenges to maintaining the required performance and service life. Electrochemical sensors will age and drift during their service life (which is the nature of this technology), resulting in frequent calibration or replacement of sensors. If you operate in a harsh environment, the performance changes will be further accelerated, as described later in this article. It is one of the key requirements for many applications to extend the service life of sensors while maintaining their performance, especially when the cost of ownership is critical.

Third, even if the technology of extending the service life is adopted, all electrochemical sensors will eventually reach the end of their life. At this time, the performance no longer meets the requirements, and the sensors need to be replaced. Effective detection of end-of-life conditions is a challenge. If this challenge can be solved, unnecessary sensor replacement can be reduced and the cost can be greatly reduced. Furthermore, if we can accurately predict when the sensor will fail, the operating cost of the gas detection system will be reduced more.

In all gas detection applications, the utilization of electrochemical gas sensors is increasing, which brings challenges to the logistics, commissioning and maintenance of such systems, resulting in an increase in total cost of ownership. Therefore, a special analog front-end with diagnostic function is used to reduce the impact of technical shortcomings (mainly the limited life of the sensor) and ensure the long-term sustainability and reliability of the gas detection system.

Signal chain integration reduces design complexity

Most of the traditional signal chains are designed with independent analog-to-digital converters, amplifiers and other building blocks, which is quite complex, forcing designers to make tradeoffs in power efficiency ratio, measurement accuracy or PCB area occupied by signal chains.

An example of this design challenge is an instrument with multiple gas configurations that can measure multiple target gases. Each sensor may require a different bias voltage for proper operation. In addition, the sensitivity of each sensor may be different, so the gain of the amplifier must be adjusted to maximize the performance of the signal chain. For designers, these two factors increase the design complexity of configurable measurement channels (simultaneous interpreting of BOM and principle diagrams) without changing the interface between sensors. 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 evolution, through which more efficient and powerful solutions can be designed. The integrated single-chip gas detection signal chain simplifies the system design by integrating TIA (mutual resistance amplifier) gain resistor or using digital to analog converter as sensor bias voltage source (as shown in Figure 2). Due to the integration of signal chain, the measurement channel can be fully configured by software to interface with many different types of electrochemical sensors and reduce the complexity of design. In addition, the power requirement of this integrated signal chain is also significantly reduced, which is very important for applications with battery life as a key factor. Finally, the measurement accuracy is improved because the noise level of the signal link is reduced and it is possible to use better signal processing devices (such as TIA or ADC).

Reviewing the example of multi gas instrument, signal chain integration enables it to:

  • To achieve fully configurable measurement channel and reduce the complexity of signal chain, it is easy to reuse single signal chain design
  • Reduce PCB area occupied by signal chain
  • Reduce power consumption
  • Improve the measurement accuracy
  • Sensor degradation and diagnosis

Although signal chain integration is an important step forward, it does not solve the fundamental disadvantage of electrochemical gas sensor, that is, its performance will decline with time. It is not difficult to understand that this is caused by the working principle and structure of the sensor. Working conditions can also cause performance degradation and accelerate sensor aging. The accuracy of the sensor will decrease until it becomes unreliable and no longer suitable for its task. In this case, the usual practice is to take the instrument offline and check the sensor manually, which is time-consuming and expensive. Then, depending on its condition, the sensor can be recalibrated and reused, or it may need to be replaced. This incurs considerable maintenance costs. By using electrochemical diagnosis technology, the health status of the sensor can be analyzed and the performance change can be compensated effectively.

Figure 1. Signal chain of typical electrochemical gas sensor (schematic diagram)

Figure 2. Dual channel integrated gas detection signal chain (diagram)

Figure 3. Correlation between sensor sensitivity (left) and impedance (right) in accelerated life test at low relative humidity

The common factors leading to performance degradation include high temperature, humidity and gas concentration or electrode poisoning. Short time exposure to higher temperature (50 ° C above) is generally acceptable. However, repeatedly exposing the sensor to high temperature will cause electrolyte evaporation and irreversible damage to the sensor, such as baseline reading offset or response time slow. On the other hand, the ultra-low temperature (– 30 ℃ ° The sensitivity and response of the sensor will be greatly reduced.

