UNITED STATES DEPARTMENT OF THE INTERIOR 
U N I T E D S T A T E S DEPARTMENT OF THE INTERIOR 

Rogers C. B. Morton, Secretary 

BUREAU OF MINES 

Thomas V. Falkie, Director 
This publication has been cataloged as follows: 

Thimons, Edkvard D 

Using sulfur hexafluoride as a gaseous tracer to study 

ventilation systems in mines, by E. D. Thimons, R. J. 

Bielicki, and F. N. Kissell. [Washington] U.S. Bureau of 

Mines [ 1 9741 

22 p. illus., tables. (U.S. Bureau of Mines. Report of investigations 7316) 

Includes bibliography. 

1. Sulphur hexafluoride. 3. Mine ventilation. I. l)ielicki, 

Kichard J., jt. auth. 11. Kissell, Fred N., jt. auth. 111. U.S. 

Bureau of Vines. IV. Title. (Series) 

TN23.U7 no. 7916 622.06173 

U.S. llept. of the Int. Library 
CONTENTS 

Page 

Abstract ................................................................ Introduction ............................................................ Analytical procedure and testing techniques ............................. Preliminary SFG tracer gas tests in a coal mine ......................... SF6 travel times ........................................................ Use of SFG in an operating limestone mine ............................... Auxiliary fan tests ................................................ Exhaust hole study ................................................. Natural draft intake study ......................................... Air exchange study in end room on working face ..................... Airflow check in nonworking area ................................... Measurement of airflow in main intake airways ...................... Recirculation tests in a vein-type metal mine ........................... Conclusions ............................................................. References .............................................................. Appendix.--Preliminary tracer gas test 19 ............................... 

ILLUSTRATIONS 

SFG concentration as a function of time for a case of poor mixing . SFG concentration as a function of time for a case of good mixing . SF6 lecture bottle with 0.006-inch hole drilled in cap ............ Schematic of exponential dilution system .......................... SF6 concentration in a room as a function of time ................. Chromatograph calibration curves ... ............................... Bureau of Mines Safety Research Coal Mine ......................... Map showing area of auxiliary fan tests ........................... Airflow pattern in exhaust hole ................................... Diagram of air exchange study in 16 room .......................... Central portion of the limestone mine ............................. Schematic of recirculation tests in vein-type metal mine .......... Curve of SF6 concentration in intake for first recirculation test . Curve of SF6 concentration in intake for second recirculation 

test ............................................................ 

TABLES 

1 . Preliminary tracer gas test results from Safety Research Coal 

Mine ............................................................ 2 . Comparison of estimated and actual SF6 travel times ............... 3 . Velocity results from auxiliary fan tests ......................... A.1 . Calculation table for test 19 ..................................... 
USING SULFUR HEXAFLUORIDE AS A GASEOUS TRACER TO STUDY 

VENTILATION SYSTEMS IN MINES 

by 

E. D. Thimons, 1 R. J. Bielicki,l and F. N. Kissell2 

ABSTRACT 

The Bureau of Mines found sulfur hexafluoride (SF6), released from a 

lecture bottle, to be an ideal gaseous tracer for studying mine ventilation 

systems. Air samples were collected in glass syringes and analyzed by 

electron-capture gas chromatography. In these studies, the lower limit of 

detection was about 1 part per billion by volume (ppb) of SFs per part of air. 

Experiments conducted in a Pennsylvania limestone mine showed this technique useful in evaluating the effectiveness of auxiliary fans, measuring low 

flow velocities, probing the air circulating near a working face region where 

ventilation appeared to be poor, and estimating volumetric flow rates in 

airways of large cross-sectional area and having low flow velocities. This 

technique was also used in a western vein-type metal mine to measure the 

amount of return air being recirculated into the intake air due to leakage 

through old stoped areas. 

INTRODUCTION 

Ventilation airflows in noncoal mines are generally very complex and 

often cannot be studied with the conventional anemometers. Multilevel 

workings, large airways, low air velocities, and leakage through caved areas 

all make it difficult to trace airflow. New procedures are needed to assess 

ventilation in noncoal mines and to attack such specific problems as the measurement of air exchange rates in areas of low airflow, average velocities in 

large low-velocity airways, recirculation, transit air time, and leakage 

across stoppings or air curtains. Recent research and development by the 

Bureau of Mines indicates that tracer gas techniques may prove useful in 

solving ventilation problems. 

Numerous organic and inorganic materials, including radioactive substances, have been used as tracers. Most have certain inherent drawbacks. 

Chemical tracers are not as easily detected at low concentrations as are radioactive substances and are often highly adsorbed on many surfaces. Radioactive 

1 Research physicist. 2 Physical research scientist. 
substances can be detected in low concentrations, but are difficult to handle 

and are likely to be unacceptable to the man working underground. An effectivetracer gas must be detectable at low concentrations, safe, odorless, 

chemically and thermally stable, and not occur normally in the environment. 

