Everything we have learnt about the flame safety lamp so far, its parts, its safety features and its care, leads to one practical skill: using it to find out whether firedamp (methane) is present in the mine air. In many coal mines this is the job of the sirdar, overman and gas tester every single shift, and it is also the part that exams love to test.
This is Part 4 of our flame safety lamp series. It covers testing of firedamp by the safety lamp: where gas is tested, the accumulation test and the percentage test, how to read the gas cap, what to do when gas is found, testing for gas layers, and how the lamp reacts to blackdamp and low oxygen. Read Part 1, Part 2 and Part 3 first if you have not already.
- Two tests: the accumulation test first, then the percentage test
- If the flame rises and flickers in the accumulation test, gas is about 3% or more: bring the lamp out slowly
- The percentage test uses a testing flame and the gas cap on top of it
- The lamp gives only a rough idea of the percentage. A methanometer gives the accurate figure
- The flame also warns of blackdamp and low oxygen, going out at around 17% oxygen
Testing of Firedamp by Safety Lamp
Q1. Where is firedamp tested in a mine?
Answer: Firedamp (methane) is mainly tested in these places:
- The general atmosphere of the roadways and workings.
- Places where gas is likely to accumulate: goaf edges, the mouths of rise workings, blind ends, holes, cavities and cracks in the roof, old workings, places where air does not move and areas near faults or dykes.
- At working places before drilling holes, before blasting and after blasting.
- In sinking inclines or drifts that are close to a coal seam or that cross one.
- Near running electric motors installed in places where gas might collect.
Q2. Can firedamp be found in stone drifts and sinking inclines too?
Answer: Yes. A stone drift driven to join two coal seams, or one that crosses a seam, can have gas near the seam. In the same way, a sinking incline that crosses a coal seam or a thin coal band can meet firedamp.
Q3. How is firedamp tested? Which method is best for everyday work in the mine?
Answer: There are three ways:
- Collect a sample of air from the place and analyse it in a laboratory.
- Use a firedamp detecting instrument, a methanometer.
- Use an oil safety lamp.
For day-to-day work the oil safety lamp is the most practical and widely used. It is simple, and besides firedamp it also shows the presence of blackdamp and a shortage of oxygen, which are very important for safety. It does not give the exact percentage of gas, but an approximate idea is enough for safety purposes. For the exact value, use a methanometer with a probe tube or take a gas sample. You can read more about methane in our post on mine gases and firedamp.
Q4. What is the flame of a safety lamp like? How is a testing flame made? How does the gas cap form?
Answer: The flame has three parts:
- A: the innermost part, blue and not bright. It is unburnt oil vapour and is the coolest part.
- B: the middle part, light yellow or white. It is the brightest part, where carbon particles become white hot and give light.
- C: the transparent outer part. It is the hottest part but gives no light, because burning is complete here.
Testing flame: Hold the lamp in front of your eyes and slowly lower the wick until the white light almost disappears and only a faint blue flame remains. This is called the testing flame (or standard testing flame). It should be made in fresh air, some distance away from the place being tested.
Gas cap: When firedamp is present, methane burns on the testing flame and forms a pale blue cap above it. The cap forms when methane and the oil vapour burn in the oxygen of the air. Its height and shape depend on the amount of gas.
Accumulation Test
Q5. How is firedamp tested with an oil safety lamp?
Answer: There are two tests:
- The accumulation test
- The percentage test (also called the quantity test)
Accumulation test: This is done to find out whether gas has collected anywhere. The lamp is taken to the place burning normally or with a slightly reduced flame. If gas is present, the flame rises and starts to flicker. It cannot tell you the exact amount, but if the flame rises, the gas is certainly 3 percent or more. As soon as you see this, take the lamp out into fresh air and do not do the percentage test.
Percentage test: If the accumulation test shows no gas, the percentage test follows. It is a more sensitive test, and it is also done where air is flowing strongly and only a very small amount of gas is likely. The wick is lowered until the light is almost gone and a faint blue cap shows. The testing flame should be made in fresh air some distance from the test place. The height and shape of the gas cap give an idea of the percentage of gas.
Q6. Why does firedamp accumulate?
Answer: When the quantity or speed of air is low, gas coming from coal and rock strata does not mix fully with the air and clear away. Since it is lighter than air (about half its weight), it collects on the rise side or near the roof. The same can happen when the gas comes out as a blower. If gas keeps adding, the whole rise heading can fill with gas, even down to the floor. This build-up is called accumulation of firedamp.
Q7. Why is it necessary to withdraw the lamp at once when the accumulation test shows gas?