Humidity is the most important factor for sensor life. The ideal working condition of electrochemical gas sensor is 20 ° C and 60% relative humidity. When the ambient humidity is lower than 60%, the electrolyte inside the sensor will dry, which will affect the response time. On the other hand, humidity higher than 60% will cause the water in the air to be absorbed by the sensor, thus diluting the electrolyte and affecting the characteristics of the sensor. Absorbing moisture can also lead to sensor leakage, which may cause pin corrosion.

The extent of the above degradation mechanism, if not very large, will affect the sensor. In other words, things like electrolyte depletion are natural and can lead to sensor aging. Regardless of the working conditions, the aging process will limit the life of the sensor, but some ecsense gas sensors can work for more than 10 years.

Electrochemical impedance spectroscopy (EIS) or timed amperometric analysis (bias voltage pulse is applied while observing the sensor output) can be used to analyze the sensor.

EIS is a frequency domain analysis and measurement of electrochemical system excited by sinusoidal signal (usually voltage). At each frequency, the current flowing through the electrochemical cell is recorded and used to calculate the impedance of the cell. Then, the data is usually displayed in the form of Nyquist diagram and Porter diagram. The Nyquist diagram shows the complex impedance data, and each frequency point is drawn by the real part on the x-axis and the imaginary part on the y-axis. The main disadvantage of this data representation is the loss of frequency information. The baud diagram shows the relationship between impedance amplitude and phase angle and frequency.

The experimental results show that there is a strong correlation between the decrease of sensor sensitivity and the change of EIS test results. The example in Figure 3 shows the results of an accelerated life test in which the electrochemical gas sensor is placed at low humidity (10% RH) and high temperature (40% RH) ° C) In the same environment. During the whole experiment, the sensor was removed from the environmental chamber regularly and placed for one hour, then the baseline sensitivity test and EIS test were carried out under the known target gas concentration. The test results clearly show the correlation between the sensitivity and impedance of the sensor. The disadvantage of this method is that it takes a lot of time, because it is very time-consuming to obtain the measurement results at very low sub Hertz frequency.

Chronoamperometry (pulse testing) is another technique that helps to analyze the health of sensors. The measurement method is as follows: a voltage pulse is superimposed on the bias voltage of the sensor, and the current flowing through the electrochemical cell is observed at the same time. The pulse amplitude is generally very low (e.g. 1mV) and very short (e.g. 200ms), so it will not interfere with the sensor itself. In this way, the test can be carried out quite frequently, and the gas detection instrument can keep normal operation. Before performing more time-consuming EIS measurements, the timed amperometric method can be used to check whether the sensor has been physically inserted into the device and to indicate changes in sensor performance. An example of the response of a sensor to a voltage impulse is shown in Figure 4.

Figure 4. Sample results of timed amperometric test

Previous sensor probing techniques have been used in the field of electrochemistry for decades. However, the equipment required for these measurements is usually expensive and cumbersome. From both practical and financial aspects, it is impossible to test a large number of gas sensors deployed on site with this kind of equipment. In order to realize the health analysis of remote built-in sensors, the diagnostic characteristics must be directly integrated into a part of the signal chain.

With the help of the integrated diagnostic function, the gas sensor can be automatically tested without manual intervention. If the gas sensor is characterized in production, the data obtained from the sensor can be compared with these characteristic data sets, so as to deeply understand the current situation of the sensor, and then use intelligent algorithm to compensate for the loss of sensor sensitivity. In addition, the sensor’s history can be used to predict when the sensor’s life will end, and remind the user when the sensor needs to be replaced. The built-in diagnostic function will eventually reduce the maintenance requirements of the gas detection system and extend the service life of the sensor.

System design challenges for industrial applications

Safety and reliability are critical, especially in industrial environments. When operating in severe industrial environment (such as chemical plant), there are strict regulations to ensure that the gas detection system meets these requirements and maintains reliable and complete functions.