Sulfur hexafluoride meets all these essential requirements. Lester and 

Greenberg (813 showed SF6 to be completely safe by keeping rats in an 80 percent SF6 atmosphere for 24 hours with no ill effects. Sulfur hexafluoride is 

odorless, chemically and thermally stable, and is convenient to handle and 

dispense in air (13). - Hunt and Moore (7) showed that no measurable adsorption 

of SF, occurs on reservoir sandstone, aGd Whisman (14) - found the same for coal. 

SF6 does not occur naturally in the environment and therefore background concentration is no problem. Finally, SFG car! be detected i:: very ?ow conceiitrations by gas-solid chromatography, using electron-capture detection (1). 

Saltzman (11) and Niemeyer and McCormick (2) showed SF, can be traced-down to 

a concentration of ppm. 

Several researchers (2-3, 13) used SF6 in meteorological studies of 

moving air masses and the dispersion of airborne pollutants. Drivas (3) used 

SF6 as a tracer gas to study ventilation systems in buildings. They conducted 

tests to measure the effectiveness of the ventilation of individual rooms and 

the building contamination resulting from the reentry of fume hood exhausts. 

SF6 has also been used to evaluate the effectiveness of plugging oil wells to 

prevent wellgas from entering coal mines (10). 

ANALYTICAL PROCEDURE AND TESTING TECHNIQUES 

Methods of releasing SF6 gas into the mine air were tried in the Bureau 

of Mines Safety Research Coal Min.e at Bruceton, Pa. In these experiments SF6 

was released in the mine's only intake and the air in the lone return was 

sampled at a distance of about 3,100 feet from the point of release. 

In the first method tried, SFG was released from a glass container by 

water displacement. A glass container of known volume was filled with pure 

SFG in the laboratory and tightly capped. In the mine, the cap was removed 

and the SFG was displaced from the container by filling it with water. Air 

samples taken simultaneously at different positions in a cross section of the 

return showed that the gas did not mix uniformly with the mine air even after 

traveling 3,100 feet. There was considerable variation between samples taken 

in the middle of the airways and about 12 inches from the roof, walls, or 

floor. A second indication of poor mixing was the very irregular multiplepeak curves obtained when SF, concentrations at the sampling point were 

plotted as a function of time after release. Both of these phenomena can be 

seen in figure 1. This can be compared with the curve shown in figure 2, 

where good mixing occurred. This second indication of poor mixing, multiplepeak curves, holds true only if a single airflow path exists. Where multiple 

airflow paths exist a series of peaks could result, but would not necessarily 

be an indication of poor mixing. 

sunderlined numbers in parentheses refer to items in the list of references 

preceding the appendix. 
0 10 20 30 40 50 60 70 80 

TlME FROM RELEASE OF SF6, min 

FIGURE 1. - SF6 concentration as a function of time for a case of poor mixing. 

TlME FROM RELEASE OF SF6, min 

FIGURE 2. - SF6 concentration as a function of time for a case of good mixing. 
required a minimum of equipment for use 

underground. The amount of SF6 released 

was calculated from the weight loss of 

the lecture bottle. 

The gas samples were taken in the 

I mine with 10-ml glass syringe bottles, 

tightly capped with rubber stoppers. 

Since several days often elapsed before 

the samples were analyzed and since 

samples were occasionally transported by 

FIGURE 3. - SF6 lecture bottle with 0.006- plane, where they would be subjected to 

inch hole drilled in cap. substantial changes in outside barometric pressure, it was important for the 

syringe bottles to be airtight. 

Bottles that appeared to give low readings with respect to other samples 

were checked for leaks in the laboratory. For this purpose the bottle was 

filled with a known concentration of SFB, which was rechecked after the bottle 

had been sealed for several days. 

F. H. Shair (12) found that the surface charge on glass can yield low 

readings for very Low SF6 concentrations. Apparently our SFe concentrations 

were high enough to avoid this problem, but consideration is being given to 

the use of plastic syringes, which appear to have no surface charge and are 

nonbreakable. 
At the sulfur hexafluoride concentrations employed (1 ppb to 1 ppm) some 

adsorption of the SFG on glass does occur. For this reason it was necessary 

to clean the glass sample syringes after each use by washing them in water or 

by flushing them with high-velocity air. Periodic spot checks of these 

cleaned syringes were made to insure that they contained no traces of SFG. 

The SFG concentration in the syringe bottles was determined with a portable electron-capture gas chromatograph. The chromatograph column was an 

8.5-foot by 1/8-inch stainless steel tube packed with Chromasorb 102 

(60-80 mesh) ,4 which was treated at 100" C for at least 2 hours. The 

operating temperature of the chromatogra h was 50" C. Carrier gas was argon D with 5 percent methane, flowing at 10 cm per 17 seconds. Samples collected 

in the TO-ml glass syringes were transferred to 0.5-ml syringes, from which 

they were injected into the chromatograph in amounts of 0.1 ml. The peak 

heights from the chromatograph analyses were read by a digitai voltneter and 

printed out at 20 voltage readings per second. The peak height for a given 

SFs sample corresponded to the highest voltage recorded for the second peak, 

the first peak being the air peak. 