Answer: Gas in the accumulation test means about 3 percent or more, and you do not know how much more. If gas exceeds about 5.4 percent, the flame will flare up and the lamp may go out, but the gas may keep on burning inside the gauze. The gauze gets hotter and hotter, and if it becomes white hot, it can no longer protect the lamp, so the flame can pass out and set off a gas explosion. That is why the law requires the lamp to be lowered slowly, without jerks, and brought into fresh air as soon as gas is detected.
Q8. How is the flame kept during the accumulation test?
Answer: The flame is kept burning normally, or slightly lower.
Q9. Why is the accumulation test done first where gas may collect? What is the problem with going straight to the percentage test?
Answer: With a normal flame, the risk of igniting gas outside the lamp is lower than with a testing flame, and the chance of the lamp suddenly going out is also less. The reason is that a normal flame produces a lot of carbon dioxide, which fills the gauze and gives protection. With the testing flame, there is less of this protective gas in the gauze.
Q10. How will you carry out the accumulation test in a rise?
Answer:
- Check that the lamp is in good condition.
- Keep the flame burning normally and enter the rise slowly.
- Sit at the mouth of the rise with the flame at eye level, and keep watching the flame as you slowly move up.
- Hold the lamp by the handle in your right hand and support the oil vessel lightly with your left hand.
- Stop for a few seconds at intervals so that the air of the place can enter the lamp.
- If no gas is found, keep moving ahead with the flame at eye level.
- If the flame begins to flicker or to rise and lengthen, understand that 3 percent or more gas has collected. Lower the lamp without a jerk and bring it out to fresh air.
Q11. What percentage of gas is indicated when the accumulation test finds gas?
Answer: About three percent or more.
Q12. What does it mean if gas explodes inside the lamp during the accumulation test?
Answer: It means that the gas at that place is 5.4 percent or more.
Q13. What is the danger if the lamp is not put out when gas burns inside it?
Answer: When gas catches fire inside the lamp, the flame of the wick goes out but the gas keeps burning. If the lamp is left in the gassy place, the hot gas goes on burning in the gauze. Once the gauze becomes white hot, it can no longer stop the flame from escaping, and a gas explosion can follow in the mine.
Q14. Why does the law require the lamp to be taken slowly and without jerks to fresh air when gas burns inside it?
Answer: A jerk can make the flame leap out through the gauze and cause danger. The same risk applies if someone suddenly throws a burning lamp away, if another person snatches it, or if someone tries to blow it out.
Q15. What will you do if you find gas in the accumulation test?
Answer: I will not raise the lamp any higher. I will slowly lower it towards the floor without a jerk and carry it out through the rise, into the fresh air in the cool airway.
Percentage Test
Q16. How do you test the percentage of firedamp with a safety lamp?
Answer: The percentage (quantity) test is done as follows:
- Check that the lamp is in good order.
- Make a testing flame. In moving air a little away from the test place, slowly lower the wick until the light is reduced and the cap above the wick is just a point. The cap height should be about one thread (around 3 mm) or less.
- At the test place, hold the lamp by the handle in your right hand and support the oil vessel lightly with your left hand. Sit down and, keeping the flame at eye level, slowly raise the lamp from the floor upwards. Always keep your eyes in line with the wick. Keep other lamps well away.
- If gas is present, a faint blue gas cap will appear above the wick. Note its height and shape carefully. Stop the lamp at several points for a few seconds so that the gas mixture can get into it.
- When the blue cap appears, estimate the percentage of gas from its shape and height (see the chart below).
- As soon as gas is detected, do not raise the lamp any higher. Carefully lower it and bring it out into fresh air.
Gas cap size and percentage (as given in the source notes):
| Gas cap shape | Height | Gas percentage |
|---|---|---|
| Top cut off, like a temple with a flat top | 6 mm | 2% |
| Top fully rounded and smooth | 8 mm | 2.5% |
| A point is clearly visible on top (cap looks like an equilateral triangle) | 12 mm | 3% |
| Taller pointed cap | 20 mm | 3.5% |
| Triangle with a very sharp tip | 25 mm | 4% |
| Top becomes long and pointed, then spreads like a fish tail | 35 mm | 4.5% |
Q17. What do the height and shape of the gas cap depend on?
Answer: On the size of the wick, the type of fuel, the amount of gas and the experience of the observer. (The cap on a flat wick looks different from that on a round wick, but flat-wick lamps are no longer approved by DGMS.)
Q18. Firedamp is colourless. Why does a blue cap appear on the testing flame?
Answer: The blue cap is not the firedamp itself. It is the result of methane and the burning oil vapour burning in the oxygen of the air, a chemical reaction. Its height and shape depend on the quantity of gas.
