DUPLICATE ANALYSES

In analyzing any type of gas sample, the most dependable results are obtained by making two separate analyses whose results are in satisfactory agreement. Ordinarily, these results may be obtained from the same sample, provided the sample was collected and preserved so as to insure its being representative of the atmosphere in question. However, a better procedure, and one that should be employed in all important cases, is to collect and analyze duplicate samples. Time may be saved by making a single analysis of each sample. In both instances the remaining portion of the sample should be preserved in the container, as it may be desired later to make duplicate analyses of the same sample. If the results of duplicate analyses are in satisfactorily close agreement, the arithmetical average of the two analyses is a suitable expression of the composition of the atmosphere sampled. If the results do not agree within practical limits, an error has been made in the analysis, the samples were not representative of the same atmosphere, or one or both of the samples have been contaminated. This will necessitate additional analyses of one or both samples until agreement is obtained or the cause of nonagreement is established. If the two samples finally are found to have different compositions and more samples of the same atmosphere cannot be obtained, the sample that contained the lowest percentage of oxygen (referring particularly to mine atmospheres) usually can be taken as the more closely representative of the condition in question, although in the absence of information to the contrary even this sample may be viewed with suspicion. If the analyst is unable to obtain duplicate results for the same sample that are in satisfactory agreement, errors are being made in the analytical technique, or the apparatus is in need of repair.

There are instances in which single samples and even single analyses are satisfactory, such as samples of essentially the same atmosphere or successive samples collected at a place where the composition is not subject to sudden marked changes, as in sealed mine-fire areas. In reality, such samples serve as a check on each other or on previous samples and need not be analyzed in duplicate unless unexpected or inconsistent results are obtained or accuracy of results is extremely important.

AVERAGING DUPLICATE ANALYSES

When duplicate analyses have been made, the arithmetical average of all satisfactory results for each constituent except carbon dioxide should be taken as the final result. For carbon dioxide, the result obtained by the first analysis should be taken if it is significantly higher than the second, because carbon dioxide is soluble in the water used to displace the sample from its container, and the second analysis usually gives a lower result because an appreciable amount of the carbon dioxide has dissolved in the water during the first analysis. The sample for the first analysis should be drawn into the burette and measured as rapidly as possible after the sample container is opened. If the second analysis indicates a higher carbon dioxide content than the first by an amount greater than the analytical error of the method, the discrepancy has been caused by improper technique or imperfections in the apparatus.

These rules for averaging results always should be followed, unless there is definite evidence that one of the analyses has been more liable to error than the other, in which case the more dependable one should be taken alone. In no event, however, should one analysis be chosen merely because its results conform more closely to the expected results, or the other discarded merely because its results do not conform to them.

ACCURACY OF THE PORTABLE ORSAT AND AGREEMENT OF

RESULTS

The limits of accuracy of the portable Orsat apparatus, when used by an analyst of average skill, are 0.2 to 0.3 in the percentage found for carbon dioxide, oxygen, and carbon monoxide and 0.2 to 0.3 for methane in the range of 0 to 25 percent, about 0.3 to 0.4 in the range of 25 to 50 percent, and 0.4 to 1 in the range of 50 to 100 percent methane. All results of duplicate samples that agree within these limits are as accurate as can be expected and are satisfactory for the intended purpose of the portable Orsat.

SOURCES OF ERROR IN ANALYSIS

The principal sources of error in the analysis of mine gases with the portable Orsat are as follows:

1. Errors or omissions in general technique and procedure of analysis.

2. Faulty or careless technique in measuring gas volumes or bringing solutions to marks in pipette stems.

3. Carelessness in purging apparatus of residual gases from a previous sample, or in avoiding contamination of sample during analysis.

4. Insufficient time of contact with absorbing solutions.

5. Insufficient oxygen present for combustion of methane.

6. Leaking stopcocks or rubber connections.

7. Errors in reading gas volumes in the burette.

8. Errors in arithmetic in calculating volume of constituent present or percentage of constituent.

9. Worn-out absorbing solutions, especially cuprous chloride.

10. Temperature of platinum coil too low to burn methane completely in the combustion procedure.

11. Traces of absorption solutions in the manifold.

12. Alkaline condition of confining liquid in the burette or combustion pipette. 13. Nonrepresentative sample.

14. Contamination of sample before analysis.

RECORDS AND DATA

The analyst usually is held responsible for analytical results, and for this reason he should keep a permanent record of the history of the sample and the original analytical data as recorded at the time of making the analysis. Only copies of these data should be given to other persons as reports. All data should be kept in legible and permanent form in a notebook or card system.

