Detailed Explanation of Underground Carbon Monoxide Sensor Calibration Procedures

Coal mine underground environments are complex and highly variable. Carbon monoxide (CO), as a typical indicator gas of coal spontaneous combustion and incomplete combustion, is directly related to miners’ life safety and production order through its accurate monitoring. As a key node in the comprehensive gas prevention and control system of coal mines, underground carbon monoxide sensors must be calibrated regularly to ensure reliable readings and timely alarms.

Relying on more than 40 years of experience in mining sensor research and development, Nexisense has systematically summarized the complete calibration procedures for underground CO sensors, including preparation, operation, verification, cycle management, and troubleshooting. This article aims to provide a clear and practical reference guide for coal mine safety inspectors, electromechanical engineers, and technical management personnel, helping to improve the overall quality and compliance of underground gas monitoring.

Comprehensive Preparation Before Calibration

The success of any calibration operation begins with thorough preparation. Underground CO sensor calibration has strict requirements for both the environment and equipment.

Environmental conditions must meet the following requirements: temperature controlled between 0–40 °C, relative humidity not exceeding 85% RH, good on-site ventilation, and no strong electromagnetic interference sources. Calibration operations are strictly prohibited in areas with excessive combustible gas concentrations or gas accumulation, in order to eliminate secondary risks.

The equipment list includes: 50 ppm CO standard gas (uncertainty ≤ 1%), high-purity nitrogen or clean compressed air as zero gas, a flow meter (range 0–1 L/min, accuracy class 2), a dedicated pressure regulator, a matching calibration hood, and a portable CO detector as a backup monitoring method. The pressure of all gas cylinders should be maintained above 5 MPa to avoid unstable flow caused by insufficient pressure.

In terms of personnel, operations must be carried out by professionally trained and certified staff, with at least two people working together: one responsible for operation and one for safety supervision. All participants must wear self-rescuers and carry emergency communication tools to ensure rapid response in case of any abnormal situation.

Nexisense provides each sensor with a detailed calibration manual and recommended gas circuit connection diagrams at delivery, enabling quick and efficient on-site operation.

Detailed Calibration Operation Steps

Calibration is divided into five interrelated stages: visual inspection, preheating, zero calibration, span calibration, and verification.

First, conduct a visual inspection: confirm that the sensor housing has no cracks or deformation, the breathable membrane is intact and unobstructed, and the cable insulation is in good condition without damage. Any visual abnormality should result in immediate suspension of use and reporting for repair.

Next, power on the sensor and perform preheating for at least 30 minutes. Observe whether the displayed value stabilizes and record the initial reading. This step effectively eliminates errors caused by temperature drift or circuit instability.

Zero calibration is the fundamental step. Connect the zero gas source (high-purity nitrogen or clean air), adjust the flow steadily to 0.5 L/min, and continue purging for more than 3 minutes until the reading is completely stable. Enter the sensor calibration menu, select the “Zero Calibration” function, and confirm that the reading is within the range of 0 ± 1 ppm before saving the parameters.

Span calibration uses 50 ppm CO standard gas. Similarly, control the flow at 0.5 L/min, supply gas for 3 minutes until the reading stabilizes, then enter the “Span Calibration” menu, input the standard concentration value of 50 ppm, and observe whether the reading falls within the range of 50 ± 2 ppm. Confirm and save once accuracy is verified.

Finally, perform verification: first reintroduce zero gas and confirm that the reading returns to 0 ± 1 ppm; then switch back to the standard gas and verify that the span point remains within 50 ± 2 ppm. The entire process must be fully documented, with calibration data before and after recorded to form traceable records.

Nexisense mining CO sensors are equipped with an intelligent calibration wizard that supports one-button menu access and automatically prompts each step, significantly reducing the probability of human error.

Calibration Cycle and Special Situation Management

According to AQ 1029-2019 “Coal Mine Safety Regulations” and related industry standards, the calibration cycle of underground CO sensors varies depending on installation location:

• Coal mining faces and development headings: once every 15 days

• Return air roadways: once every 30 days

• Main intake air roadways: once every 90 days

In addition, recalibration must be performed immediately under the following special circumstances: after sensor repair or component replacement; after an alarm event has occurred; before reactivation after long-term shutdown (more than 30 days); or whenever abnormal readings are suspected.

Strict implementation of calibration cycle management effectively controls sensor drift risks and ensures reliable data at critical moments.

Key Precautions During the Calibration Process

Safety always comes first. Before calibration, it must be confirmed that on-site ventilation is normal and gas concentrations meet requirements; personnel must wear self-rescuers throughout the operation and work in pairs; single-person operation or work in high-risk areas is strictly prohibited.

From a technical perspective, attention should be paid to the following: maintaining gas cylinder pressure above 5 MPa; strictly controlling flow at 0.5 ± 0.1 L/min; ensuring airtight gas circuit connections to prevent leakage; opening valves slowly during gas supply to avoid high-pressure gas impact on the sensor; and avoiding contact with sensitive parts of the sensor during calibration.

