After the vent pipe has been purged of its original air the sample containers may be attached to the sampling tube and filled with gas. If the outward pressure is almost imperceptible, it may again be necessary to use an aspirating device to fill some types of containers. (See fig. 7, A, C, and D.)

One should always remember that the oxygen content in samples of air from mine fires is of utmost importance, and all sources of contamination from atmospheric air should be excluded. This

A

After purging hose tightly insert shoulder of container.

B

Break tip by grasping tip in one hand and container in the other and bend hose

Allow gas to flow past loosely fitting end of broken tip into container FIGURE 8.-Method for filling vacuum containers through extension tubes leading to inaccessible places

should be considered by making the very fundamental observation that the seals are not under reduced pressure and drawing air inward and by making tight sampling connections, by purging the sampling tracts, by filling the containers, and finally by sealing these containers so that no contamination will occur between the time of sampling and analysis. To reiterate, there should be full realization of the fact that the samples are collected, preserved, and analyzed in atmospheric air, a medium that contains markedly higher percent

ages of oxygen than are ordinarily found in gases from sealed areas. For this reason the prevention of contamination by air is not always an easy matter; it has been found to constitute the largest and most general source of error and difficulty in this kind of work.

Vacuum tubes.-In using the vacuum tube for obtaining samples from inaccessible places it may be joined to a sampling-tube extension by a short piece of medium-wall rubber tubing of a size that will fit tightly over the shoulder of the tube but will leave a free space around the capillary neck, as shown in Figures 7 and 8. Insert the tip and shoulder of the vacuum tube into the end (noting the approximate position of the file scratch on the tip) and when tightly in place

To aspirator

if necessary

B

Water

Extension tube to prevent back flow of air

D

FIGURE 9.-Suggestions for taking samples from inaccessible places by water displacement

purge the entire system by the aid of a foot pump or aspirating device. When the system has been completely purged close the screw clamp between the T connection and aspirator and then by bending the hose break the tip at the mark. The gas will rush past the broken piece of tip and fill the bottle. Before removing the container rub the broken end (by bending the rubber tubing as before) to remove any grains of glass that might have lodged in the capillary opening and might cause impediment to the flow of gas. After waiting about one-half minute remove the tube and quickly seal with wax. If there is no perceptible flow and a T connection for the aspirator bulb as shown in Figure 7, B, is not available, a straight tube may be

used. The hose connection to receive the sample container is then pinched near the end with the fingers after purging and the sample container carefully inserted, air contamination being avoided by forcing the tip and neck into the opening while the tubing is held pinched together.

Open-end tubes and ordinary bottles; gas displacement.—Open-end tubes and ordinary bottles can be filled by gas displacement by adopting convenient methods from those previously described under sampling from accessible places and by referring to Figure 7 for suggestions. After the sampling tube is purged, the sample container is tightly connected in series and approximately 10 to 15 times the container volume of gas is drawn through the system. If the seal is under distinct pressure, the gas will flow through without the aid of an aspirating device. If there is a distinct flow of gas from the sample container, it should be left in place for about five minutes to obtain a satisfactory sample. If there is only a moderate or just perceptible flow, the time should be extended.

When using aspirating devices the sample containers of the types shown in Figure 2 should be between the sampling tube and aspirator, to avoid contaminating the sample in case air leaks into the aspirator. Making a water seal at the outflow end of the aspirator bulb or pinching off the exit tube with the fingers during the intake stroke of the aspirator is an added precaution.

When the gas-displacement method is used for filling magnesium citrate bottles or other types of bottles with gas from inaccessible places the aspirator must be placed between the container and the sampling tube. If means for a water seal are available the neck of the bottle should be inserted about 1 inch under water and the gas led under the water and into the bottle, as shown in Figure 7, D. If a seal is not available, the hose leading into the bottle should be pinched off with the fingers during the intake stroke of the bulb to avoid contamination from leaky valves. Aspirating should be continued while the hose is slowly withdrawn from the bottle and a stream of gas played on the top while closing.