CARE AND STORAGE OF APPARATUS

The portable Orsat and sampling equipment should be kept in usable condition. If this is not done and an emergency arises, such as a mine fire or explosion, the apparatus may be beyond repair, or an unreasonable amount of time and work may be required to put the

equipment in shape for use. The following are suggestions regarding the care and storage of the apparatus:

RUBBER CONNECTIONS

If the apparatus is to be out of use for some time, it is best to remove all rubber-tubing connections to pipettes, manifold, and burette and pack all detached glass parts carefully with the apparatus to protect them from breakage or loss. If rubber connections are left in place for several months, they are likely to adhere tightly to the glass and will be difficult to remove. Rubber connections should be replaced frequently when the apparatus is in use, and those connecting the pipettes to the manifold should be replaced whenever a pipette has been removed for cleaning or refilling,

In renewing and removing the rubber connections, breakage can be avoided by not trying to slip the connections from the stems of the pipettes but by shaving them down the side with a sharp knife and then peeling off the tubing. Never use gasoline to loosen adhering pieces of rubber, as some of the gasoline may enter the pipettes or manifold and cause erroneous results for methane or even cause an explosion in the combustion pipette.

SOLUTIONS

Pour out all solutions and wash, drain, and replace all pipettes when the apparatus is to be stored. Although the solutions may not be worn out, it is not good practice to save old solutions, as their quality is always uncertain. Solutions that have been used but slightly should be preserved in magnesium citrate bottles.

STOPCOCKS

Before putting the apparatus in storage, remove stopcocks and wipe the grease from the barrels and plugs. Clean any accumulation of grease from the bore of the plug and from the barrel outlets. A piece of small-diameter copper wire with a short L bend at one end is convenient for cleaning the outlets. After flushing the manifold and barrels with slightly acidic water, wipe the stopcocks dry, insert a strip of paper about one-fourth inch wide the entire length of the barrel and over the outlet leading to the pipette, grease the plug lightly, and place it in the barrel. This will prevent the plug from sticking. Secure the plugs against dropping out and breaking by fastening them in place with a copper wire looped around the constriction below the handle and passed over the small end of the barrel or over the branch of the manifold. As stopcock plugs are not interchangeable, it is best to remove, clean, and replace only one stopcock at a time.

REMOVING "FROZEN" STOPCOCKS

If a stopcock becomes stuck, warm the barrel rather quickly by placing it in hot water for a few seconds, and then, before it cools appreciably, hold the stopcock in one hand and attempt to turn the plug or tap it gently with a piece of wood. The stopcock may be tapped on the small end of the plug if it projects through the barrel or on the handle if it is not the hollow-tube type. Repeat the heating

from time to time. If this does not loosen the plug, the device shown in figure 26 often may be used successfully. This device consists of a block of hardwood bored to receive the handle of the stopcock and

FIGURE 27.--Method of operating stopcock-plug remover.

to support the barrel on the rim so that the plug may be forced out by applying pressure with a vise, as shown in figure 27. Similar devices may be purchased from laboratory-supply houses.

The method for using the device as shown is to select a hole just slightly larger than the diameter of the plug, so that the barrel will fit over the rim, place the bottom of the block against one jaw of an ordinary vise, set a hardwood pin having a diameter slightly less than the small end of the plug against the end of the latter, and close the vise until pressure is exerted on the plug. Avoid strain and allow free movement of extending parts of the stopcock. Continue tightening until the plug is loosened or breaks, because if it cannot be loosened in this manner it will be useless to try other means.

STORAGE

Store the apparatus in an upright position. Other items to be considered in storing it are (a) minimize danger of breakage through rough handling and (b) keep it away from excessive heat, which causes rapid deterioration of the rubber tubing and expansion bags.

SYSTEMATIC INSPECTION

It is good practice to inspect the portable Orsat apparatus at intervals, prepare it for use, and analyze a sample of gas. The sample preferably should be from a sealed-fire area; but if it is not convenient to obtain that type of sample, a sample of automobile exhaust gas may be used, as it contains all of the constituents ordinarily determined in mine-fire gases—that is, CO2, O2, CO, CH4, and N2. Occasionally a sample containing an amount of methane above the explosive limit should be analyzed to test combustion analysis equipment and technique.

The practice of periodically analyzing a sample of gas accomplishes two purposes: (1) It assures the usability of the apparatus and discloses the need for repairs or new parts; (2) it gives the personnel whose duty it may be to analyze samples opportunity to learn and keep familiar with the technique involved.

LIST OF EQUIPMENT FOR FIELD WORK

Following is a list of the equipment that should be on hand for sampling and analyzing mine-fire gases. Articles marked with an asterisk are essential for analyzing gases.

*One portable Orsat apparatus.

*One copy of these instructions-Bureau of Mines Miners' Circular 34 (revised).

*One set of solutions for portable Orsat apparatus.

*One 150-watt transformer for transforming available alternating current (specify voltage, as 110 or 220) to 6 to 8 volts, or batteries to give 6 to 8 volts. *Two feet of No. 22 B. & S. gage nichrome wire for variable resistance in electrical circuit of combustion pipette, with three binding posts for mounting. This resistance wire is not needed if a suitable rheostat is included in the Orsat assembly.

*One semicircular wire-gauze shield to place around combustion pipette to protect operator from flying glass, should an explosion occur during the determination of methane.

*One U-tube (8-inch arms) made from 1/4-inch outside diameter soft-copper tubing for withdrawing samples from vacuum or citrate bottles into Orsat apparatus.

*One small box or tube of stopcock grease.

*One bundle of ordinary pipe cleaners for removing grease from bores of stopcocks.