Samples were prepared in concentrations ranging from 1 pprn SFG down to 

1 ppb SF6. For chromatograph calibration purposes, two methods of preparation 

were used. The first method made use of an experimental dilution system 

(fig. 4) based on systems described by Saltzman (11) and Drivas (4). - 

To obtain a 1 pprn concentration of SFs, 0.85 cm3 of pure S& was injected 

with a syringe into the large vessel, which contained air at atmospheric pressure. This vessel was then pressurized with air to 10 atmospheres. Since the 

large vessel had a volume of 85,000 cm3, the resulting SFs concentration was 

1 ppm. This vessel was used both in the exponential dilution system and in 

performing the daily calibration checks on the chromatograph. During months 

of use, the chromatographic response to this concentration was unchanged. 

The operation of the exponential dilution system began with the evacuation of the entire system. The mixing chamber was filled with the prepared 5 1 pprn SFEj concentration at atmospheric pressure. Air was then passed through 

the mixing chamber at a rate that caused one complete air change in the 

chamber every 20 minutes. A small electric fan in the chamber insured 

thorough mixing. When complete mixing occurs, the SFB concentration follows 

the dilution curve given by C = S e-N, where N is the number of complete air 

exchanges in the mixing chamber, C is the SFG concentration after N air 

exchanges, and C, is the initial SF6 concentration (1 pprn). To obtain a gas 

sample at any point in the dilution process, a syringe needle was inserted 

through a rubber septum on the exhaust side of the chamber. This exponential 

dilution process was rerun several times with better than 95-percent repeatability. Chromatograph calibration curves were developed from these results. 

4Reference to trade names is made for identification only and does not imply 

endorsement by the Bureau of Mines. 

6~t was found that if pure SF6 was introduced into the mixing chamber, the 

dilution system became contaminated by SFG adsorption. For this reason the 

dilution process was started with a concentration of 1 ppm. 
Vacuum 

Q=T 7To power source 

Mixing chamber 

Gas bubbling into 

water 

manometer 

Air 

source s 

Gas flowmeter 

Large pressure vessel 

FIGURE 4. - Schematic of exponential dilution system. 

The second method of obtaining calibration samples was to prepare them in 

rooms of known volume. This method was originally intended as a check on the 

results from the exponential dilution system, but proved to be just as 

accurate and much simpler than the dilution method. For example, to prepare a 

sample of concentration 100 ppb we released 9.3 cm3 of pure SF, into a room 

3 having an approximate volume of 9.3 x 10' cm . The room air was thoroughly 

mixed by several fans placed around the room. Gas samples were withdrawn from 

the room with syringes. The only problem with this method is that the samples 

had to be taken in the room shortly after release, before leakage affected the 

SF6 concentration of the room atmosphere. Figure 5 shows a typical curve of 

SF, concentration in a room versus time for the first 15 minutes after the SF, 

was released. The rate of decrease of SFG concentration varied from room to 

room depending upon how well the room was sealed. Samples were usually taken 

between 1 and 2 minutes after release of the SF6. The difference between the 

chromatograph results from prepared room samples and exponential dilution 

samples was less than 3 percent at the same concentration level. 

Figure 6 gives the chromatograph calibration curves for the ranges 10 ppb 

to 100 ppb and 100 ppb to 1 ppm, respectively. The peak height response is a 
FIGURE 5. - SF6 concentration in a room as a function of time. 

FIGURE 6. - Chromatograph calibration curves. 

straight-line response from 10 ppb up to 100 ppb, but becomes nonlinear above 

this. Concentrations up to 100 ppb were run at attenuation 1024. It is 

interesting to note in figure 6 that at the change of scale concentration 

(lo"), the attenuation apparently increases by a factor of 4 while the peak 
height drops only by a factor of about 3 (from 330 mV to 120 mV). The reason 

for this inconsistency is uncertain, but since the calibration points were 

confirmed by numerous calibration samples, it appears that the cause is an 

inherent characteristic of the chromatograph attenuator. 

PRELIMINARY SF6 TRACER GAS TESTS IN A COAL MINE 

Preliminary experiments were conducted in the Bureau of Mines Safety 

Research Coal Mine at Bruceton, Pa. (fig. 7), to determine how quantitatively 

SF6 gas can be traced in a mine. The mine has a single intake and a single 

exhaust, which offered an ideal situation for testing the use of SF6 as a 

tracer gas in ventilation studies. Before each trial, the airflow in the mine 

was measured with an anernometer traverse. The average fl~w is about 

16,500 cfm. 

The testing procedure began with the release of SF6 into the intake air 

at the point labeled R in figure 6. In most experiments, SF, was continually 

released over a predetermined period of 2 to 5 minutes. This made it possible 

to release a fairly large volume of SF6 without producing a concentration in 

the mine air higher than that for which the chromatograph was calibrated 

(1 ppm), and also to improve mixing of the - SFs with the mine air. Return air 

FIGURE 7. - Bureau of Mines Safety Research Coal Mine. 
samples were taken every 2 minutes at position (fig. 7) located about 

3,100 feet from R. Since the maximum air velocity measured in the mine was 

305 feet per minute, this meant that the SF6 could not possibly reach the 

sampling location until at least 10 minutes after release, as shown in table 1. 

