Use of Local Methane Detector (LMD) in Underground Coal Mines – DGMS Rules, Operation & MCQs

🧾 DETAILED NOTES

1️⃣ Introduction

Methane gas (CH₄) is highly explosive (5–15% range) and lighter than air, accumulating near the roof. Its early detection is crucial to prevent fires, explosions, and ignitions. Local Methane Detectors (LMDs) are mandatory for gas-checking in underground coal mines as per DGMS.

2️⃣ Types of Methane Detectors

Catalytic Type: Measures heat produced during methane oxidation. Requires oxygen for function.
Infrared (IR) Sensor Type: Highly accurate. Works without oxygen. Less affected by dust/moisture.
Portable Hand-Held LMD (Used by officials, overmen, gas-checkers).
Fixed continuous methane monitoring sensors.

3️⃣ Statutory Requirements (DGMS / CMR 2017)

Continuous methane monitoring is essential. Cutting, drilling, blasting must stop if concentration exceeds limits.

Methane Danger Limits	Action Required
1.0%	Cut off electrical supply
1.25%	Withdraw persons
2.0%	No one permitted (explosive risk)

4️⃣ Use of Local Methane Detector

Before starting: Switch ON in fresh air, allow warm-up, check battery & confirm zero calibration.
Checking: Hold detector near the roof. Move slowly to detect pockets.
Machine operations: MUST check methane before restarting equipment.

5️⃣ Care & Maintenance of LMD

Daily: Zero calibration, Battery check, Clean sensor.
Weekly: Bump test (with methane test gas) to confirm sensor response.
Monthly: Full calibration.
Annual: Factory calibration (mandatory), Sensor replacement (as per manufacturer).

⚡ QUICK ONE-LINERS (REVISION)

Methane explosive range = 5–15%.
LMD must be switched ON in fresh air.
Check methane near the roof.
1% → power OFF.
1.25% → withdraw persons.
Catalytic LMD requires oxygen.
IR LMD works in oxygen-deficient zones.
Daily zero check is compulsory.
Bump test done weekly.
Calibration required monthly.

🧠 DESCRIPTIVE MODEL ANSWER

Q. Describe the use and care of Local Methane Detector (LMD) in underground coal mines.

Answer:
Local Methane Detectors (LMDs) are essential instruments for detecting methane concentration in underground coal mines. They are used before machine operation, blasting, re-entry, and during routine gas checks. The detector must be switched ON in fresh air, held near the roof, and moved slowly to avoid missing pockets of gas.

Proper care involves daily zero check, battery check, and cleanliness. Weekly bump tests (using test gas) and monthly full calibration are mandatory. Annual factory calibration and replacement of faulty detectors are required to prevent unreliable readings, thereby mitigating the risk of methane explosions and ensuring DGMS compliance.

🧮 25 MCQs (Dynamic Options A–E)

Q1. Methane is:

Solution: Methane (CH₄) is a highly flammable, odorless gas and the main explosive hazard in coal mines.

Q2. LMD must be switched ON in:

Solution: Detectors must be initialized in fresh air (0% gas) to set the correct zero baseline.

Q3. Methane accumulates near the:

Solution: Methane is lighter than air (specific gravity 0.55), causing it to rise and accumulate near the roof.

Q4. Explosive range of methane:

Solution: Methane is explosive between its Lower Explosive Limit (LEL) of 5% and Upper Explosive Limit (UEL) of 15%.

Q5. At 1% methane:

Solution: 1.0% is the statutory limit for immediately cutting off power supply (switching off electricity) to prevent ignition.

Q6. Catalytic sensors need:

Solution: Catalytic sensors work by oxidation (burning), a process that requires the presence of oxygen.

Q7. IR detectors measure methane without:

Solution: IR sensors rely on light absorption, a physical property, and can operate without ambient air or oxygen.

Q8. Before restarting a machine, methane must be:

Solution: Checking methane is mandatory before operating any electrical machine, as the spark could cause ignition.

Q9. Daily requirement:

Solution: A daily zero check (or zero calibration) in fresh air is mandatory to ensure the instrument is stable and accurate.

Q10. Weekly requirement:

Solution: The weekly bump test confirms the sensor's functionality and alarm response using certified test gas.

Q11. Monthly requirement:

Solution: Full calibration (adjusting the instrument to precise standards) is typically required on a monthly basis.

Q12. LMD must be kept:

Solution: LMDs must be protected from water and moisture to prevent sensor damage and electrical shorts.

Q13. LMD failure may occur due to:

Solution: Dust blocking the sensor port or lens is a very common cause of inaccurate or failed readings.

Q14. Methane must be checked during:

Solution: Drilling and cutting operations liberate methane and can also cause ignition sources (sparks).

Q15. Withdrawal required at:

Solution: 1.25% is the statutory limit mandating the immediate withdrawal of all persons from the affected area.

Q16. LMD should show:

Solution: The detector provides a direct reading of the concentration, measured as a percentage of methane volume in the air.

Q17. LMDs must be:

Solution: In gassy mines, all electrical devices must be intrinsically safe (IS) to prevent sparks from causing explosions.

Q18. Accuracy depends on:

Solution: Regular calibration against a known test gas is the only way to verify and maintain the device's accuracy.

Q19. Before blasting, methane must be:

Solution: Statutory rules require officials to ensure the methane concentration is below explosive limits before initiating the blast.

Q20. LMD displays methane in:

Solution: Gas concentrations are typically measured and displayed as a percentage of volume in the air.

Q22. Battery failure causes:

Solution: Battery drain is the most common reason the LMD will shut down or fail to provide a reading.

Q23. Sensor must be:

Solution: Regular cleaning is essential to prevent blockages and inaccurate readings.

Q24. LMD used mainly in:

Solution: LMDs are specific to coal mines where methane liberation is a constant hazard (gassy mines).

Q25. Main purpose of LMD:

Solution: The core function is to provide an early warning of methane build-up to prevent explosions.

🔗 INTERNAL LINKS

Related Topic	Read More On...
Gas Testing	Gas Testing & Methane Regulations
Mine Ventilation	Mine Ventilation & Gas Control
Intrinsic Safety	Intrinsically Safe Equipment in Mines
Explosion Prevention	Fire & Explosion Prevention in Gassy Mines

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