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for shipment to engineers in the field. The shipping containers are wooden boxes (11 by 712 by 412 inches, outside measurement) which have two compartments lined with corrugated paper board to minimize the possibility of breakage during shipment, and are of a size to be shipped by parcel post. Figure 1 shows a box containing two vacuum bulbs. When the samples have been taken the bulbs are wrapped in paper so as to fit snugly in the compartments, and more paper is packed around the necks. Each compartment contains a form card on which information can be noted. Definite directions for sampling gases by the vacuum-bulb method can not be given to fit all circumstances, because of the many varying conditions encountered in different mines or even in different parts of the same mine.
588 It must be understood that
300 the vacuum bulb takes a sample of gas at one point only—where the tip of the bulb is broken off. This condition largely limits its use to these places where
FIGURE 1.—Shipping box containing vacuum bulbs there is enough air movement to make the composition of the gases uniform across a given cross section in a return airway or entry.
SAMPLING WHERE THERE IS APPRECIABLE AIR MOVEMENT
To sample where there is appreciable air movement, stand facing the direction of air movement—that is, with the air blowing against the face and hold the vacuum bulb, with arms outstretched (to prevent contamination of the sample with expired air from the breath), as nearly as possible in the center of the place to be sampled. While the bulb is held in this position, break off the end of the capillary tube at a file mark or scratch thereon. When a file scratch has been made at the desired place, glass tubing is easily broken by holding the tube with one hand on each side of the file
mark with the mark facing upward and pressing down slightly on · each side of the mark. A small piece of wood with a small hole bored at each end is included in the sampling outfit to provide a better handhold for the glass tube. Insert the latter in the deeper of the two holes almost up to the scratch; slight pressure downward will then break the tip. If a wooden plug is not available, use a small pair of pliers or slip the tube through the hole in the handle of a common cabinet lock key.
As soon as the tip is broken air rushes into the bulb; a few seconds later close the tube by forcing some specially prepared wax into the end broken. (Prepare the wax at the laboratory by heating two parts of beeswax with one part of Venice turpentine. During the warm summer months, or for the collection of samples in hot places in deep metal mines, increase the proportion of beeswax 20 per cent. Melt the wax and pour it into empty 38-caliber short brass cartridge shells, of which there are two in each shipping container.) Force a shell over the capillary opening for one-fourth to one-half inch. Samples should always be taken in duplicate.
SAMPLING IN STILL AIR If the vacuum-bulb method is used for sampling atmospheres that are not of uniform composition, take several samples across a cross section to get an average value for the given area. If the composition is thought to vary from bottom to top or from side to side, divide the cross section as nearly as can be judged by the eye into equal rectangles, and take a sample of the gas from the center of each rectangle. Under such conditions, where the composition of the gas is thought to vary, the number of samples taken on the cross section should not be less than four.
After the samples have been taken and notes recorded on the blue cards provided in the shipping box, replace the bulbs with the cards and mail them to the gas-analysis laboratory.
SAMPLING BY AIR DISPLACEMENT There are some places in mines where sampling by the vacuum method is difficult, if not impossible, such as behind brattices built to confine an area during fires. To sample such places, fasten a rubber tube and a sufficiently long glass or small metal tube to the suction end of an aspirator bulb. Pass the tube through a small hole in the stopping and pack the hole tightly with clay or other suitable material. Place the other extension from the bulb in the sampling bottle; compress the bulb, forcing the air from behind the stopping into a magnesium citrate or other bottle used as a container. The authors have found that compressing the bulb 50 times more than suffices to replace completely the air in an 8-ounce bottle. When the air in the bottle has been entirely replaced with gas, withdraw the rubber tube slowly and close the bottle at once to prevent access of
air. A margin of safety is provided because the bulbs become less efficient with use, although their usefulness can be fairly well determined by compressing the bulb and closing one end to determine whether or not there is leakage. A little dust sometimes lodges on the valve seats.