Sampling generally began 8 minutes after the release. An air sample was taken 

in the center of the return airway every 2 minutes, and periodically a simultaneous sample was taken near the roof, walls, or floor to check uniformity of 

mixing, Generally, the measurable quantities of SF6 were exhausted past the 

sampling point within 90 minutes after SF6 release ended. 

Once an SF6 release technique was found that resulted in good mixing of 

the SF6 with the mine air, it was possible to release a known quantity of SF6 

into the intake air and, by numerically integrating the chromatograph results 

from the samples taken in the return air, account for approximately this same 

quantity of SF6 in the return air (table 1). This can be done using the equation of Drivas, Simmonds, and Shair (4): - 

Here, qF6 is the SF, volume, & ,, if the volumetric airflow rate at the 

sampling point, and c is the SF6 concentration at time t. Integration of this 

equation is simply done by multiplying C,,, the average SF6 concentration in 

those sample bottles that contain SF6, by the total time T during which measurable amounts of SF6 were found in the return air. This total time T is the 

sampling interval (2 minutes in this case) multiplied by the number of samples 

with a measurable SF6 concentration. The results of experiments in which SF6 

was released from a lecture bottle with a 0.006-inch hole in the cap are 

presented in table 1. An example of complete analysis of one of these tests 

(No. 19) is presented in the appendix. 

TABLE 1. - Preliminary tracer gas test results from 

Safetv Research Coal Mine 

These tests showed that if the SF6 is thoroughly mixed in the mine air, 

it can be quantitatively traced through the mine, thus confirming its potential value for studying underground ventilation systems. 

Test 

19 

20 

21 

SF6 TRAVEL TIMES 

A problem encountered in this work was to determine when to begin and end 

sampling. The time required for the SF6 to initially arrive at the sampling 

Time from 

release to 

arrival at 

sample point, 

minutes 

15 

14 

16 

S F~ 

released, 

liters 

10.5 

16.1 

1.9 

S FG 

monitored, 

liters 

10.7 

15.9 

1.8 

Error, 

percent 

2 

1 

5 
location and the time lapse before all the SF6 passes this location depend on 

the length of travel, the air velocity, the passage of the air through stopes 

and cooling plants, and recirculation of the air. 

Some idea of how difficult it is to estimate these travel times can be 

obtained from the results of several tracer gas experiments presented in 

table 2, which shows the expected arrival times based on velocity and distance 

calculations and the actual arrival times for various travel distances and 

conditions. It also gives the time required for SF6 to completely pass the 

sampling locations in each experiment. 

TABLE 2. - Comparison of estimated and actual SF6 travel times 

Distance 

traveled, 

feet 

The ratio of actual travel time to expected travel time (TIT') varies 

from 0.62 to 3.57; showing that the estimated travel times are not very 

accurate, especially when stopes are being ventilated. The results of table 2 

suggest that it would be advisable to begin taking samples after about onehalf of the estimated travel time has elapsed. It is even more difficult to 

decide how long to continue sampling. The only conclusion that can be drawn 

from the data is that it takes much longer for SF6 to completely pass the 

sampling location if it travels by way of stopes. 

5,000 

7,000 

8,000 

6,000 

8,000 

3,100 

USE OF SF6 IN AN OPERATING LIMESTONE MINE 

Conditions 

Several tracer gas ventilation tests using SF6 were conducted in a singlelevel underground limestone mine in Pennsylvania. The general ventilation 

system of the mine is simple. Most of the air (about 140,000 cfm) enters the 

ISF, still passing after 90 minutes. 

Straightaways and 

stopes ............. ..... do ............. ..... do ............. ..... do ............. Mostly straightaways 

All straightaways ... 

mine through an intake fan in a drift entry, The remainder enters the mine 

through a vertical natural draft hole from the surface. All the mine air is 

exhausted through a single vertical exhaust hole. Several auxiliary fans are 

employed in the mine to aid air movement. 

Actual 

travel time 

(T), minutes 

The large number of intake and exhaust airways and the large cross 

sections of these airways (600 to 700 ft2 average) made conventional anernometer methods of studying ventilation impractical in most areas of the mine. 

The air velocities were generally well below 100 ft/min; locally, air velocities were so low that even smoke clouds dissipated before traveling far enough 

to permit velocity estimates. 

36 

25 

5 

10 

15 

14 

Expected 

travel time 

(T'), minutes 

11 

7 

8 

4 

9 

12 

Time till 

last SF6 

arrived, 

TIT' 

minutes 

1 90+ 

110 

150 

115 

60 

55 

3.27 

3.57 

.62 

2.50 

1.66 

1.17 
Working face 

10 butt 

9 butt 

LEGEND 

12 Entry numbers 

Curtain stopping 

D Fan 

x Release location 

V Sampling location 

-Return air 

i and smoke tests taken 

here) 

To exhaust 

0 100 200 

Scale, feet 

Six different types of 

ventilation experiments were 

conducted in this mine using 

the SFG tracer gas. They 

demonstrate the versatility 

of the tracer gas as a tool 

for examining underground 

ventilation systems. 

