A Hardened Carbon Dioxide Sensor for Continuous In-Ground Monitoring in Perennial Grass Systems

Core Concepts

A hardened, low-cost, and compact NDIR-based CO2 sensor was developed to enable continuous monitoring of subsurface CO2 levels in perennial grass systems, addressing the limitations of existing solutions.

Abstract

The article presents the design and development of a hardened CO2 sensor for continuous in-ground monitoring in perennial grass systems. The key challenges addressed include the need for a robust sensor that can withstand harsh winter conditions, including ice encasement, as well as the limitations of existing solutions in terms of cost, size, and ease of deployment.

The sensor design utilizes a Sensirion SCD30 NDIR CO2 sensor, which is interfaced with a custom PCB and housed in a 3D-printed enclosure. The enclosure is designed to protect the sensor from mechanical damage, water, and soil intrusion, while allowing gas exchange through Gore-Tex vents. The use of a removable cable and a robust M12 connector helps mitigate issues with rodent damage experienced in previous deployments.

The sensor has been extensively field-tested in turfgrass monitoring applications, where it has demonstrated reliable performance in harsh winter conditions. Laboratory evaluations have also confirmed the sensor's steady-state accuracy and response time characteristics.

The key features of the design include:

• Low system cost (under $100 USD)
• Highly robust and immersion-rated enclosure
• Easy assembly and repair
• Wide operating range (0 to 40,000 ppm CO2)

The hardened CO2 sensor has enabled continuous monitoring of subsurface CO2 levels, which is crucial for understanding the impact of factors like ice encasement on perennial grass health, particularly in the context of winterkill. The design can also be applied to other applications, such as grain storage, produce cultivation, air quality monitoring, and carbon storage.

Stats

Carbon dioxide levels below the soil surface can range from near atmospheric (∼400 ppm) to saturating the sensor (40,000 ppm).
The sensor has a time constant of approximately 7 minutes, resulting in a response time of around 35 minutes.
The sensor's steady-state error was found to be within the limits of accuracy, with a mean error of less than 17% across the tested conditions.

Quotes

"Carbon dioxide levels below the soil surface are an important measurement relating to plant health, especially for plants such as perennial grasses in northern climates where ice encasement can occur over winter."
"Many off the shelf CO2 sensors exist, but they are not sufficiently hardened for in ground deployment over winter."
"To combat this problem we have taken an established NDIR CO2 sensor and hardened it for use in winter and ice encased environments to allow for continuous automated sampling of subsurface CO2 levels to better understand ice encasement damage in perennial grass systems."

Key Insights Distilled From

A Hardened CO$_2$ Sensor for In-Ground Continuous Measurement in a Perennial Grass System

by Bobby Schulz... at arxiv.org 09-12-2024

https://arxiv.org/pdf/2409.06828.pdf

Deeper Inquiries

How could this hardened CO2 sensor design be adapted for monitoring in other harsh environmental conditions, such as high-altitude or deep-sea applications?

The design of the hardened CO2 sensor, originally developed for in-ground monitoring in perennial grass systems, can be adapted for high-altitude and deep-sea applications by addressing the unique challenges presented by these environments.

High-Altitude Adaptations: At high altitudes, the primary concerns include lower atmospheric pressure, temperature fluctuations, and potential exposure to UV radiation. To adapt the sensor for these conditions:

Pressure Compensation: Incorporate a pressure sensor to adjust CO2 readings based on the ambient pressure, ensuring accurate measurements despite the lower atmospheric conditions.
Thermal Insulation: Use materials with high thermal resistance for the enclosure to mitigate the effects of extreme temperature variations. This could involve using advanced polymers or composites that can withstand both high UV exposure and low temperatures.
UV Protection: Apply UV-resistant coatings to the sensor enclosure to prevent degradation from increased UV radiation at high altitudes.

Deep-Sea Adaptations: For deep-sea applications, the sensor must withstand high pressures, corrosive saltwater, and low temperatures. Adaptations could include:

Pressure-Resistant Enclosure: Design the enclosure to withstand the immense pressures found at great depths, potentially using thicker walls or specialized materials like titanium or high-strength polymers.
Corrosion Resistance: Utilize corrosion-resistant materials and coatings to protect the sensor from saltwater damage. This could involve using marine-grade stainless steel or specialized coatings that prevent corrosion.
Waterproof Sealing: Enhance the sealing mechanisms, possibly using O-rings or gaskets made from materials that can withstand prolonged exposure to saltwater, ensuring no water intrusion occurs.

