FOR DETECTING THEM1

By J. J. Forbes2 and G. W. Grove 3

Revised by

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G. E. McElroy, H. A. Watson,3 E. J. Coggeshall, D. D. Dornenburg, and L. B. Berger3

INTRODUCTION

This publication is a revision of the first of a series of four Miners' Circulars originally prepared and issued in 1929 for use in a course of training designed to prepare mine officials to organize men for rescue and recovery operations. These circulars were revised in 1937 and 1938 and again in 1948.

This circular (Miners' Circular 33) discusses mine gases and methods for detecting them. The second of this series (Miners' Circular 34) gives instruction in Bureau of Mines methods of sampling and analysis of mine gases; the third (Miners' Circular 35) describes methods and equipment used for protection against mine gases; and the fourth (Miners' Circular 36) explains procedures used in sealing and unsealing mine fires and in recovery operations after mine fires or mine explosions.

In this publication, detailed information is given on the nature and occurrence of mine gases and mixtures of gases. Detection of gases is so important that the various methods are treated at length. Particular attention is given to flame safety lamps; methane-indicating instruments; various types of carbon-monoxide detectors; methods for detecting atmospheres deficient in oxygen or containing accumulations of blackdamp; and methods for detecting hydrogen sulfide, oxides of nitrogen, and sulfur dioxide.

SOURCES, CHARACTERISTICS, AND PHYSIOLOGICAL EFFECTS OF MINE GASES

The sources, properties, characteristics, and physiological effects of the gases found in mines during normal operations and under abnormal conditions, such as after mine fires and explosions, are discussed in some detail.

1 Revision completed February 1954.

3 Director, Bureau of Mines; formerly chief, Health and Safety Division.

3 Deceased 1950: formerly chief, Accident Prevention and Health Division, Region VIII, Bureau of Mines, Pittsburgh, Pa.

Pa.

Chief, Mine Ventilation Section, Health Branch, Bureau of Mines, Pittsburgh, Pa. Chief, Gas Analysis and Research Section, Health Branch, Bureau of Mines, Pittsburgh, Electrical engineer, Electrical-Mechanical Branch, Bureau of Mines, Pittsburgh, Pa. Mining health and safety engineer, Mine-Ventilation Section, Health Branch, Bureau of Mines, Pittsburgh, Pa. 8 Chief, Health Branch, Bureau of Mines.

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AIR

According to Humphreys pure, dry air at sea level contains the following gases: Oxygen (O2), 20.95 percent; nitrogen (N), 78.09 percent; carbon dioxide (CO2), 0.03 percent; and argon (A), 0.93 percent by volume. Traces of helium (He), neon (Ne), krypton (Kr), and xenon (Xe) are present also, but these gases, as well as argon, are usually included with the nitrogen because they are chemically and physiologically inert and for practical purposes may be considered as part of the atmospheric nitrogen. Mine air also contains 1 percent or more of water vapor, depending on the temperature, barometric pressure, presence of liquid water underground to form the vapor, and the mosture content of the intake air entering the mine. The weight of a gas, or a mixture of gases, varies with temperature and pressure and, to a slight extent, with the associated water vapor present. Dry air at sea level at 70° F. weighs 0.075 pound per cubic foot,10 a figure that is quite generally used as a standard for air density. The specific gravity of a gas, or a mixture of gases, is the ratio of the weight of a specific volume of the gas to the weight of an equal volume of air at the same temperature and pressure. Thus, the specific gravity of air is stated arbitrarily to be 1; a gas lighter than air has a specific gravity less than 1; and the specific gravity of a gas heavier than air is greater than 1.

The quantities of air breathed and of oxygen consumed by averagesize men with various degrees of physical activity 11 12 are given in table 1.

TABLE 1.-Approximate rate and volume of respiration and of oxygen consumption by man

From table 1 it is evident that rate and volume of respiration and consumption of oxygen increase with the degree of physical effort or exertion that a man puts forth. The rate of oxygen consumption is the same, for a given degree of exertion, whether normal air or pure oxygen is breathed.

Air exhaled from the lungs contains,13 on the average, 16.3 percent of oxygen, 79.3 percent of nitrogen, and 4.4 percent of carbon dioxide,

9 Humphreys, W. J., Physics of the Air: McGraw-Hill Book Co., Inc., New York, 3d ed., 1940, p. 67.

10 McElroy, G. E., Engineering Factors in the Ventilation of Metal Mines: Bureau of Mines Bull. 385, 1935, 196 pp.

11 Henderson, Y., and Paul, J. W., Oxygen Mine Rescue Apparatus and Physiological Effects on Users: Bureau of Mines Tech. Paper 82, 1917, 93 pp.