Electromagnetic compatibility (EMC) refers to the ability of different electronic equipment to operate normally in the common electromagnetic environment without mutual interference. EMC involves electromagnetic radiation emission or radiation immunity. The radiated emission test studies the harmful radiation of the system to help reduce the radiation, while the radiated immunity test checks the ability of the system to maintain its function under the interference of other systems.

The structure of EC gas sensor itself has a negative impact on EMC performance. The sensor electrode acts as an antenna and can pick up the interference from the nearby electronic system. For wireless connected gas detection equipment (such as portable worker safety instrument), this effect is more obvious.

EMC testing is usually a very time-consuming process, and the system design may need to be iterated many times before the requirements are finally met. This test has a great impact on the cost and time of product development. Reduce time and cost with a pre tested integrated signal chain solution that meets EMC requirements.

Functional security is another aspect that needs to be considered seriously, and it is also a technical challenge. By definition, functional safety refers to the activation of protection or correction mechanisms to prevent any hazardous event when a potentially hazardous condition is detected. The relative degree of risk reduction provided by this safety function is defined as safety integrity level (SIL). Functional safety requirements are of course included in industry standards.

In industrial gas detection applications, the importance of functional safety is mainly related to the safe operating environment, because there may be explosive or flammable gases in the environment. Chemical plants or mining facilities are good examples of such applications. In order to meet the functional safety standards, the system must pass the functional safety certification to achieve satisfactory safety integrity level.

ADI’s single chip electrochemical measurement system

In order to solve the above challenges and enable customers to design more intelligent, accurate and competitive gas detection systems, ADI company launched aducm355, a single-chip electrochemical measurement system for gas detection and water analysis applications.

Aducm355 integrates two electrochemical measurement channels, an impedance measurement engine for sensor diagnosis, and an ultra-low power mixed signal arm for running user applications and sensor diagnosis compensation algorithm ® Cortex ®- M3 microcontroller. Figure 5 shows a simplified functional block diagram of aducm355.

Figure 5. Simplified functional block diagram of aducm355

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

  • A 16 bit 400k SPS ADC
  • Two dual output DACs are used to generate the bias voltage of the electrochemical cell
  • Two ultra low power and low noise potentiostat with TIA amplifier
  • A high speed 12 bit DAC with high speed TIA
  • Analog hardware accelerator supporting diagnostic measurement (waveform generator, digital Fourier transform module and digital filter)
  • Internal temperature sensor
  • 26MHz arm Cortex-M3 microcontroller

Aducm355 provides a means to overcome the technical challenge of electrochemical gas detection. The two measurement channels support not only the most common 3-electrode gas sensor, but also the 4-electrode sensor configuration. The fourth electrode can be used not only for diagnostic purposes, but also as the working electrode of the second target gas in the dual gas sensor. Any potentiostat can also be configured in sleep mode to reduce power consumption while maintaining the sensor bias voltage, thus reducing the stabilization time that the sensor may need before normal operation. The analog hardware accelerator module supports sensor diagnostic measurements such as electrochemical impedance spectroscopy and chronoamperometry. The integrated microcontroller can be used to run compensation algorithms, store calibration parameters and run user applications. The design of aducm355 also considers the EMC requirements and meets the en50270 standard after pre-test.

If the application does not need an integrated microcontroller, it can use ad5940, which is only a front-end version.

conclusion

Thanks to technological innovation, we now have all the necessary knowledge and tools to effectively meet the technical challenges of electrochemical gas sensors and remove the obstacles for us to enter the era of universal detection. From low-cost wireless air quality monitors to process control and worker safety applications, signal chain integration and built-in diagnostic features will enable these sensors to be widely used, while reducing maintenance requirements, improving accuracy, extending sensor life, and reducing costs.

About the author

Michal raninec is a systems application engineer in the industrial systems division of ADI’s automation and energy division. Its professional fields include electrochemical gas detection and wireless sensor networks. Michal graduated from Brno University of science and technology in the Czech Republic with a master’s degree in electronic engineering.

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