Walk into the lamp room of an underground coal mine and you may still find a small, old-fashioned oil lamp on the shelf. It gives very little light and nobody carries it to see their way. Yet this lamp has saved countless lives, and it still turns up in almost every mining exam. It is the flame safety lamp.
In this Topic we look at how the lamp is built, how it developed over time, the principle behind it and what each part does. The notes follow a simple question-and-answer format, which makes them easy to revise before DGMS, Mining Sirdar or GATE Mining. Since the lamp is mostly used today for gas testing, we have also linked the notes to our posts on mine gases.
- Invented: Sir Humphry Davy, 1815 (single 28-mesh gauze)
- Double gauze: Marsaut, 1882
- Principle: the gauze cools flame gases below the ignition temperature of methane
- Today's use: testing for firedamp and oxygen deficiency, not lighting
- DGMS standard (1988): inner gauze 28 mesh, outer gauze may be 20 mesh
Construction of the Flame Safety Lamp
Q1. What types of lamps are used in underground mines?
Answer: Four kinds of lighting are used underground:
- Fixed electric lights installed at various places in the mine.
- Electric cap lamps, which every worker carries on the head. They are also called electric safety lamps.
- Cell-operated safety torches, which are hardly used today.
- The flame safety lamp (oil safety lamp). When it was first made, it was meant to give light. Today it is used for other jobs: testing for firedamp (methane), detecting blackdamp and finding out whether the mine air is short of oxygen.
Many kinds of methanometers have been developed for gas testing, but the flame safety lamp is still one of the simplest and most practical tools for the purpose. To read more about methane itself, see our post Mine Gases and Firedamp: Questions and Answers.
Q2. Does the flame safety lamp have other names?
Answer: Yes. It is called the oil safety lamp because it burns oil, and the wick safety lamp because it burns with a wick, much like an ordinary lantern.
Discovery and Development of the Safety Lamp
Q3. When and by whom was the oil safety lamp made, and how did it reach its modern form?
Answer: In the early days, miners worked with open flames. Any firedamp (methane) in the mine caught fire, and the explosion that followed killed many people.
Sir Humphry Davy (1815). The English scientist Sir Humphry Davy covered the flame with a wire gauze. His idea was simple: the gauze would quickly carry away the heat of the flame, so the flame could not pass through to the surrounding gas.
Dr. Clanny. The gauze, however, cut down the light. A hospital doctor named Clanny used a glass to improve the light, with a shorter gauze above it. Packing was provided above and below the glass so that the flame could not escape.
Marsaut (France). Both of these lamps went out in a strong air current. The French mining engineer Marsaut placed an iron bonnet around the gauze, fitted to the pillars of the oil vessel. For extra safety he used two gauzes, one inside the other.
After this, the lamp was improved step by step:
- Magnetic lock. The oil vessel was made of brass, and a magnetic lock was added so that nobody could open the bonnet inside the mine. In the lamp room, a magnet is used to pull the pin and turn the bonnet.
- Five pillars. Five pillars (rods) were fitted around the glass in the lower part of the bonnet to protect it from knocks.
- Screw-type wick adjuster. A wire pin was fitted under the oil vessel and made into a screw, so the wick could be moved up and down by turning it.
- Filling arrangement. Earlier, the oil vessel was filled through the wick opening. Later a small spring-loaded ball was fitted in a hole, and the oil was filled with a filling syringe by pushing the ball inward.
- Petrol with cotton wool. Petrol came to be used in place of mineral colza or kerosene oil. As petrol is highly inflammable, the oil vessel was packed with cotton wool, which absorbs the petrol and prevents it from spilling.
- Chimney and small glass. A chimney, with a small glass below it, was added to carry the smoke and gases of the flame out properly.
- Lamp for gas testing. When cap lamps came into use, the safety lamp became a gas-testing tool rather than a source of light. The small glass was therefore removed, because it interfered with gas testing, and the chimney was made a part of the bonnet. This gave the double-layer bonnet.
