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sary in order to establish a third known quantity for insertion in the equation, in order to obtain the three equations needed to calculate the quantity of each gas present. The readings in a combustion analysis of a mixture of three gases must be made with precision, because a small error is greatly magnified in the calculations. When these three gases are present the following relations are obtained from equations (1), (2), and (3):
(18) Oxygen consumed=1/2H2+2CH.+1/2C0.
(21) CO=CO2-CH. A typical analysis of a gas containing carbon monoxide, hydrogen, and methane follows:
Analysis of mine gas containing Co, H2, and CH.
C. c. Volume of sample taken -----
----------------------------------- 20. 430 Volume after carbon dioxide absorption---
From oxygen by second analysis (3.820:-20.270) X100=18.85 per cent. If this oxygen value in the first analysis is used for determining the oxygen consumed (20.430X18.85):-100=3.851 c. c.—3.790 C. c. (residual oxygen)=0.061 c. c. oxygen consumed during combustion. By equation (19) : Hydrogen=contraction-oxygen consumed.
=0.067-0.061=0.006 c. C.
To estimate the black damp and air in the mixture, the results can be calculated by grouping with the carbon dioxide the nitrogen in excess of that necessary to form atmospheric air with oxygen, as follows: Per cent
Per cent 0 m2 Carbon dioxide---
1. 18 Nitrogen ----------
8. 55 (Carbon dioxide-----
.03 Air ---
PRECAUTIONS IN MANIPULATION OF APPARATUS
Keep the mercury clean by occasionally filtering it through chamois skin and allowing it to drop in a fine stream through dilute nitric acid, passing air through it; or by agitating a mixture of mercury and dilute nitric acid with air.
NECESSITY FOR MOISTURE
Slightly moisten the inside of the burette and compensating tube with water. Accurate work can not be done unless the inclosed air is continuously saturated with moisture.
Do not allow the caustic hydrate or alkaline pyrogallate solution to be drawn into the capillary manifold. If that happens, draw dilute sulphuric acid into the burette, pass it through the manifold to neutralize the caustic, and then wash the manifold again with water. One very small drop of caustic will completely ruin the accuracy of the combustion analysis, and this condition must be carefully avoided if results are to be accurate.
TEMPERATURE OF BURETTE JACKET
Maintain all the water in the jacket surrounding the burette at a uniform temperature. Keep the apparatus in a room of uniform temperature, free from air currents, and stir the jacket water during the analysis by bubbling air through it. If compressed air is not available, use a small rubber syringe bulb by introducing into the jacket a glass tube extending to the bottom of the water jacket, the bulb being attached to the upper end of the tube. The bulb should be provided with a valve, which causes it, when squeezed, to act like a pump, forcing air down through the glass tube and into the water in the jacket.
CHECKING THE APPARATUS
Test the working qualities of the apparatus occasionally by an analysis of atmospheric air. Pure dry air contains 0.03 to 0.04 per cent of carbon dioxide and 20.93 of oxygen, but in large cities these proportions may differ slightly.
EFFECT OF DRAFTS
Protect the apparatus from drafts as far as possible and use it in a room where the temperature change is slight. Although the compensating device corrects for change of temperature and pressure, sudden temperature changes may cause the compensation leveling tube to be raised or lowered beyond the working range during an analysis.
CORRECTIONS FOR WATER VAPOR
The analysis is given on a dry basis and must be corrected for content of water vapor, if this is desired, either by testing the air with some satisfactory type of psychrometer when sampled, or by making a special determination, with special apparatus, for watervapor content.
CLEANING THE APPARATUS
The burette becomes dirty after several weeks of use, due to finely suspended dusts that are usually present in the samples and to a less extent to some of the constituents combining with the mercury and to particles of stopcock grease finding their way into the burette.
