confining samples of mine gases, especially when analysis will be made within a few days. These containers are not as convenient for filling with gas from accessible places or as durable for shipping as vacuum tubes. They are often more convenient, however, for sampling gases in inaccessible places, as behind fire seals, tanks, etc. These tubes may be filled by either gas or liquid displacement, as described later.

The container shown in Figure 2, B, is similar to the type A container except that the rubber tubing and screw clamp are replaced by a glass stopcock. This container has the same field of use for sampling mine gases as the type A. Also, when the stopcocks are well greased there appears to be less danger of leaks during storage. On the other hand, the type B container is more expensive and less durable for shipment. Purchasers of this type should insist that the stem between the body of the container and the stopcock be as short as possible, preferably entirely eliminated, and not as shown in Figure 2, C. By eliminating this stem and cutting the extension beyond the stopcock to not more than 14 inches breakage will be markedly reduced.

The ordinary size of types A and B containers for mine gases is 250 to 300 c. c. capacity. The tubes should be made from heavywall glass with 2 to 3 mm. inside diameter and approximately 7 to 8 mm. inside-diameter capillary-tubing stems.

METAL-TYPE GAS-SAMPLE CONTAINERS

Metal containers, similar in other respects to types A and B glass containers, may be purchased from laboratory-supply houses. However, iron or tinned-iron sample containers are not recommended. The gas laboratory at the Pittsburgh Experiment Station has received samples in tinned-iron containers in which the oxygen had been reduced from 20 to less than 10 per cent, due to rusting of the metal on the inside of the tube. However, copper and zinc tubes are used by a number of laboratories for samples to be analyzed shortly after collection. Metal containers with stopcocks have the disadvantage of being more liable to leakage than the corresponding ones of glass design; however, if they are fitted with good stopcocks properly greased and kept in good condition, they are fairly satisfactory for routine work when the samples are analyzed within a day. If it is necessary to keep them for several days before analysis can be made, the ends may be coated with paraffin, as described later.

Hydrogen sulphide, sulphur dioxide, and oxides of nitrogen react with metal containers and accordingly distort results for these gases. However, no type of container suitable for routine sampling of mine air has been found which is entirely satisfactory for preserving these reactive gases for extended periods. Clean glass containers may be used for short periods, but even then the amount of hydrogen sul

phide, sulphur dioxide, or oxides of nitrogen ordinarily found in mine air is so small that the 250 to 300 c. c. sample bottle contains insufficient gas for making a satisfactory determination. A method in which the gas is determined at the time of sampling or absorbed in a suitable reagent for later determination is more desirable.

ORDINARY GLASS-BOTTLE SAMPLE CONTAINERS

Botiles used by druggists for magnesium citrate make satisfactory containers. These bottles are closed by a rubber washer under a cap held in place by a strong spring. A supply of extra rubber washers or gaskets should be kept on hand for renewals. Unless the samples are to be analyzed within a day or two the tops of the bottles should be dipped in molten paraffin or sealing wax after collection. This coating should be applied by successive rapid dips and subsequent cooling to produce several thin layers. Thin layers make a more effective seal and avoid undue heating, which would cause the gas to expand and possibly to leak outward; on cooling, the gas would then contract and tend to draw in air.

It may sometimes be necessary to use ordinary bottles with cork or rubber stoppers for taking a gas sample, although this is not recommended. Under such circumstances the end of the stopper should be cut off a little below the top of the neck and the recess filled with sealing wax or paraffin, as described above.

ASPIRATING DEVICES

When filling containers in accessible places by gas displacement and when taking samples through tubes leading into inaccessible places an aspirating device may be needed to purge or sweep the original air or gas content from the tubing and to create a stream of gas through the container. This procedure is obviously unnecessary when gas is being sampled from behind seals that are under enough positive pressure to cause a flow of gas through the tubing and the container. Many convenient devices, such as motor-driven air pumps and steam injectors, are used for aspirating gases, but the most convenient for underground use is a double-acting rubber-bulb aspirator, a double-acting foot pump, or aspirator bottles.

RUBBER-BULB ASPIRATORS

Rubber-bulb aspirators of the type shown in Figure 3 may be obtained from apparatus-supply houses and may be used for either suction or pressure. As a rule, the aspirator is connected with the valves so that the pressure and relief on the bulb will draw the air out of the container and admit the gas as shown in Figures 5 and 6, A, although in some cases it is used to force the gas into the container as shown in Figure 7, C and D. Fifty compressions of an ordinary 40 to 60 c. c. capacity bulb will replace the air in a 250 ̊c. c.

sample container. A factor of safety is provided, because the bulbs become less efficient with use. Their usefulness can be fairly well determined, however, by compressing the bulb, closing the intake end, and noting whether there is any leakage, and then by repeating the process endeavoring to compress the bulb with the outflow end

FIGURE 3.-Rubber-bulb hand aspirator

closed. A little dust sometimes lodges on the valve seats, causing them to leak. A seemingly defective bulb is often remedied by the removal of such dust and by wetting the valve seats with water. An added precaution during sampling is to connect a hose to the outflow end and place the exit under a water seal or water trap of 1 or 2 inches. This prevents air contamination through leaky valves.

