In comparison, the application of an analytical method having an accuracy no better than 0.3 to 0.4 in determining the percentage of methane in a return air containing less than 1 percent of this gas would produce results that would mean little or nothing in regard to rate of methane liberation by the mine.

The important consideration in regard to the accuracy of any analytical method is whether or not that method will yield results sufficiently accurate to serve as the basis for intelligent and useful interpretation in the particular situation under investigation.

PORTABLE ORSAT APPARATUS

The portable Orsat apparatus may be used in the field to analyze samples of mine atmosphere similar to those analyzed in the laboratory by the use of the laboratory-type Orsat. These samples constitute atmospheres that contain certain gases in such proportions that they cannot be determined with the Haldane apparatus, such as samples collected from behind fire or gas seals, samples containing high concentrations of the gases produced by mine fires or explosions (the so-called after-damp), and atmospheres markedly depleted in oxygen and containing percentages of methane within or above the explosive limits.

The chief advantage of the portable Orsat is that it may be taken to the scene of operations in critical situations, such as when a mine fire is being brought under control by sealing or when knowledge of the efficiency of the seals, and consequently the progress of the fire towards extinguishment, is important. In such situations, where conditions may change from day to day or even from hour to hour, it is imperative that information on these changes be available in the shortest possible time. As the planning and conduct of operations may depend largely on the composition of the atmospheres in the affected areas of the mine, it is evidence that shipment of samples to a distant laboratory for analysis would involve so much delay between the time of sampling and receipt of the analytical results that the information would, on many occasions, be of little or no value when received. It is in such situations that portable gas-analyzing apparatus plays an important part in keeping those who direct rescue or recovery operations informed of changing conditions that may have a bearing on the manner in which the work is carried on or that may affect the safety of those doing this work.

The portable Orsat may be used to determine carbon dioxide, carbon monoxide, oxygen, methane, and nitrogen with an accuracy of 0.2 to 0.3 percent for carbon dioxide, oxygen, and carbon monoxide, and an accuracy 0.2 to 0.3 for 0 to 25 percent methane, 0.3 to 0.4 for 25 to 50 percent, and 0.4 to as much as 1.0 for 50 to 100 percent methane. The error in determination of methane increases with the percentage of this gas present in the sample, because the volume of sample that may be used for analysis is decreased proportionately and causes multiplication of the unavoidable errors of manipulation.

Some of the specific uses to which the portable Orsat may be applied are as follows:

1. Determination of "blackdamp," or depletion of oxygen and increase in carbon dioxide in either coal or metal-mine atmospheres from the standpoint of their effect upon the health of the workmen.

2. Accumulations of methane below, within, or above the range that can be determined with the flame safety lamp. It is especially useful in verifying flame safety-lamp tests or examining atmospheres above the range of a safety lamp. 3. Determination of progress of mine fires and effectiveness of sealing. Also, oxygen and methane content with respect to safety from explosive mixtures, either in the case of mine fires or in recovery work.

The portable Orsat cannot be used for―

1. Determination of methane in return air or air from splits to calculate volume of methane emitted, unless the percentage of methane is exceedingly high (2.0 percent or more).

2. Determination of carbon monoxide from the standpoint of health and safety or progress of a fire other than to show a general trend toward increase or decrease in activity. The lower limit of detection of carbon monoxide is plus or minus 0.2 to 0.3; that is, pure air may show 0.3 percent, although no carbon monoxide is present, and contaminated air may show 0.0 when 0.3 percent is present. As 0.3 percent of carbon monoxide is enough to cause unconsciousness in half an hour or less, with exercise, the apparatus obviously is unsuitable; likewise, it is entirely too inaccurate for determining the proper time for unsealing mine fires on the basis of complete disappearance of carbon monoxide from the atmosphere of the sealed area.

In instances such as those cited above, it is desirable to have analyses for carbon monoxide that are accurate to at least 0.01 percent. For this purpose, other methods for determination of carbon monoxide must be used. 11

The portable Orsat apparatus is described, with detailed instructions for its use in the analysis of mine atmospheres, in the appendix to this publication.

INTERPRETATION OF ANALYSES OF MINE
ATMOSPHERES

The correct interpretation of the results of analysis of mine atmosphere is of equal importance with accuracy in analysis, for the interpretation or meaning of the results is the ultimate reason for which the analysis is made.

SIGNIFICANCE OF COMPOSITION OF ATMOSPHERES IN OPEN OR ACTIVE WORKINGS

In considering the composition of the atmosphere in active ventilated workings the analytical results often are compared with values for the respective gases that are defined by codes or standards 12 that state the limiting percentages of these gases that may be present in a mine atmosphere considered safe or suitable. In such instances, interpretation of the analytical results consists mainly of ascertaining whether the atmosphere in question conforms with the limiting values set forth in the standards or of recognizing the hazard indicated by the presence of flammable or toxic gases in dangerous amounts.

In considering the composition of the atmospheres in sealed fire areas, the situation is somewhat different, as many factors may influence the composition of the gases, and interpretation of the analyses of samples collected from such places often may be the deciding factor in planning the safest and most advantageous methods for conducting rescue or recovery operations.

