DESCRIPTION OF PORTABLE ORSAT

The apparatus shown in Figures 10, 11, and 12 consists essentially of a burette, e, and four pipettes, a, b, c, and d. The burette has a total capacity of 100 c. c. and is divided into 100 large divisions, each of which is further subdivided into 5 small or 0.2 c. c. divisions. The pipettes contain the following solutions: Pipette a, potassium hydroxide or sodium hydroxide solution for the removal of carbon dioxide; b, alkaline pyrogall..te solution for the removal of oxygen; and c, acid cuprous chloride solution for the removal of carbon monoxide. Pipette d is the slow-combustion pipette for burning the methane. Pipettes a and b are filled with glass tubes to increase the absorption surface of the reagents. Pipette c may be similar to a and b, but preferably it should be of the bubbling type, because cuprous chloride solution is not viscous enough to spread out a large absorption surface on the glass tubes as does caustic or pyrogallate but rapidly drains and leaves the tubes nearly dry. The bubbling pipette is provided with a 2-way cock, one branch of which communicates with a capillary tube extending to within about five-eighths inch of the bottom of the pipette and the other with the top. The cock should be turned first so that the liquid is just displaced from the short tube and then turned through 180° and the remainder of the gas slowly forced out the long tube in a stream of small bubbles. The cock is then turned through 180° and the gas withdrawn through the short arm to a point just below the short-arm outlet. This procedure should be repeated five or six times. The gas is drawn up each arm in turn to a mark on the tubes.

The platinum combustion element inside of the slow-combustion pipette d is made from about 22 to 3 inches of No. 28 or 30 B. & S. gage wire formed into a 6-inch coil and supported by two glass tubes. As a rule, the platinum wire is hermetically sealed through the upper ends of the supporting tubes and welded to copper wires that pass down through the tubes to the exterior. Connections to a storage battery or other source of electric current are made with the lower ends of these copper wires. A current of about 5 amperes at approximately 4 to 6 volts is desired and may be obtained by connecting a number of cap-lamp batteries or dry cells in series. Where electric current, as from lighting circuits, is available the desired current may be obtained by the use of a lamp-bank resistance, or (if alternating current) a more satisfactory device is a toy or electric-bell transformer that will give enough current to heat the platinum coil. Most of these transformers are made for 110-volt primary current and will burn out if used for 220 volts.

When cap-lamp batteries or dry cells are used the temperature may be regulated by a combination of partly used units with fully

charged ones. A variable rheostat may be made by placing a 12-inch piece of 20-gage nichrome wire in series with the coil. One end of the wire is fixed to a copper lead from the coil, and the other is made variable by slipping it through the eye of an ordinary screw binding post to which the corresponding lead from the source of current is attached. A small variable resistance similar to that used for radio apparatus can be used, provided that it has enough resistance and is so constructed that the mounting will not burn or melt.

The solution reservoirs of pipettes b and c are closed with 1-hole rubber stoppers to which are attached air bags or rubber expansion bags, shown in Figures 10 and 11. These bags prevent interchange of air during the operation of the pipette and thereby extend the life of solutions that are affected by constituents of the air. They may be connected singly-that is, one to each pipette or a common manifold may be made, in which case only one bag is required for all of the pipettes. Pipette a does not need protection while in use and can be closed with a rubber or cork stopper when not in use.

The marks on the pipette stems that indicate the height to which the solution is to be drawn are usually etched into the glass. If not, a suitable mark can be made by pasting a narrow strip (about onesixteenth inch wide) of gum label, the sealing edge of an envelope, or adhesive tape around the stem about one-fourth to one-half inch below the lower end of the rubber connections. This distance gives space for observing and adjusting the level of the liquid and eliminates dead space as much as possible. "Dead space" is the term usually applied to the space in the manifold and capillary glasstubing connections that can not be conveniently filled with displacing liquid during the operation of the apparatus.

READING GAS VOLUMES WITH THE BURETTE

DESCRIPTION OF BURETTE

The gas-measuring vessel (burette e), for the type of apparatus shown in Figure 10, is a tube that has a total volume capacity of 100 c. c. and is divided by horizontal calibration marks into 100 equal whole portions called cubic centimeters and marked in even numbers, 0, 2, 4, 6, etc. (see fig. 13), beginning with 0 at the top or bottom and reading both ways in some cases. The odd figures, 3, 5, 7, etc., are generally omitted, due to lack of space. These unit portions are then further divided into smaller portions, usually five equal parts, each one of which represents one-fifth of 1 c. c. or, expressed in decimals, 0.2 c. c. For laboratory apparatus the markings are usually consecutive, as 0, 1, 2, 3, 4, 5 (see fig. 13, C), and the small units are subdivided into 10 equal portions, each representing 0.1 c. c. The horizontal scratches or marks generally correspond in length to the magni

the laboratory type is not a significant advantage in analyzing the types of mine-gas samples for which the portable apparatus is recommended, because the errors of sampling are usually as large as the error of analysis. Even if it were possible to collect better samples, these limits are within the range of practical use and interpretation of results.

Also, the use of the laboratory type of apparatus in most cases involves shipment of samples, which results in delay and allows time for leaks, in many cases causing far greater error than if the samples

FIGURE 11.-Front view of portable Orsat gas apparatus

were immediately analyzed by the portable type of apparatus. Because the oxygen content is of major importance in mine-fire gases more samples of that type are vitiated by air contamination after taking than by all other causes.

Another advantage of the portable apparatus is the possibility of obtaining immediate results. In the case of samples taken from sealed areas which contain active mine fires, the samples represent only the composition at the time of sampling, and the composition may be entirely different the next day. For this reason a lapse of even a few days may change the condition to such an extent that the

tude of the division; that is, the 1 c. c. units are the longest and may extend half or entirely around the burette, whereas the 0.2 or 0.1 c. c. marks are only across a portion of the face.

In reading the volume of gas in an ordinary clear-glass burette the lowest point of curvature of the water level, termed "meniscus,"

FIGURE 13.-Sections of various types of burettes for portable Orsat gas apparatus

is always compared to a mark on the burette. However, with burettes of the Schellbach type, which have a milk-glass back with a vertical blue or colored line (fig. 13, D), the liquid in the burette magnifies the line below the meniscus, but the portion above appears