SAMPLING BY WATER DISPLACEMENT
WATER AS DISPLACING FLUID Water may be used as the displacing fluid in the collection of samples of air that are to be examined for methane only, because water has no appreciable absorbent action upon methane; but the content of carbon dioxide in a sample collected by this means will be less than actual. In order to show the loss of carbon dioxide the following experiments were performed:
Prepared mixtures of carbon dioxide and air were collected in clean bottles by water displacement and were analyzed daily for carbon dioxide. In collecting the samples the bottles were almost completely emptied of the water so that only a thin film remained on the inner side. The progressive decrease of the carbon dioxide within the bottles is shown as follows:
Decrease of CO2 in a prepared mixture of CO2 and air contained within bottles wet
with distilled water
The bottles (sample containers) had a capacity of about 300 c. c. It was found that after they had been drained of distilled water in the ordinary operation of collecting a sample, about 0.7 c. c. of the water still remained on the inner surface. The following calculation shows how small a quantity of carbon dioxide could be . absorbed by this volume of pure water:
Pure water absorbs approximately its own volume of carbon dioxide at normal temperature and pressure, so the 0.7 c. c. of water remaining on the inside of these containers should absorb about 0.7 c. c. of carbon dioxide at normal pressure. The partial pressure of the carbon dioxide was 760X0.0053, or 4 mm. The amount of carbon dioxide absorbed by the water becomes then
A =0.0036 C. c., or 0.0012 per cent of the entire quantity of the gas sample.
This is within the error of making the analysis (0.01 per cent); hence the appreciable loss of carbon dioxide noted above must have been due to some other cause than absorption of carbon dioxide by water. It is probable that absorption of carbon dioxide in the alkali dissolved out of the glass by the water was responsible. Haldane' has called attention to the absorption of carbon dioxide by alkali dissolved out of glass by water, and has pointed out that in a wet, dirty bottle an appreciable amount of carbon dioxide may appear and oxygen may disappear by bacterial action. :
PROCEDURE FOR SAMPLING
Figure 2 shows twelve 250-c. c. glass sampling tubes assembled in a box. Before a sample of the gas is taken fill the tubes with water. Pinch clamps over short pieces of rubber tubing, make the tubes gas-tight, connect the upper end with the place where a sample of gas is to be taken, and open the pinch clamps; the water then flows out and draws in the sample of gas. When all of the water has run out, close the pinch clamps, and the sample is ready for analysis. Analyze samples taken by this method as soon as possible, because rubber tubing is not impervious to gases when left over a
? Ilaldane, J. S., Methods of air analysis. 1912, p. 3.
long period. The box serves as a convenient means of carrying the sample containers to and from the laboratory. Each tube in the box is protected by strips of felt, and when the box is closed each is held firmly in place.
CHECKING THE READING OF A FLAME SAFETY LAMP It is frequently desired to obtain samples of mine atmospheres to check the percentages of methane indicated by a flame safety lamp. To collect such a sample after the safety lamp has been “read,” hold the bottle so that the point at which the mine atmosphere enters is close to the place where the air inlet of the safety lamp has been. If there is a still accumulation of methane and air, especially a pocket of gas near the roof, it is obvious that even with the greatest care some disturbance of the atmosphere will occur in sampling by some of the methods already mentioned, so that the sample collected in the container may not have exactly the same composition as the air that entered the lamp.
Minimum disturbance of the atmosphere will result if the sample is taken by the vacuum-bulb method, the bulb carefully raised to the place to be sampled, and the tip broken with care.
DETERMINATION OF MOISTURE USE OF WET AND DRY BULB THERMOMETERS Plate I shows the wet and dry bulb thermometers (psychrometer) used by the Bureau of Mines for determining the amount of moisture in the air. The thermometers are mounted as a unit in an aluminum frame. Opposite the thermometer bulbs are slots to allow free circulation of air. To use the hygrometer, remove the leather case, hold the apparatus firmly by the handle shown, and swing it in a circular path facing the air current.
To make standard readings with the hygrometer, or sling psychrometer, described above, the speed of the revolving bulb must be about 15 feet per second. Whirl the psychrometer for 15 or 20 seconds and then read it, the wet bulb first. Repeat this process until successive readings agree closely.
The relative humidity of the atmosphere can be determined from the wet and dry bulb for different observations of barometric pressure by the psychrometric tables 8 issued by the Weather Bureau.
Figures 3 and 4 show curves' from which the relative humidity of a mine atmosphere can be found from the wet and dry bulb read
& Marvin, C. F., Psychrometric tables for obtaining the vapor pressure, relative humidity, and temperature of the dew point: Dept. of Agriculture, Weather Bureau Bull. 235, 1910, 84 pp.
Rice, G. S., The explosibility of coal dust: Bull. 20, Bureau of Mines, 1911, pp. 59 and 66.