Auxiliary Fan Tests 

In an attempt to speed 

up the air exchange rate at 

the working face, two auxiliary fans were placed in the 

return airway leading from 

the working face (fig. 8). 

Fan No. 1 was mounted on a 

small platform in the center 

of the airway. Fan No. 2 

was hung about a foot from 

FIGURE 8. - Map showing area of auxiliary fan tests. the and feet the 

rib. No tubing was used on 

either of the fans. Two SF6 

tracer gas tests were run to determine if the auxiliary fans were helping tc 

move the air along faster or if they were just recirculating the air. The air 

velocity in this area, at least with the fans on, was high enough that we 

could check the tracer gas results against vane anemometer and smoke cloud 

results. The volume of air exhausted from the face was also measured. 

With both fans in operation, 3.64 liters of SF6 were released just behind 

fan No. 1 over the time interval from 5:12 to 5:13 p.m. Gas samples were 

taken every minute from 5:12 to 5:50 p.m. at the sampling location shown in 

figure 8. 

The first detectable trace of SF6 appeared in the sample taken at 

5:16 p.m. Measurable quantities of SF6 were monitored for 28 minutes, from 

5:18 to 5:45 p.m. The average concentration of SFG in these 28 samples was 

97.6 ppb. This information was used in the following equation of continuity: 

QF, 

"" ) (28.2 liters/ 

ft3 air 

ft3) (28 min) (8 ,R). 

From this, the volume flow of air, QA IR , being exhausted from the working area 

past the auxiliary fans is approximat~ly 47,000 cfm when the fans are 

operating. The cross-sectional area at the sampling point was about 640 ft2, 

making the average calculated velocity at this location about 74 ft/min. 
A smoke cloud test conducted at the sampling location gave a velocity in 

the center of the airway of 88 ft/min, while a vane anemometer at the same 

location showed a reading of 94 ft/min. SF6 was detected in a sample taken 

4 minutes after release. This means that the first SF6 actually arrived 

somewhere between 3 and 4 minutes after release. Assuming that it arrived 

after 3.5 minutes and knowing that the total distance traveled was about 

340 feet, the velocity is about 97 ft/min. This compares rather well with the 

velocities determined by the two other methods. Actually, the maximum 

velocity calculated from the SF6 travel time should be slightly higher, 

because the cross-sectional area at the sampling location was, in general, a 

little larger than the cross-sectional areas along the path of SF6 travel, 

reflecting higher velocities through smaller cross sections. 

Hartman (5) shows when the Reynolds number exceeds 10,000, which is the 

case here, thewaverage velocity should be 0.8 of the maximum velocity. For a 

maximum velocity of 97 ft/min the average velocity would be about 77 ft/min, 

as compared with the average velocity of 74 ft/min measured above. A summary 

of the results is given in table 3. 

TABLE 3. - Velocity results from auxiliary fan tests 

Technique I Velocity, 

Smoke cloud (centerline velocity) .................................. 1 88 

Anemometer (centerline velocity) ................................... 

Maximum velocity from trace gas travel time.. ...................... 1 97 

f t /min 

94 

Average velocity from trace gas (continuity equation) .............. 1 74 

Average velocity (0.8 of centerline velocity) ...................... I 77 

The auxiliary fans were turned off at 5:53 p.m. After allowing the 

airflow to stabilize for 30 minutes, another 3.07 liters of SF6 were released 

behind fan No. 1. Samples were again taken at 1-minute intervals at the same 

location as before for 50 minutes. The first sample taken showed an SF6 concentration of 32 ppb and the concentrations of the succeeding samples varied 

from 20 ppb to 90 ppb for the entire 50 minutes. It was concluded that after 

the auxiliary fans were turned off the air in the exhaust airway began to back 

up slowly, so that the SFEI sampled in the second portion of the test was the 

gas that had passed the sampling point in the first part of the test. An 

anemometer held in the center of the airway at the sampling location gave no 

reading, but a smoke cloud indicated a slow air movement in the reverse direction together with a great deal of erratic transverse air movement. These 

results show that the auxiliary fans were necessary to remove air from the 

working area of the mine. 

Exhaust Hole Studv 

The entire supply of ventilation air for the mine is exhausted to the 

surface through a single vertical hole about 20 feet in diameter and 50 to 60 

feet deep. The hole surfaces on a hillside, so that the back of the hole is 

6 to 8 feet higher than the front. Often a mild wind blows across the exhaust 

hole from the low to the high side. This wind acted as a resisting force 
FIGURE 9. - Airflow pattern in exhaust hole. 
down the exhaust. To check the effect of this brattice cloth we repeated the 

tracer gas test on a day when the wind was about the same as when the first 

test was conducted. We released 16.7 liters of SF6 around the surface perimeter of the exhaust during a 2-minute interval and repeated the previous test. 

No trace of SF6 appeared in any of thbe samples taken. The brattice cloth 

around the front portion of the exhaust hole had evidently eliminated the 

downdraft . 