Q19. The percentage test is called a sensitive test. Can it give the exact percentage of firedamp?
Answer: No. The size and height of the gas cap only give an approximate percentage.
Q20. How do you test for inflammable gas in a roof cavity with a safety lamp?
Answer: Check that the lamp is in good condition and that its lock is fitted. Then:
- If it is a top-feed lamp, close the shut-off ring below so that air is drawn in from the top only.
- Reduce the flame to a testing flame in fresh air.
- Rest the lamp's oil vessel on the palm of your right hand and slowly raise the lamp into the roof cavity, watching the flame. Do not put your head into the cavity.
- Keep raising the lamp until (a) a gas cap forms on the testing flame, (b) the lamp reaches the roof of the cavity, or (c) the lamp is so high that the flame can no longer be watched.
- If a blue cap appears, estimate the gas percentage from its size and height.
- As soon as gas is detected, do not raise the lamp further. Carefully lower it into fresh air.
Q21. What will you do if you find gas in the percentage test?
Answer: I will not raise the lamp any higher and will lower it slowly to fresh air. Then I will inform the mining sirdar, who will:
- inform the overman, so the gas can be removed;
- cut the electric power at that place;
- bring the workers on the return side over to the intake side;
- stop blasting;
- take steps to remove the gas as the overman directs; and
- fence off the place.
If I am the mining sirdar, I will do all of the above. If I am a shot-firer, I will stop hole drilling and blasting at that place and in the places on its return side. If I operate a drill, a pump or a coal cutting machine, I will stop it and cut the power.
Fencing, Equipment and Gas Layering
Q22. What kind of fencing is put at the mouth of a place where gas has been found?
Answer: So that nobody enters a gassy place by mistake, a “U-1” type fence is set up at its mouth. Two posts are stood on either side and ropes, rope strands, barbed wire or poles are fixed across them. The lowest wire is 0.25 m (10 inches) above the floor, and the wires are 0.30 m (one foot) apart. The top wire should be at least one metre above the floor, so four wires are used. The figure in the source gives the total height as 1.15 m (0.25 + 0.30 + 0.30 + 0.30).
Q23. What equipment should a gas tester have?
Answer: An oil safety lamp and a cap lamp. While testing, the head-piece of the cap lamp is covered, so that the gas cap can be seen clearly in the dark.
Q24. What is gas layering? What is the danger?
Answer: Gas layering means that a layer of warm gas forms. In places where the air speed is low, warm gas coming out of cracks in the roof does not mix with the air and leave the mine. Instead, it collects along the roof as a thin sheet or layer.
This layer can even flow against the direction of the air and spread over a long distance. Close to the roof the gas can be 20 to 30 percent, yet one foot below it you may find only about 1 percent. An ordinary safety lamp cannot test the layer, so it spreads far without anyone knowing. If a fire or an electric spark reaches it, an explosion can follow. That is the danger of gas layering.
Q25. In a 3 m high rise, 3% gas is found 0.6 m below the roof. How much gas can be found at the roof?
Answer: In a rise, the higher you look, the more gas you find. We cannot say exactly what percentage there will be right at the roof, but there will certainly be more than 3 percent.
Q26. How is a gas layer tested, and how is it removed?
Answer: A gas layer can be tested with a top-feed safety lamp or with a methanometer. It can also be checked with an aspirator, a rubber bulb fixed to the end of a copper tube, which draws air from near the roof into the lamp's inlet hole. A special lamp called the Garforth lamp is made for this. A gas sample can also be drawn off and analysed.
To remove the layer, a screen (No. 1 in the figure) is erected in the roadway a little away from the layer, reaching to about one foot (30 cm) from the roof. Two lower screens (No. 2 and No. 3) are set up at some distance beyond it. This raises the speed of the air near the roof, and the gas in the layer slowly mixes with the air and is carried off. A compressed air injector near the layer can also be used to stir the air and clear it.
Q27. How do you test for gas in roof cavities, holes and machine-cut kerfs with a safety lamp?
Answer: An aspirator is used for gas testing in roof cavities, holes and machine kerfs. It is a rubber bulb with a thin copper tube 2 to 3 metres long on one side and a thin tube on the other side that enters the lamp's inlet hole. The end of the copper tube is put into the roof cavity, hole or kerf, and the rubber bulb is pressed and released several times. This draws gas into the safety lamp, and the presence of gas is detected. The Garforth lamp was made specially for this work. In a roof cavity, though, gas can also be tested with an ordinary safety lamp without an aspirator.
Q28. In an old working, the rise is 5 m high. How will you test for gas there?