Filling by liquid displacement.-Containers of the types shown in Figure 2 and ordinary bottles can be filled by liquid displacement, but in so doing the carbon dioxide content will be affected. This topic was discussed under the use of liquid displacement for sampling from accessible places. The containers shown in Figure 2 also have the advantage that where aspirating devices are not available the need for them can be obviated by filling the sample container and the sampling tube with water. The latter can then be thrust through the opening in the seal and the water drained out of both the tube and container through the bottom outlet of the container. Thus, the system is completely purged with the water. When the

last portion of water is draining from the sample container care should be taken to prevent air from entering at the bottom due to negative pressure in the container. To guard against this use a 6 or 8 inch small-bore tube extension at the lower end of the container, as shown in Figure 9, C. Such an extension will remain filled with water and in effect will be a hydrostatic head that will prevent air from entering the container unless the negative pressure is greater than the weight of the water column in the extension.

Ordinary bottles are filled by an arrangement similar to that shown in Figure 9, D. In all sampling by water displacement the water should be completely drained from the container. If the sample is taken rather slowly, say in two or three minutes, the walls will drain satisfactorily during the sampling period.

INFORMATION TO BE COLLECTED AT TIME OF SAMPLING

The value of collecting and analyzing samples of mine air lies in the interpretation of the results in terms of the conditions at the time the sample was collected. It is essential therefore to collect accurate detailed information regarding each sample. Attention should be paid not only to the information needed to answer immediate questions but to making the analytical results as far-reaching as possible and to contributing information that will be of fundamental value to the mining industry when similar conditions are met elsewhere. In the past the value of many analyses of mine gases made by the Bureau of Mines gas laboratory has been greatly curtailed by lack of detailed information regarding the sample.

BUREAU OF MINES BLANK FORMS FOR SAMPLERS

Experience has shown that there is certain fundamental information that the collector should endeavor to get for each sample in addition to special detailed notebook information which may be specific to the immediate problem. As a consequence, the Bureau of Mines gas laboratory requires the collector to fill out a form sample card and submit it to the laboratory with each sample sent to the laboratory for analysis. When vacuum sample containers are sent out in the regular mailing case the blank card (blue form 562) is usually placed in the case compartments between the corrugated pasteboard liner and the side wall. After it has been filled out it should be replaced in this position and returned with the sample.

When properly filled out the sample card embodies the information that is needed for

1. Identification of the sample.

2. Record and filing.

3. Interpretation of results.

4. Assistance to the laboratory in making the analysis.

The portion applying to each of these items is in most cases obvious from the nature of the data; however, certain essential items will be discussed in detail. In this discussion reference is made to the symbols in parentheses shown in the following reproduction of the form sample card:

(FILL OUT THIS CARD AND SEND TO THE LABORATORY WITH GAS SAMPLE)

Referring to the letters on the face of the card

(a) This should be the number shown on the bottle and should always be filled in by the collector for identification, record, and filing.

(b) The source of sample, as: Coal-mine air, iron-mine air, railroad-tunnel air, etc., should be given.

(c), (d), (e), (f), and (g) The type of information desired is obvious and should always be filled in completely for identification, reports of analyses, record, and filing. If this is not given, the gas laboratory must look it up in directories or communicate with the collector.

(i) The exact location in the mine of the place of sampling should be given in detail and made as descriptive as possible. If taken through a stopping or seal, over a fall, from a borehole, etc., be sure to add that information; also whether sampled close to roof, to floor, etc. It has been observed frequently that collectors confuse "Location in mine" with "Location of mine", inserting the name of a town in this space, whereas it should contain information describing the entry, room, or other location where the sample was taken. () Insert method of sampling and type of container.

(k) The date and exact time of sampling should be given as they often have a very important bearing on interpreting or explaining the results of analysis. (1) and (m) The velocity as measured by an anemometer in linear feet per minute and the square feet in the cross-sectional area of the place of sampling are essential data in calculating the quantity of gas emitted or produced and in explaining variations in composition of the air. If the air is still or is moving too slowly to be measured, a descriptive statement should be made instead of leaving the space blank. Where at all feasible velocity and area should be taken at the point of sampling and recorded on the sample card.

(n) This is obviously the value obtained by multiplying the cross-sectional area in square feet by the velocity in linear feet per minute. The result is cubic feet of air per minute.

(0) If the sample is of gases from behind a seal or stopping it should be recorded whether the gas pressure on the seal is positive (blowing out), negative (drawing in), or neutral (pressure the same on both sides of seal). If the pressure measurement is made by means of a water gage the reading, in inches of water, positive or negative, should be recorded.

(p) The barometric pressure or change in barometer very often has an important bearing on the emission of gas, condition of seals, etc., and should, if feasible, be taken as constituting important information relative to the sample.