Natural Draft Intake Study 

About 600 feet from the exhaust hole there is a second vertical hole to 

surface from an inactive area of the mine. This hole is about 70 feet deep 

and varies in diameter from about 16 to 20 feet. This hole was known to act 

as a natural draft intake, but the volume of air entering the mine through 

this intake was not known. As the cross section of the airway at the bottom 

of the hole was too large (E 800 ft2) and the air velocities too low to use an 

anemometer traverse, the trace gas was used to measure the intake air volume. 

Around the top perimeter of the hole, 12.2 liters of SF6 were released, 

holding the lecture bottle of SF6 over the intake hole and directing the jet 

spray into the hole to insure that all the SF6 entered the mine. Starting at 

the time of release, gas samples were taken for 35 minutes at a location about 

70 feet from the bottom of the hole along the intake airway. Twenty-seven of 

the samples contained SF6. The average concentration was 470 ppb. Assuming 

that all the SF6 released (12.2 liters) entered the intake hole, the volume of 

intake air q,, can be calculated from the continuity equation: 

In this case, g ,, is the airflow rate, Qs F6 is the volume of SF6 released, 

Ca, is the average SF6 concentration in the 27 samples containing SF6, and T 

is the 27-minute time interval that these samples represent. Therefore: 

ft3 SF6 

12.2 liters = 470 x 10" (28.2 liters/ft3) (27 min) ( , ) , from which 

ft3 air 

we can calculate that &*, = 34,000 cfm and the average velocity is about 

42 ft/min. This technique is similar to the method used by Higgins and 

Shuttleworth (6). - 

Air Exchange Studv in End Room on Working Face 

At the time these ventilation tests were conducted, 16 room was the end 

room on the working face (fig. 10). There was an obvious buildup of dust and 

diesel fumes in this area. It was assumed that the intake air, which was 

intended to sweep the entire working face, did not reach 16 room in sufficient 

amounts to clear the dust and fumes. To check this assumption, about 

30 liters of sF6 were released into the intake air in 5 room at 9:45 a.m. (see 

fig. 10). At the same time, gas sampling was started simultaneously at the 

junction of 16 room with 10 butt, and in 9 butt about 50 feet downstream from 
FIGURE 10. - Diagram of air exchange study in 16 room. 

intended to exhaust the air from the working area. Samples were taken every 

minute for 30 minutes. 

After 30 minutes, the samples taken in 16 room at 10 butt showed at the 

most slight traces of SF6, the concentrations were around 1 ppb. On the other 

hand, samples taken in 9 butt, downstream from the fan, exceeded 100 ppb SFB 

after 2 minutes of sampling and continued to show these high concentrations 

throughout the 30 minutes of sampling. Assuming the concentrations in 16 room 

to be 1 ppb or less, and since the average concentration in the return air in 

9 butt was generally greater than 100 ppb, this indicates that only 1 percent 

or less of the intake air reaches 16 room. 

Airflow Check in a Nonworking Area 

Figure 11 shows a central portion of the mine with the general intake 

airflow pattern. Most of the intake air flows along 8 and 9 north entries, 

but some may circulate around the area of 4-7 north entries. This area is not 

being mined at present, but because future mining operations are planned, a 

tracer gas test was run to determine if there was any airflow in 4-7 north 

entries. 

At R1 in 7 north entry, 13.4 liters of SFG werereleased at 4:33 p.m. Gas 

samples were taken in the intake air at every 2 minutes from 4:35 until 
FIGURE 11. - Central portion of the limestone mine. 

6:30 p.m. At 6:50 p.m., a gas sample was taken in 8 north entry just below 

the SF6 release location in 7 north entry. None of the gas samples showed any 

evidence of SFG, indicating that little or no intake air circulated through 

4-7 north entries. 

Measurement of Airflow in Main Intake Airways 

As stated, most of the intake air passes through 8 and 9 north entries. 

To determine the velocities in these entries, we released 16.8 liters of SF6 

into a crosscut between 8 and 9 entries, at position R2 (fig. 11). The time 

of release was 7:04 p.m. Samples were taken every minute from 7:04 to 

8:00 p.m. at the same sampling location, S, used in the previous test. The 

distance between - R2 and - S is about 1,600 feet. 

The first sign of SF6 arrived at the sampling location at 7:17 p.m., 

13 minutes after the gas was first released. Dividing the 1,600 feet by 

13 minutes gives a maximum velocity of 123 ft/min. Anemometer readings taken 

in the airway centers in 8 north entry and 9 north entry were 115 ft/min and 

121 ft/min, respectively. 
If we assume the average velocity to be 0.8 of the maximum velocity, then 

the average velocity in these airways is 0.8 x 123 ft/min = 98 ftlmin. 

The continuity equation could not be used to calculate the airflow volume 

because all of the released SF6 did not pass the sampling location, as shown 

in the airflow pattern in figure 11. However, the total intake air volume 

through 8 and 9 north entries can be estimated. Assuming the combined average 

cross-sectional area of these two entries to be 1,200 ft and the average 

velocity to be the calculated 98 ftlmin, the intake volume in this area is 

about 118,000 cfm. Since the volume of air being exhausted from the working 

areas past the auxiliary fans was found to be only about 47,000 cfm, much of 

the intake air must be lost through leakage before it ever reaches the working 

area. 