11 References to such methods are given in a subsequent section.

12 Bureau of Mines, Federal Mine Safety Code for Bituminous-Coal and Lignite Mines of the United States: 1946, 84 pp.

FLAMMABILITY OF ATMOSPHERES IN SEALED AREAS

In many instances it is important to know whether the gases in a sealed area would create an explosive mixture (see flammable limits of mine gases in table 1) if mixed with air, as would occur if a stopping should be knocked down by a roof fall or if the area is intentionally opened and ventilated. Figure 15 13 demonstrates the possibilities of forming explosive mixtures when atmospheres containing various proportions of oxygen, nitrogen, and methane are mixed with normal air (20.93 percent O, 0.03 percent CO2, and 79.04 percent N2). In the figure, the straight line AD represents the composition of all mixtures

FIGURE 15. Relation between quantitative composition and explosibility of mixtures of methane, air, and nitrogen.

of methane and normal air that contain up to 20 percent methane. No mixture of methane and normal air can fall above this line, and all mixtures of methane, air, and nitrogen must fall below it. The line BE represents the lower limits of flammability of methane, and line CE the upper limits. As the oxygen content of the mixture decreases, BE and CE approach each other until they meet at E, which means that no mixture that contains less oxygen than that corresponding with point E, or about 12.1 percent, is explosive of itself, but all mixtures in the area BEC are within the limits of flammability and are explosive. Next, consider any mixture to the right of the line

18 For the original of this figure and the discussion thereof, see Coward, H. F., and Jones, G. W., Limits of Inflammability of Gases and Vapors: Bureau of Mines Bull. 279, 1939 revision, 136 pp.

CEF; for example, the mixture represented by the point G. If the dotted line joining points G and A is drawn, then line GA represents the mixtures formed, in succession, as the mixture_represented by point & is diluted with increasing proportions of air. Because line GA passes through the area BEC, the mixture, as it is diluted with air, becomes explosive and remains so as long as its composition is represented by any point between H and J.

The line FE is the boundary between mixtures that will and will not form explosive mixtures with air. Any mixture of methane and nitrogen that contains more than 14.3 percent methane can form explosive mixtures with air. If oxygen is present in the original mixture, a smaller percentage of methane may suffice to form an explosive mixture with air.

The presence of carbon dioxide in mixtures that also contain methane, oxygen, and nitrogen exerts an effect upon the flammable limits of the mixture, tending to decrease the upper limit appreciably but having only slight effect upon the lower limit. However, the percentages of carbon dioxide ordinarily found in coal-mine atmospheres will have little significant effect upon limits of flammability of the methane in such mixtures. For practical purposes, the carbon dioxide in such mixtures (seldom more than 6 percent) may be considered as additional nitrogen, and the data shown in figure 15 may be used to determine whether or not the atmosphere will create an explosive mixture when mixed with air. Actually a small factor of safety will be introduced by such procedure, because the limits of the explosive range will be narrowed somewhat from those shown in figure 15.

The gaseous mixture produced in a sealed fire area in a coal mine is likely to contain hydrogen and carbon monoxide in addition to methane, and the relative proportions of these gases affect the limits of flammability of the mixture. These limits are affected, also, by the proportions of nitrogen and carbon dioxide in the atmosphere. The flammable limits of such mixtures with air may be calculated with usable accuracy. As these calculations are rather involved, space is not devoted to their presentation or discussion in this publication. For such information the reader is referred to other Bureau of Mines publications.14 15 16

Both hydrogen and carbon monoxide form explosive mixtures with much lower percentages of oxygen than does methane, it being necessary to reduce the oxygen content of such mixtures to below approximately 5 percent to eliminate the possibility of explosion. Gases from fires in bituminous coal mines seldom contain more than a total of 2 to 3 percent of hydrogen and carbon monoxide, which gases may be associated with various percentages of methane. Even small percentages of hydrogen and carbon monoxide have some effect in lowering the percentage of oxygen necessary for an explosion of the mixture, and it is the safest policy, when such percentages of hydrogen and carbon monoxide are present, to consider that the mixture in the sealed area may be explosive until the oxygen content is well below 12 per

14 Coward, H. F., and Jones, G. W., Limits of Inflammability of Gases and Vapors : Bureau of Mines Bull. 279, 1939 revision, 136 pp. 15 Jones, G. W., Inflammability of Mixed Gases: Bureau of Mines Tech. Paper 450, 1929, 38 pp.

18 Ash, S. H., and Felegy, E. W., Analyses of Complex Mixtures of Gases. Application to Control and Extinguish Fires and to Prevent Explosions in Mines, Tunnels, and Hazardous Industrial Processes: Bureau of Mines Bull. 471. (To be published.)

cent-the minimum oxygen content that will support an explosion of methane.

If the percentages of either hydrogen or carbon monoxide should be at or near their lower flammable limits (4.1 and 12.5 percent, respectively), then the mixture most certainly should be considered explosive until the oxygen content of the atmosphere is reduced to well below 5 percent.

SIGNIFICANCE OF COMPOSITION OF ATMOSPHERES IN SEALED AREAS

Figure 16 illustrates some of the conditions that may occur and that must be taken into consideration when a fire is sealed off in a gassy coal mine. The curves in this figure show the changes in the

1 1를 2 3 4 5 6 7 14 21 28 35 42 49

DAYS AFTER SEALING

FIGURE 16.-Changes in composition of atmosphere behind seals after seating a fire in a gassy coal mine.

percentages of oxygen, methane, carbon monoxide, and carbon dioxide that might occur in the gaseous mixture behind a fire seal on successive days after sealing. The condition of the fire at various times may be interpreted from the curves as follows: At point A the oxygen dropped below 12 percent before the methane increased to 5 percent, which eliminated the possibility of an explosion in the sealed area. If the methane had increased rapidly, as shown by the dotted line at point B, while the oxygen still was above 12 percent, an explosion might have occurred. At point C the increase in oxygen and the decrease in methane show dilution of the atmosphere behind the seal with air, which might be caused by a leaking seal, or by negative pressure on the seal, which makes it difficult to obtain a sample that is not contaminated with air, or by improper sampling technique or leakage of air into the gas-sample container. Such condition should be investigated