- Air inlets. Three ways of letting air into the wick were developed, giving the bottom feed, middle feed and top feed lamps.
- Internal relighter. A lamp that goes out inside the mine cannot be relit with matches, so an internal relighter was added. First this was a spring wire with a rotating flint, like a cigarette lighter. Later a dry cell was fitted inside the oil vessel to supply current to a filament wire. Turning the relighter pin completes the circuit, and the spark lights the wick.
- Round wick. A round wick replaced the flat wick because the flat wick got in the way of gas testing.
Q4. What is a safety lamp?
Answer: A safety lamp is an oil-burning lamp that can be used safely in a mine even when hot gas (firedamp) is present.
Q5. On what principle is the oil safety lamp built?
Answer: The gauze surrounding the flame cools the hot smoke and gases below the ignition temperature of methane. The heat taken up by the gauze spreads through all its parts by conduction, so the burning inside the lamp cannot spread to the gas outside.
The gauze itself is kept cool by the smoke and gases rising past it (convection), and it also loses some heat by radiation. In short, the lamp works on the ability of the gauze to cool hot gases by conduction, convection and radiation, and thereby not allow the flame to pass out.
For this purpose the gauze is made of a particular kind of iron or steel, with a fixed height and shape. The wires are spaced so that there are 28 openings in every inch. This is called a 28 mesh gauze, and it has 28 × 28 = 784 openings per square inch.
| Gauze | Openings per inch | Openings per sq. inch | Wire gauge |
|---|---|---|---|
| Inner gauze | 28 | 784 | 28 SWG |
| Outer gauze | 20 | 400 | 27 SWG |
Q6. What do conduction, convection and radiation mean?
Answer: Heat travels from one object to another in three ways:
- Conduction: in a solid, heat moves from particle to particle. When one end of a spoon is heated, the heat travels along to the other end.
- Convection: a liquid (like water) or a gas heated by a source rises, and cooler liquid or gas moves in to take its place.
- Radiation: a solid, liquid or gas can also give out heat in the form of rays.
How this applies to the lamp:
- Part of the heat from the flame is absorbed by the gauze, which is a good conductor, and it spreads through the whole gauze by conduction. Smoke and gases touching the gauze take heat from it as they rise (convection). Some of the heat is also lost by radiation.
- The flame is surrounded by a gauze rather than a sheet of iron because the wire gauze has many times more surface in contact with the heat. The finer the mesh, the more heat it can absorb. A gauze finer than 28 mesh is possible, but the thinner wires would make it weak.
Q7. Explain the construction and work of each part of the oil safety lamp.
Answer: The figure below shows the main parts, followed by a short explanation of each.
1. Bonnet. The steel bonnet covers roughly the upper half of the gauze. It protects the gauze from outside blows and keeps the flame from being blown out by a strong current of air. The top cover of the bonnet guards the gauze against knocks, stops water from dripping onto the flame and protects the hand of the person holding the lamp from the heat of the fumes.
2. Two gauzes. Inside the bonnet there are two gauzes, a bigger one and a smaller one, with the smaller one placed inside the bigger. The inner gauze is about 5 inches long and the outer about 5.5 inches. The inner gauze is 28 mesh, made of 28 SWG wire, and the outer is 20 mesh, made of 27 SWG wire.
The job of the gauze is to take up part of the heat produced when firedamp (methane) gets into the lamp and burns, and to spread it quickly, so that the temperature of the gases inside stays below the ignition point of methane. This way, even the hot gases coming out through the gauze cannot light the gas outside the lamp.
The layer of burnt gases between the two gauzes (the blanket of burnt gases) separates the mine air from the air inside the lamp and gives extra protection. The French engineer Marsaut began using two gauzes for exactly this reason in 1882. The gauzes are made of the best charcoal-annealed iron or steel. From the safety point of view, they are the most important part of the lamp.