RECIRCULATION TESTS IN A VEIN-TYPE METAL MINE 

Recirculation of air from returns into intake airways is a major hazard 

in many multilevel mines. Usually this results from leakage across old stoped 

areas. To control recirculation, its origin must be located and the amount of 

recirculated air measured. SF6 trace gas can be used for both purposes. Two 

LEGEND 

To adjoining mine 11 I CIA 

--c - Intake air 

Return olr 

---, Possible leakage . , .+. E 

tracer gas experiments were 

conducted in a western metal 

mine, where recirculation 

was known to occur from the 

return into the intake 

(fig. 12) . To measure the 

quantity of return air 

leaking through the stopes 

into the intake, a known 

amount of SFe was released 

into the return air and air 

in the intake was sampled 

for a given time. The percentage of SF6 monitored in 

the intake should be the 

same as the percentage of 

return air recirculating 

through the stoped areas 

into the intake. In the 

first test, 32.4 liters of 

SF, were released into the 

return air at Rl, from 10:45 

to 11:OO a.m. The return 

air rate at this point was 

86,000 cfm. At 10:45 a.m., 

gas sampling every 2 minutes 

was begun in the intake 

airway at S1 and continued 

until 11:4Ta.m. The rate 

GI Fan 

X Release location t A Sampllng locat~on 

FIGURE 12. - Schematic of recirculation tests in vein- here was 87,000 cfm. 

type meta 1 mine. 

V) 

0) a 

0 

t 

0 

U 

0 

0 

U) 

: 
The first SF6 was 

detected at S1 at 10:59 a.m. 

and the lastat 11:19 a.m. 

(fig. 13). A total of 

2.1 liters of SF6 was 

calculated to have passed D 

Q 40 - - S1, making the amount of a - 

recirculation in this region 

slightly over 6 percent. 

20 - In the second recirculation experiment, 8.6 1' ~~ters of SF6 were 

released in the return air5 10 15 20 2 5 30 35 shaft, at g, from 1:10 to 

TIME FROM RELEASE OF SFg, min 1:23 p.m. Sampling began at 

the bottom of the intake 

shaft, at s. Samples were FIGURE 13. - Curve of SF6 concentration in intake for taken every minutes until 

first recirculation test. 2:45 p.m. The intake rate 

was 58,000 cfm. The return 

rate was 7,000 cfm at - R2, but increased considerably further up the return 

shaft. Recirculated SF6 was detected first at 1:40 p.m. and last at 2:30 p.m. 

(fig. 14). The total quantity of SFG recirculated was 5.3 liters, which gives 

a recirculation of 61 percent. Whether this means that 61 percent of the 

7,000 cfrn at R2 was recirculated or that 61 percent of some larger volume of 

return air wasrecirculated depends upon where the leakage occurred between 

20 

0 10 20 30 40 50 60 70 80 90 100 

TI ME FROM RELEASE OF SF6, min 

FIGURE 14. - Curve of SF6 concentration in intake for second recirculation test. 
the return and intake airways. If some or all of the leakage occurred above 

the level where the 25,000 cfm enters the return shaft, then it is 61 percent 

of the larger air volume. Further tracer gas tests would be required to 

determine this. 

CONCLUSIONS 

The SF6 tracer gas has been shown to be a useful and versatile tool for 

studying mine ventilation systems, To date, it has been used successfully to 

measure low velocities (less than 100 ft/min), to determine airflow volumes in 

airways of large cross-sectional areas (up to 1,000 ft2), to measure the air 

exchange rate in areas of little or no airflow, and to measure air recirculation due to leakage across old stopes. Continuing experiments in other types 

of mines with different ventilation problems will no doubt expand its region 

of application. 
REFERENCES 

Clemens, C. A., and A. P. Altshuller. Response of Electron-Capture Detector to Halogenated Substances. Anal. Chem., v. 38, 1966, pp. 133-136. 

Clemens, C. A., A. I. Coleman, and B. E. Saltzman. Concentration and 

Ultrasensitive Chromatographic Determination of Sulfur Hexaflouride for 

Application in Meteorological Tracing. Environ. Sci. Tech., v. 2, 

No. 7, 1968, pp. 551-556. 

Collins, G. F., F. E. Rartlett, A. Turk, S. M. Edmonds, and H. L. Mark. 

A Preliminary Evaluation of Gas Air Tracers. J. Air Pollution Control 

Assoc., v. 15, 1965, pp. 109-112. 

Drivas, P. J., P. G. Simmonds, and F. H. Shair. Experimental Characterization of Ventilation Systems in Buildings. Environ. Sci. Tech., v. 6, 

No. 7, 1972, pp. 609-614. 

Hartman, H. L. Mine Ventilation and Air Conditioning. Ronald Press Co., 

New York, 1961, pp. 86-87. 

Higgins, J., and S. E. H. Shuttleworth. A Tracer Gas Technique for the 

Measurement of Airflow in Headings. Colliery Eng., v. 35, No. 417, 

1958, pp. 483-487. 