FOOT PUMPS

From sample

container or

sampling
tuber

Gas

Water

Tight stopper

Where it is necessary to purge long tubes leading to inaccessible places, as through seals and bulkheads, the use of a rubberbulb aspirator may be laborious if the dead space is rather large. Then a doubleacting foot pump (also procurable from laboratory supply houses) may be used. The principle and use of this device are identical with that of the rubber-bulb aspirator, although it is not thought to be as free from sources of air contamination when the sample container is on the discharge outlet, as in Figure 7, C and D, When this is necessary it is good practice to use the pump page 21. to purge the connections and a rubber bulb for filling the container.

FIGURE 4.-Aspirator bottle or can

ASPIRATOR BOTTLES

If a foot pump is not available, the gas may be aspirated by means of a bottle or can filled with water, as shown in Figure 4. As the

water escapes from the can gas is drawn through the tube to replace the water. The size of the can is determined by the volume of gas necessary to purge the tube and sample container. The can may, of course, be refilled several times during the sampling, the only precaution necessary being that of closing the connection between the container and the can during the filling process.

FILLING SAMPLE CONTAINERS

Filling a sample container with gas is merely replacement of the original content of the container with the gas to be sampled, and the most satisfactory method for accomplishing this is one that will insure complete replacement and not change the composition of the sample. The latter has particular reference to the partial solubility of some of the constituents in liquids and consequently their loss to the sample.

The methods for filling sample containers in accordance with the medium displaced in each case are (1) vacuum displacement, (2) air or gas displacement, and (3) liquid displacement.

Vacuum displacement is the principle employed in vacuum tubes, in which the tube from which the original gas content has been previously removed is allowed to fill with the gas desired for analysis.

Air or gas displacement is the method in which the original gas content is swept out by causing a stream of the gas to be sampled to flow through the container.

Liquid displacement refers to replacement of the original gas or air in the container with a liquid, such as water, salt solution, or mercury, and later replacement of the liquid with the gas to be sampled. This method obviates the necessity of sweeping large volumes of gas through the container and often avoids the need of an aspirator to produce a stream of gas.

SAMPLING IN ACCESSIBLE PLACES

APPRECIABLE AIR MOVEMENT

Moving air in main air courses, splits, etc., is usually well mixed, and a sample of average composition can be obtained without special precaution in selecting the sampling point. However, in all cases the air should have traveled under conditions favorable for mixing since it passed the last point or source of significant contamination with gas of markedly different composition (as a large feeder), gas from abandoned or poorly ventilated sections, the junction of two splits of different composition, etc. Such sources of gas cause streaming currents of varying composition which may persist for a considerable distance. Turns and irregularities in airways cause turbulence, which obviously aids mixing and decreases the distance that streaming will persist.

31650°-29- -2

In sampling air where there is appreciable movement, select a rather straight section of the air course and stand facing the air current with the tube at arm's length. Avoid exhaling breath into the zone to be sampled. Open the sample container and allow it to fill rapidly. As a precaution against possible stratification, the tip or incoming opening to the container may be rapidly moved in a plane at right angles to the current and gas thus obtained from several points in the cross section. If the time of sampling is short, as when using vacuum tubes, the inlet to the container may merely be passed in a vertical line across the current.

SLOWLY MOVING OR STILL AIR

The composition of still air or slowly moving air that has had no opportunity to mix may vary widely in vertical cross section at a particular place. In still air methane almost invariably accumulates near the roof of the place where it is being emitted. In relatively still air methane will also flow along the roof to places of higher elevation and fill recesses and potholes in the roof. If issuing in dead ends of rise workings, it will fill the entries and workings in a manner almost exactly opposite to that in which water fills dips, and with a difference of only 1 foot in elevation a sample of gas may vary in composition from a low percentage of methane and almost normal oxygen content to as much as 50 to 75 per cent of methane and a marked deficiency of oxygen.

On several occasions the gas laboratory of the bureau received samples of mine atmospheres that the collector stated indicated less than 2 per cent of methane when tested with a flame safety lamp, whereas the samples actually contained an explosive mixture (5 to 15 per cent) to as high as 65 per cent of methane. In all probability the discrepancy was due to the fact that the point of sampling and the point of making the lamp test were not identical, because samples may be collected several inches nearer the roof than it is possible to test with a flame safety lamp.

When the air is still, however, black damp or atmospheres depleted of oxygen by oxidation unlike methane and like water generally collect at the bottom of the workings. They also flow along the bottom to places of lower elevation. Places have been observed where enough oxygen was present to sustain life when the air 3 or 4 feet from the bottom was breathed, but if men sat down or fell down so that their heads were below that level they very quickly became unconscious from lack of oxygen, much the same as if falling into

water.

SAMPLING PRODUCTS OF COMBUSTION OF EXPLOSIVES

The procedure for sampling mine air to determine slight degrees of contamination by gaseous products of the combustion of explosives