12 Sayers, R. R., and others, Mine Rescue Standards: Bureau of Mines Tech. Paper 334, 1923, 44 pp.

13 See footnote 12.

which is produced in the oxidation reactions that take place in the tissues of the body. The residual or alveolar air in the depths of the lungs contains 15 to 16 percent of oxygen and 5 to 6 percent of carbon dioxide. Under these conditions the hemogoblin of the blood is virtually saturated with oxygen as the blood leaves the lungs and is circulated to the body tissues where the vital oxidation reactions take place.

Normal air, when circulated through a mine, loses some of its oxygen when breathed by men and animals, and a certain amount is absorbed by the coal and timber. The absorbed oxygen is largely replaced by carbon dioxide, which is given off by the breathing of men and animals, by the coal, and by decaying timbers. To this may be added large or small quantities of methane (CH). Most coal mines give off methane in large or small amounts or are likely at any time to give off methane. The air from the return of 275 bituminouscoal mines was found 1 to have the following average concentrations: Carbon dioxide, 0.17 percent; oxygen, 20.53 percent; methane, 0.18 percent; and nitrogen, 79.12 percent. As the air travels through mines small amounts of carbon monoxide (CO) also may be added, even during normal operations, because of the gases from the explosives used when coal or rock is blasted. Trouble is seldom experienced from this source, however, unless men return to their places too soon after blasting, when dangerous concentrations of carbon monoxide may be present, especially when certain types of explosives are used or certain methods of blasting are employed. Dangerous concentrations of carbon monoxide may occur, owing to abnormal conditions, such as a mine fire or an explosion. If stoppings have been blown out and the ventilation destroyed by an explosion, carbon monoxide is likely to remain in the mine until ventilation has been restored. In addition to the gases already mentioned as being present during normal or abnormal conditions in mines, others may be found occasionally in mine air. These include oxides of nitrogen (NO, NO2) after blasting with certain high explosives or as the result of afterburning of such explosives rather than detonation; sulfur dioxide (SO2) during metal-mine fires in sulfide ores; hydrogen (H2) during coal-mine fires and after explosions; and hydrogen sulfide (H2S) released from stagnant ground water in coal mines or emitted directly from strata.

OXYGEN (O2)

Oxygen is necessary for the support of life and of combustion. It is colorless, odorless, tasteless, nonpoisonous at ordinary concentrations and pressures, and the most important to man of all the gases in the air. Its specific gravity is 1.1054,15 and its weight per cubic foot at sea level pressure (29.92 inches of mercury) and 70° F. is 0.083 pound. Men breathe mostly easily and work best when the air contains about 21 percent of oxygen, the amount usually present in air; yet they can live and work, though not so well, when there is less oxygen. If about 17 percent 16 of the air is oxygen, men at work will

14 Yant, W. P., and Berger, L. B., Methane Content of Coal-Mine Air: AIMME Tech. Pub. 44, December 1927, 7 pp.

15 Burrell, G. A., and Seibert, F. M. (revised by G. W. Jones), Sampling and Examination of Mine Gases and Natural Gas: Bureau of Mines Bull. 197, 1926, 108 pp.

18 Sayers, R. R., Sanitation in Mines: Bureau of Mines Miners' Circ. 28, 1924, 16 pp.

breathe a little faster and a little deeper, about the same as when they first go from sea level to a height of 5,000 feet. Men breathing air that has as little as 15 percent of oxygen usually become dizzy, notice a buzzing in the ears, have a rapid heartbeat, and often suffer from headache. Very few men are free from these symptoms when the oxygen in the air falls to 10 percent. Men sometimes may faint or become unconscious when the air contains 9 percent of oxygen. Although men do not usually lose consciousness until very much less than 13 percent of oxygen is present, no one should try to enter or remain in an atmosphere in which a flame safety lamp or candle will not burn unless he wears a self-contained breathing apparatus.17 flame of a flame safety lamp or candle is extinguished at about 16.25 percent and the flame of a carbide lamp at about 12.50 percent. When the air contains only about 7 percent of oxygen, life is greatly endangered. When the oxygen is diminished below 6 percent, movements are convulsive, and respiration consists of intermittent gasps that finally cease, followed in a few minutes by the stopping of the heart action.