Hunt, A. E., and H. A. W. Moore. Private communication, 1971. Available 

upon request from A. E. Hunt, Bureau of Mines, Morgantown, W. Va. 

Lester, D., and L. A. Greenberg. The Toxicity of Sulfur Hexafluoride. 

Arch. Ind. Hyg. and Occupational Med., v. 2, 1950, pp. 348-349. 

Niemeyer, L. E., and R. A. McCormick. Some Results of Multiple-Tracer 

Diffusion Experiments at Cincinnati. J. Air Pollution Control Assoc., 

v. 10, 1968, pp. 403-405. 

Rennick, G. E., J. Pasini 111, F. E. Armstrong, and J. R. Abrams. Demonstration of Safety Plugging of Oil Wells Penetrating Appalachian Coal 

Mines. BuMines TPR 56, 1972, p. 6. 

Saltzman, B. E., A. E. Coleman, and C. A. Clemens. Halogenated Compounds 

as Gaseous Meteorological Tracers. Anal. Chem., v. 38, 1966, 

pp. 753-758. 

Shair, F. Private communication, 1973. Available upon request from 

F. H. Shair, California Institute of Technology, Pasadena, Calif. 

Turk, A., S. M. Edmonds, H. L. Mark, and G. F. Collins. Sulfur Hexafluoride as a Gas-Air Tracer. Environ. Sci. Tech., v. 2, No. 1, 1968, 

pp. 44-48. 

Whisman, M. L. Private communication, 1972. Available upon request from 

M. L. Whisman, Bureau of Mines, Bartlesville, Okla. 
APPENDIX.--PRELIMINARY TRACER GAS TEST 19 

SF6 was released into the intake air of the mine from 9:01 to 9:06 a.m. 

The volume of SF6 released was determined as follows, in grams: 

Initial lecture bottle weight = 1328.2, 

weight after release = 1266.6; 

thus weight of SF6 released = 61.6 

TT uSLng - 2 PT,7 = "1"" --L nnl w~~ere, P = 0.98 atm, T = 300" K, ?. = 0.082 liter-atrn/mole - " K; 

61.6 grams - 

- 61'6grams weobtain 

146.07 grams/moleY 

61.6 grams 0.082 liter-atm (0.98 atm) V = 146.07 grams/mole mole - " K x (300" K). 

Therefore, V = 10.6 liters of SFG released. 

Sampling began 3,100 feet downstream from the release point at 9:08 a.m., 

and continued every 2 minutes until 10:30 a.m. The results from the analyzed 

air samples are given in table A-1. 

TABLE A-1. - Calculation table for test 19 

Concentration 

6.6 x lo-' , 5.2 x lo-' 

4.3 x lo-' 

3.2 x lo-' 

2.5 x 10" 

2.3 x lo-' 

2.0 x lo-' 

1.6 x lo-' 

1.4 x lo-' 

1.3 x 10" 

1.2 x lo-' 

9.5 x low8 

8.2 x lo-' 

7.1 x 10'~ 

6.2 x loe8 

3.6 x 

1.5 x 10'' 

Near zero. 

Sample 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

We obtained the average concentration by dividing the sum of column 3 by 

the number of samples in which a measurable quantity of SF6 was present. In 

this case SF6 was measurable in 31 samples. Therefore, the average concentration was 11,692 x 10-9/31 = 377 x or 377 ppb. 

Time, a.m. 

9:08 

9: 10 

9: 12 

9: 14 

9: 16 

9: 18 

9: 20 

9: 22 

9: 24 

9: 26 

9: 28 

9: 30 

9: 32 

9: 34 

9: 36 

9: 38 /; 1 9:40 

18 9:42 

Concentration 

0 

0 

0 

0 

6.1 x lo-' 

1.0 x lo-' 

1.0 x lo-' 

1.2 x lo-' 

1.8 x lo-' 

3.2 x lo-' 

7.8 x lo-' 

9.5 x lo-' 

9.7 x lo-' 

1.0 x 10'~ 

Sum of concentrations = 11,692 x 10-9. 

9.6 x 10" 

9.3 x lo-' 

8.5~10-' 

8.5 x lo-' 

Sample 

19 

20 

21 

22 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

Time, a.m. 

9:44 

9:46 

9:48 

9:50 

9: 52 

9:54 

9:56 

9:58 

1O:OO 

10: 02 

10: 04 

10: 06 

10:08 

10: 10 

33 

34 

35 

36 -42 

10: 12 

10: 14 

10: 16 

10: 18-10: 30 
The anemometer traverse in the return showed an average air velocity of 

220 ft/min in a cross-sectional area of 74 ft2. This results in a volume 

ft3 SFs 

airflow of 16,280 cfm. Thus, SF6 = ) (16,280 

ft3 air ft3 min air i 

(62 min) (28.2 liters ) = 10.7 liters 

f t3 

We also obtain 10.7 liters if we use Simpson's rule. Recall that the amount 

released was 10.6 liters. 

A graph of the SFc: concentration in the return air as a function of time 

is given in figure 2. The SFG concentration in the return built up and slowly 

dropped off in a smooth manner.