To clear the apparatus remove the rubber tubing connecting the leveling bulb with the bottom of the burette and draw in from the bottom a hot solution of 5 per cent nitric acid, completely filling the burette. To do this most easily put a short length of rubber tubing on one of the upper connections of the burette, connecting an aspirator bulb to it and producing suction. The dilute acid removes the mercury. Remove the organic impurities with hot cleaning solution. Allow the dilute acid to run out, and then wash the burette with distilled water, fill, it with hot cleaning solution, and allow to stand over night. Prepare the cleaning solution by saturating concentrated sulphuric acid with potassium bichromate.
ATTENTION TO STOPCOCKS
If any stopcocks are not well ground and leaks continually develop make the cocks fit more tightly by grinding with moistened carborundum powder. To make a suitable stopcock grease, melt 50 parts by weight of pure gum rubber, and as it melts add a melted mixture of 10 parts of vaseline and 25 parts of beeswax. If the resulting grease is too soft add more rubber, and if too sticky add beeswax and vaseline. Use a heavier grease in summer than in the winter. The proportions given above will necessarily vary, depending upon the brands of rubber, beeswax, and vaseline used.
APPARATUS FOR THE DETERMINATION OF METHANE IN
In spite of the fact that the apparatus described above and shown in Plates III and IV has been simplified as much as is consistent with accuracy, its proper manipulation requires more than ordinary experience; consequently it may not be practicable for use around a mine. A mine manager particularly desires an apparatus that will determine methane accurately and quickly so that he can control the methane given off each day in different parts of his mine.
GAS DETECTOR A safety lamp may be used to detect a proportion of methane likely to become quickly dangerous, as the appearance of a cap 17 on
17 Paul, J. W., Ilsley, L. C., and Gleim, E. J., Flame safety lamps: Bull. 227, Bureau of Mines, 1924, 212 pp.
its flame indicates the presence of such an amount; but in places where the safety lamp fails to indicate gas, mine officials are ignorant of the actual percentage which may be present. When the fact becomes better appreciated that enough methane is always present in some mines to make a dangerous condition should a fan stop suddenly, or should the ventilating system be deranged, more care will probably be taken to ascertain the amount of the gas in individual mines.
The Burrell methane indicator,18 a portable device for giving the percentage of methane below that at which the safety lamp gives an indication, was developed at the Pittsburgh experiment station of the Bureau of Mines. It will detect methane when present in amounts greater than about 0.1 per cent. This device, however, is not intended to displace gas analysis in the study of mine ventilation, but rather to fill the gap between the safety lamp and precise gas analysis.
Regular analyses of samples taken from the ventilating current in different sections of a mine, such as the main returns, the splits, and the goave, and at the face, will indicate whether there is danger of a fire-damp explosion. If a split ventilating a certain section of a mine contains a large amount of methane at any time, more air can be forced through that part with consequent decrease in the amount of air passing through a less gaseous district, thereby equalizing conditions. As a final resort it may be possible to increase the total quantity of air passing through the mine. In some mines much of the methane found in the returns is liberated only in certain sections. Systematic analyses of the air in different parts of the mine will define these sections. Curves may be drawn to show the percentage of methane produced each day in certain areas, and a rule can be established that a certain maximum of methane shall not be exceeded.
VERIFYING SAFETY-LAMP TESTS Gas analysis is also useful at the mine for checking the degree of accuracy with which fire bosses and other employees can determine percentages of methane by means of the safety lamp. Great difference of opinion exists as to the exact proportion detectable under the same conditions. Some men use a high flame, whereas the majority lower the flame for cap indications; but the results are seldom verified at the mine by analyses of the particular mixture examined. The kind of lamp, the illuminant used, and many other factors may affect the height of the cap. Under exactly the same
18 Burrell, G. A., A new fire-damp indicator : Coal Age, vol. 9, 1916, p. 157. Milligan, L. H., A critical study of the Burrell indicator for measuring combustible gases in air : Tech. Paper 357, Bureau of Mines, 1925, 40 pp.