3. Chimney. This is a cylinder of thin iron sheet fitted inside the inner gauze. After burning, it takes the hot fumes straight towards the outlet. The cover above the chimney stops the hot gases from going directly to the roof of the gauze, which would otherwise become red hot and be dangerous.
4 and 5. Small and large glass. There are two glasses. The inner (small) glass hangs from the lower end of the chimney and rises slightly above the wick. It helps pull the smoke upwards and improves the light. The outer (large) glass is about two inches high and two and a quarter inches in diameter. It keeps the flame steady even in a faster air current and spreads the light outward. It is very thick and strong so that it does not break even if a gas explosion takes place inside the lamp. It also carries the maker's mark and a serial number. The small glass increases the light but makes gas testing harder, so modern lamps no longer have it.
6. Packing ring (gasket). Round rings, traditionally made of asbestos, are placed above and below the large glass so that the joints are air-tight and the flame cannot escape. The rings also act as a cushion for the glass to sit on. They are about 5 mm thick.
7. Glass retainer ring (lock ring). This is a flat brass ring that screws onto the threads in the lower part of the frame fixed under the bonnet. The large glass rests on it. The ring holds the glass, gauzes and chimney firmly in their places.
| Part | Main function |
|---|---|
| Bonnet | Protects the gauze from blows and keeps the flame steady in air currents |
| Two gauzes | Cool flame gases below the ignition point of methane |
| Chimney | Carries hot fumes straight to the outlet and shields the gauze roof |
| Glass | Gives light and keeps the flame steady |
| Packing rings | Make joints air-tight and cushion the glass |
| Lock ring | Holds glass, gauzes and chimney in place |
| Oil vessel (brass) | Holds the fuel and wick, with a magnetic lock against tampering |
What the Flame Tells You During Gas Testing
Because today the flame safety lamp is mainly a testing tool, it helps to know how the flame behaves in different atmospheres:
- Methane (firedamp): the flame becomes longer and a pale bluish cap appears above it. Read more in our post on firedamp.
- Oxygen deficiency (blackdamp): the flame becomes dim and may go out. Our post on stinkdamp, nitrous fumes and afterdamp covers related mine gases.
- Carbon monoxide (white damp): the lamp gives no warning at all, so it cannot be used to detect this gas. See Carbon Monoxide (White Damp) and use proper detectors.
Gas movement also depends on ventilation, so you may find our post Mine Ventilation Formulas for GATE useful as well.
Quick Revision Points
- Davy invented the lamp in 1815 with a single 28-mesh gauze.
- Marsaut introduced the double-gauze lamp and the bonnet (1882).
- The gauze cools flame gases below the ignition temperature of methane.
- 28 mesh = 784 openings per square inch. 20 mesh = 400 openings per square inch.
- DGMS standard (1988): inner gauze 28 mesh, outer gauze may be 20 mesh.
- Flame lengthens in methane, dims in oxygen deficiency and shows nothing for carbon monoxide.
- Main parts: bonnet, two gauzes, chimney, glass, packing rings and lock ring.
Frequently Asked Questions
Why is the flame safety lamp still used?
It is simple, cheap and gives a visible indication of methane and low oxygen, which is why it is still used for testing in many places and taught in every mining course.
Who invented the flame safety lamp?
Sir Humphry Davy made the lamp in 1815 by covering the flame with a wire gauze. Later improvements by Clanny and Marsaut gave the lamp its modern form.
What does 28 mesh mean?
It means there are 28 openings in every inch of the gauze, or 28 × 28 = 784 openings per square inch.
Can the flame safety lamp detect carbon monoxide?
No. The flame shows no change in the presence of carbon monoxide, so other instruments must be used for that gas.
Keep Learning
Want to test yourself? Try the gas and rescue questions in Mining Safety MCQ Part 02 and Part 04, or visit the Indian Minerology homepage for more notes.
If this Topics helped you, share it with your study group and leave your doubts in the comments.
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