Mine Ventilation

Mine Fires & Explosions

Methane explosibility limits, Graham's ratio for early heating detection, and the oxygen-deficiency calculation behind it.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Underground fires and explosions hinge on gas chemistry. Methane is explosive only within its flammable band (roughly 5–15% in air), most violently near 9.5%. To detect a heating before it flames, mine atmospheres are monitored for carbon monoxide: Graham's ratio expresses the CO produced per unit of oxygen consumed, rising sharply as spontaneous combustion develops. The oxygen actually consumed is estimated from the measured nitrogen and oxygen via the oxygen-deficiency formula.

The Core Formula Matrix

Methane flammable range: ≈

5%

(LEL) to

15%

(UEL); most explosive near

9.5%

.

Graham's ratio (CO index):

GR = \frac{CO}{O _{2} deficiency} \times 100

Oxygen deficiency:

O_{2} def = 0.265 N_{2} - O_{2}

(gas concentrations in %).

The ‘IIT Traps’

⚠
Lower explosive limit of CH₄ is 5%, not 9.5%. 9.5% is the *most violent* mixture, not the lower limit.
⚠
Graham's ratio uses O₂ *deficiency*, not the measured O₂. Dividing by raw O₂ understates the index.
⚠
The 0.265 factor reconstructs the original O₂ from the (inert) nitrogen; omit it and the deficiency is meaningless.

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

The lower explosive limit (LEL) of methane in air is approximately:

Q2MEDIUM2 Marks · NAT

A return-airway sample shows

CO = 0.05%

with an oxygen deficiency of

0.40%

. Graham's ratio is ______. (Round off to two decimal places.)

Q3HARD2 Marks · NAT

An air sample contains

N_{2} = 80%

and

O_{2} = 20%

. The oxygen deficiency is ______ %. (Round off to two decimal places.)