Quality records are indispensable. Each calibration should document the date, personnel, environmental conditions, pre- and post-calibration readings, and standard gas batch numbers, establishing comprehensive sensor archives for traceability and auditing.

Common Issues and On-Site Handling Methods

The following typical issues may occur during calibration:

1. Readings cannot stabilize or fluctuate: usually caused by gas circuit leakage, loose fittings, or internal sensor contamination. Check gas circuit sealing thoroughly and, if necessary, disassemble for cleaning or replace the sensor.

2. Persistent zero offset (>2 ppm): commonly due to long-term dust exposure or sensor aging. It is recommended to purge with clean air for an extended period; if ineffective, clean the breathable membrane or send for repair.

3. Significantly prolonged response time (>30 s): typically caused by dust accumulation or blockage of the breathable membrane. Promptly clean the membrane surface with dedicated tools or directly replace the filter membrane assembly.

4. Low span point reading: may result from expired standard gas, pressure regulator malfunction, or reduced sensor sensitivity. First verify the validity of the gas source, then check flow stability.

Through systematic troubleshooting and timely intervention, most issues can be resolved on-site, minimizing downtime.

Nexisense mining CO sensors adopt high-stability electrochemical cells combined with laser-assisted compensation technology, significantly reducing the occurrence of zero drift and slow response.

Scientific Selection and Maintenance of Underground CO Sensors

When selecting sensors, priority should be given to explosion-proof rating (Ex d I Mb or higher), measurement range (0–100 ppm or 0–500 ppm), response time (T90 ≤ 30 s), temperature and humidity adaptability, and whether built-in automatic compensation functions are available.

Daily maintenance recommendations include: weekly inspection of appearance and breathable membrane; monthly purging with clean air; quarterly zero verification; and comprehensive calibration by a professional organization every six months. Avoid prolonged exposure of sensors to high CO concentrations to extend service life.

Nexisense provides full-chain support from sensing elements to complete instruments, including free technical training, remote calibration guidance, fast spare parts supply, and on-site services, ensuring equipment remains in optimal condition at all times.

Frequently Asked Questions (FAQ)

1. Why must underground CO sensors be calibrated regularly? Electrochemical sensors experience zero drift and sensitivity degradation over time and environmental exposure. Without calibration, missed alarms or false alarms may occur, directly affecting safety.

2. Must the standard gas concentration be 50 ppm? 50 ppm is a common calibration point, but other concentrations (such as 20 ppm or 100 ppm) may be used depending on the actual range. The key requirement is accurate concentration with uncertainty ≤ 1%.

3. Can compressed air replace high-purity nitrogen as zero gas? Yes, provided that the CO content in compressed air is <1 ppm and it has been filtered and dried; otherwise, errors will be introduced.

4. Why must the flow be controlled at 0.5 L/min during calibration? This flow rate is the optimal sampling rate designed for most mining sensors. Excessive flow may cause impact, while insufficient flow prolongs response time.

5. Why must sensors be recalibrated after an alarm? Alarm events may expose sensors to high CO concentrations, causing temporary polarization or contamination of the sensing cell, requiring recalibration to restore accuracy.

6. How should a blocked breathable membrane be cleaned? Use a dedicated soft brush or low-pressure clean air for gentle cleaning. Liquids or hard objects are prohibited to avoid damaging the membrane structure.

7. Can the calibration cycle be extended based on actual conditions? Not recommended. AQ 1029-2019 clearly specifies calibration intervals; in special environments, intervals may even need to be shortened. Any extension requires approval and documentation by safety authorities.

8. How can it be determined that a sensor needs replacement? If sensitivity degradation exceeds 20% after multiple calibrations, zero drift cannot be adjusted to ±1 ppm, or response time significantly exceeds limits, replacement should be considered.

9. Do laser-based CO sensors require the same calibration? Laser absorption sensors exhibit lower drift, but still require periodic verification according to manufacturer requirements. Cycles are usually longer, but zero and span calibration steps are similar.

10. How do Nexisense mining sensors perform in high-humidity and high-dust underground environments? With multi-layer protection design and automatic compensation algorithms, they maintain high stability even at 95% RH and high dust concentrations, with drift rates far below industry averages.

Conclusion

Although underground carbon monoxide sensor calibration may appear to be a routine task, it is directly related to the reliability of every warning and the protection of every life. Standardized preparation, rigorous procedures, strict cycle management, and timely fault handling together form a solid defense for coal mine gas monitoring.

Nexisense always places safety and accuracy first, providing more reliable monitoring solutions for coal mining enterprises through continuous technological innovation and a comprehensive service system. We hope this article serves as a practical handbook for on-site operators, and we welcome industry peers to exchange experiences and jointly promote the improvement of underground safety management. If your mine faces challenges in CO monitoring, Nexisense is willing to work with you to safeguard every space of safe production.