By implementing these adaptations, the hardened CO2 sensor can effectively monitor CO2 levels in both high-altitude and deep-sea environments, expanding its utility beyond terrestrial applications.

What are the potential limitations or drawbacks of using a single NDIR-based CO2 sensor for monitoring, and how could a multi-sensor approach improve the reliability and accuracy of the system?

While the NDIR (Non-Dispersive Infrared) CO2 sensor offers several advantages, including cost-effectiveness and compact design, there are notable limitations when relying on a single sensor for monitoring:

Single Point of Failure: A single sensor represents a single point of failure. If the sensor malfunctions or becomes damaged, the entire monitoring system is compromised, leading to data gaps and potentially misleading conclusions about CO2 levels.

Environmental Variability: Soil CO2 levels can vary significantly across different spatial and temporal scales due to factors such as soil composition, moisture content, and root respiration. A single sensor may not capture this variability, leading to inaccurate assessments of the overall CO2 dynamics in the ecosystem.

Calibration and Drift: NDIR sensors can experience drift over time, affecting their accuracy. Regular calibration is necessary, and relying on one sensor may not provide a comprehensive understanding of CO2 levels if calibration is not performed consistently.

To enhance the reliability and accuracy of the monitoring system, a multi-sensor approach could be employed:

Spatial Redundancy: Deploy multiple NDIR sensors across different locations within the monitoring area. This spatial redundancy allows for a more comprehensive understanding of CO2 levels, capturing variations that a single sensor might miss.

Sensor Fusion: Integrate data from various types of sensors (e.g., NDIR for CO2, electrochemical sensors for O2, and temperature sensors) to create a more holistic view of the environmental conditions. This approach can help correlate CO2 levels with other critical factors affecting plant health.

Data Validation: Use multiple sensors to cross-validate readings. If one sensor reports an anomalous value, data from other sensors can help identify whether it is an outlier or a genuine environmental change.
By implementing a multi-sensor approach, the monitoring system can achieve greater robustness, accuracy, and reliability, ultimately leading to better insights into the dynamics of CO2 levels and their impact on plant health.

Given the importance of understanding the interplay between soil CO2 levels, plant health, and environmental factors, how could this sensor be integrated with other monitoring technologies (e.g., soil moisture, temperature) to provide a more comprehensive picture of the turfgrass ecosystem?

Integrating the hardened CO2 sensor with other monitoring technologies can significantly enhance the understanding of the turfgrass ecosystem by providing a more comprehensive view of the interactions between soil CO2 levels, plant health, and environmental factors. Here are several strategies for achieving this integration:

Multi-Parameter Data Collection: Deploy additional sensors to monitor key environmental parameters alongside the CO2 sensor. This could include:

Soil Moisture Sensors: To measure the volumetric water content in the soil, which directly affects plant health and CO2 dynamics. Understanding moisture levels can help correlate CO2 fluctuations with drought stress or waterlogging conditions.
Temperature Sensors: To monitor soil and air temperatures, providing context for CO2 levels. Temperature influences plant respiration rates and microbial activity, both of which affect CO2 production and consumption.

Data Integration and Analysis: Utilize a centralized data logger or cloud-based platform to collect and analyze data from all sensors. This platform can:

Correlate Data: Analyze the relationships between CO2 levels, soil moisture, and temperature, helping to identify patterns and trends that affect turfgrass health.
Real-Time Monitoring: Provide real-time data visualization, allowing for immediate insights into the conditions affecting the turfgrass ecosystem. Alerts can be set up for critical thresholds, such as low moisture levels or high CO2 concentrations.

Modeling and Simulation: Use the integrated data to develop predictive models that simulate the interactions between CO2 levels, moisture, temperature, and plant health. These models can help:

Forecast Conditions: Predict how changes in one parameter (e.g., increased CO2) might affect others (e.g., plant growth or stress).
Guide Management Practices: Inform turf management decisions, such as irrigation scheduling or fertilization, based on real-time environmental conditions.

Field Trials and Research: Conduct field trials using the integrated sensor system to gather empirical data on how different environmental factors interact. This research can lead to:

Improved Understanding: A deeper understanding of the specific conditions that lead to winterkill or other stressors in turfgrass.
Best Practices Development: Development of best management practices tailored to specific environmental conditions, enhancing turfgrass resilience and health.

By integrating the CO2 sensor with other monitoring technologies, researchers and turf managers can gain a holistic understanding of the turfgrass ecosystem, leading to more informed decisions and improved plant health outcomes.