The

Oxygen higher than the normal 20 or 21 percent has no injurious effect on men under the usual conditions of exposure. Pure oxygen, as breathed by wearers of self-contained oxygen breathing apparatus, causes no noticeable effect, even after several successive periods of wear. However, it is dangerous to breathe pure oxygen, as used in oxygen breathing apparatus, at a pressure much higher than 15 pounds per square inch, or approximately atmospheric pressure.1 This hazard should be taken into account in regard to wearing oxygen breathing apparatus in caissons, in subaqueous tunneling operations, or in going to any considerable depth under water.

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The effects of breathing oxygen in concentrations and pressures above normal have been studied by many investigators,19 beginning as early as 1774, when Priestley discovered the gas. Some of the early observers reported that breathing pure oxygen irritated the lungs. Later investigations showed that an atmosphere containing 60 percent oxygen, at atmospheric pressure, may be breathed almost indefinitely without ill effects, that 80 percent oxygen produces lung irritation only after 48 hours' continuous exposure, and that essentially pure oxygen, at atmospheric or normal pressure, may be breathed for about 24 hours without producing symptoms.20 Although the effects of increased concentration of oxygen at atmospheric pressure are characterized mainly by lung irritation after prolonged exposure, inhalation of oxygen at higher pressures produces symptoms of the central nervous system. At pressures of oxygen greater than 2 atmospheres 21 the effects on the central nervous system are said to

17 See footnote 16.

18 Griffith, F. E., and Schrenk, H. H., Use of Respiratory Protective Devices Under Abnormal Air Pressure: Bureau of Mines Rept. of Investigations 3488, 1940, 9 pp.

19 Berger, L. B., and Davenport, S. J., Effects of the Inhalation of Oxygen: Bureau of Mines Inf. Circ. 7575, 1950, 36 pp. (Contains summaries and reviews of numerous investigations.)

20 See footnote 19.

21 In discussing the effects of inhalation of oxygen at pressures greater than 1 atmosphere (14.7 pounds per square inch at standard sea level conditions) it is convenient to designate the concentration of oxygen in terms of atmospheres partial pressure rather than as percent by volume. For example, pure oxygen (100 percent) at a pressure of 1 atmosphere may be designated as 1 atmosphere of oxygen; and pure oxygen at a pressure of 2 atmospheres may be designated as 2 atmospheres of oxygen, rather than by the confusing expression 200 percent oxygen.

predominate. Symptoms of oxygen poisoning by high partial pressures of oxygen are convulsive seizures of the head, neck, and limbs, irregular and labored breathing, nausea, and in some cases complete temporary cessation of breathing. Death may result. Air at 5, 10, 15, or 20 atmospheres has about the same effect as oxygen at 1, 2, 3, or 4 atmospheres. Table 2 presents a summary of the observations of many investigators on oxygen pressure and duration of inhalation that may be tolerated by the average man without development of significant symptoms.?

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TABLE 2.-Tolerable exposures to pure oxygen at 1 to 9 atmospheres

In table 2 the range of exposure times considered safe at any given pressure represents the differences of opinions of various investigators. It is said that susceptibility to oxygen poisoning varies with different individuals and that one person may vary in his susceptibility from day to day. Therefore, a practical margin of safety would be provided by considering only the lowest exposure time cited for any given pressure.

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Liquid oxygen boils or vaporizes at 297° F. below zero.2 Liquid oxygen has received limited use in some forms of self-contained oxygen breathing apparatus and also is used to some extent as an explosive for blasting overburden in surface mining or stripping; liquid-oxygen explosives, however, should not be used for underground blasting in coal mines.

CARBON DIOXIDE (CO2)

Carbon dioxide is a product of the oxidation and combustion of organic compounds and also of the respiration of men and animals. It is a colorless and odorless gas, which, when breathed in high concentrations, may cause a distinctly acid taste. It will not burn or support combustion. Its specific gravity is 1.5291,24 and its weight per cubic foot at sea-level pressure and 70° F. is 0.115 pound. Carbon dioxide usually is found along the floor in low places and abandoned workings and is a normal constituent of mine air. The proportion of carbon dioxide in mine air is increased naturally by the process of breathing of men and animals, by oxidation of coal and decay of timber, by the burning of flame lamps, by fires and explosions, and by blasting. It has been found emanating from the rock strata in underground workings of certain mines, notably in the Cripple Creek,

22 Stadie, W. C., Riggs, B. C., and Haugaard, N., Oxygen Poisoning: Am. Jour. Med. Sci., vol. 207, 1944, pp. 84-114.

23 International Critical Tables, vols. 1 and 3, 1928.

24 Burrell, G. A., and Seibert, F. M. (revised by G. W. Jones), Sampling and Examination of Mine Gases and Natural Gas: Bureau of Mines Bull. 197, 1926, 108 pp.