Answer: First test from below, up to the height at which you can still watch the flame. Then climb on a stand or stool and test again. If gas must be tested right up against the roof, draw the gas through an aspirator into the lamp and estimate the amount, which is possible only with a Garforth lamp. For the exaact quantity, use a methanometer (with a probe tube) or take a gas sample.
Blackdamp and Oxygen Deficiency
Q29. Which gases can be tested with an oil safety lamp?
Answer: Firedamp, the presence of blackdamp and a shortage of oxygen. It does not detect carbon monoxide. See Carbon Monoxide (White Damp).
Q30. How does the lamp show a shortage of oxygen?
Answer: For every 1 percent fall in oxygen, the light of the flame drops by about 25 to 30 percent, and at around 17 percent oxygen the safety lamp goes out.
Q31. Why is blackdamp (a cold gas) found near the floor?
Answer: Because it is about one and a half times heavier than air.
Q32. What effect does blackdamp have on an oil safety lamp?
Answer: The flame gets shorter and the light is reduced. For every 5 percent of blackdamp, the oxygen falls by 1 percent and the light drops by about 30 percent. When blackdamp reaches 15 percent, the oxygen falls to about 17 percent and the lamp goes out.
Q33. How is cold gas (blackdamp) tested?
Answer: With an oil safety lamp. At the test place, take the lamp slowly from above towards the floor. While lowering it, keep your head above the lamp and watch it from a distance. The flame reduces according to the amount of gas, and the lamp goes out if there is a lot. As soon as the flame dims and starts to go out, raise the lamp at once and bring it back to fresh air. For more on blackdamp and other gases, see our mine gases posts.
Q34. At what percentage of carbon dioxide will the oil safety lamp go out?
Answer: As per the source notes, a lamp goes out in an atmosphere of about 4 percent carbon dioxide. Note that an oil safety lamp goes out when oxygen falls to about 16.5 to 17 percent.
Q35. According to the regulations, what is the maximum percentage of carbon dioxide in a mine that is considered harmless?
Answer: 0.5 percent (half a percent).
Q36. A rise has 10% blackdamp. Will the safety lamp go out there?
Answer: 10 percent blackdamp means the air there is reduced by 10 percent. Since one-fifth of air is oxygen, the oxygen falls by 10 ÷ 5 = 2 percent. The remaining oxygen is a little less than 19 percent (air has about 21 percent), so the lamp will not go out. However, its light will drop by about 60 percent, because each 1 percent fall in oxygen reduces the light by about 30 percent.
Quick Revision Points
- Test in the general atmosphere, at accumulation points, before and after blasting, near coal seams and near running motors.
- Accumulation test first with a normal flame. Flame rising and flickering means 3% or more.
- Gas exploding inside the lamp means 5.4% or more. Lower the lamp slowly, without a jerk, to fresh air.
- Percentage test uses a testing flame of about 3 mm made in fresh air. Read the gas cap by shape and height.
- Gas cap chart: 6 mm = 2%, 8 mm = 2.5%, 12 mm = 3%, 20 mm = 3.5%, 25 mm = 4%, 35 mm = 4.5%.
- The lamp gives only an approximate percentage. The methanometer gives the accurate value.
- Gas layers can be 20 to 30% at the roof and only about 1% a foot below. Use a top-feed lamp, an aspirator or a methanometer.
- The lamp goes out at around 17% oxygen. Blackdamp lies near the floor, and the lamp does not detect carbon monoxide.
Frequently Asked Questions
What is the difference between the accumulation test and the percentage test?
The accumulation test uses a normal flame to find out if gas has collected (it shows 3% or more). The percentage test uses a testing flame and the gas cap to estimate how much gas is present.
What is a gas cap?
A pale blue cap that forms above the testing flame when firedamp is present. Its shape and height give an idea of the percentage of gas.
Can a flame safety lamp give the exact methane percentage?
No. It gives only a rough figure. A methanometer or gas analysis gives the accurate value.
At what oxygen level does the safety lamp go out?
At about 17 percent oxygen.
What is gas layering?
A layer of gas that collects along the roof in places where air speed is low. It can be far richer in gas than the air a foot below it.
Keep Learning
- Flame Safety Lamp Part 1: Construction, Parts and Development
- Flame Safety Lamp Part 2: Parts, Types and Internal Relighter
- Flame Safety Lamp Part 3: Maintenance and Examination
- Mine Gases and Firedamp: Questions and Answers
- Carbon Monoxide (White Damp)
- Stinkdamp, Nitrous Fumes, Afterdamp, Sulphur Dioxide and Hydrogen
- Mine Ventilation Formulas for GATE
- Gas testing exam posts
Test yourself with the questions on gases and the flame lamp in Mining Safety MCQ Part 02, or visit the Indian Minerology homepage.
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