1 

METHODS TO DETERMINE THE STATUS OF MINE ATMOSPHERES – AN OVERVIEW 

R. J. Timko, National Institute for Occupational Safety and Health, Pittsburgh, PA 

R. L. Derick, Twentymile Coal Co., Oak Creek, CO 

Abstract 

 On a regular basis, mine personnel should obtain air samples 

directly from active underground areas as well as remotely from sealed 

locations. These samples provide mine officials with data relative to the 

makeup of the various atmospheres. Should conditions underground 

change, these results can provide a baseline that can assist officials in 

determining the extent of change. Relating the concentrations of 

individual gases to time, applying gases to equations, and examining 

their change over time are tools that have been successfully used to 

determine if a heating event exists and, if so, the extent of the 

emergency. These atmospheric status equations have either been 

developed for the mining industry or adapted from other industries. 

They serve as tools to assist personnel in determining the condition of 

underground mine atmospheres. As long as the problem area 

continues to be ventilated, certain gases, gas ratios, and equations can 

be used to determine if a fire exists, what type of material could be 

burning, and what quantity of coal is burning. Once the mine or the 

involved section has been temporarily sealed, gases, ratios, and gasderived equations can assist personnel in determining how conditions 

within the sealed volume are changing over time. To alleviate potential 

explosions when fresh air is once again introduced into the sealed 

volume, equations and a diagram have been developed to predict the 

likelihood of an explosive atmosphere. Similarly, prior to commencing 

recovery of a mine or section, equations exist that can assist in 

determining if the fire will rekindle when air is reintroduced. This paper 

serves as an overview to remind and/or instruct readers about gassampling methodologies and gas analyses to assist in determining the 

status of underground atmospheres. 

Introduction 

 Obtaining atmospheric samples on a regular basis during normal 

operations establishes a reliable baseline. Based on these numbers, a 

mine operator can effectively determine if the atmosphere has changed. 

In the event of a fire-related emergency, gas samples become critical to 

those persons delegated to fighting the fire. These samples can provide 

rescue persons with vital information on the status of the atmosphere 

(Koenning, 1987). 

 It is important to understand that individual sample results often 

mean little. It is typically more important to follow the trends that 

multiple samples produce. This does not mean that individual samples 

should be ignored. At times, especially when the atmosphere is 

relatively static, an individual sample can indicate a change in activity. 

Again, however, when a change is detected, the sampling frequency 

should be increased so that adequate backup data are obtained. 

 Mine personnel should understand what type of fire is likely and 

how this potential source could influence the spread of fire. For 

example, if the mine is prone to spontaneous combustion or rapid 

oxidation, tests for coal gases such as carbon monoxide (CO), carbon 

dioxide (CO2), and hydrogen (H2) should be performed. Coal samples 

can be extracted and examined in a laboratory capable of simulating 

different stages of heating. From these tests, gas samples can be 

obtained for the different heating phases. 

 Faced with a fire, mine officials, responsible for the safety of 

miners and mine rescue personnel, must make difficult and potentially 

life-threatening decisions. Almost immediately, one must decide if 

conditions are suitable for fighting the fire directly, or if the affected area 

should be sealed and the fire fought indirectly. The difficulty increases if 

miners are trapped or missing. 

 Preparation should be made for just such an occurrence. This 

article has been written to instruct readers about methods available to 

assist in determining the status of the mine atmosphere. Specific pieces 

of hardware, such as gas sample containers, manometers, sample 

pumps, flexible tubing, handheld gas detectors, and a recording 

barometer should be on hand. 

 

Sampling Techniques 

 There are two methods for obtaining gas samples. Real-time 

instruments are designed to provide nearly instantaneous results. They 

range from hand-held sample tubes to more complex, larger, and less 

portable instruments. A number of different technologies perform realtime gas analyses. These include stain-tube chemical sensors, electrochemical reactions, and infrared sensors. 

 The second technique for analyzing sampled gases is the gas 

chromatograph (GC). These instruments are typically housed in a 

transportable laboratory or a building. Unlike real-time instruments, the 

period required for analysis results in a time interval between sample 

acquisition and analysis results. The GC is often used to compare 

results obtained with real-time samplers. This is especially true when 

interfering gases and high background gas levels (such as carbon 

dioxide background when using infrared instruments) are present. 

Instantaneous and GC results should be compared on a regular basis. 

McNider (1985) provides a description of the importance of having a gas 

chromatograph available as well as obtaining gas samples on a regular 

basis. 

 There are two GC types, each based on gas concentration. The 

flame ionization detector-type gas chromatograph is used for low gas 

concentrations. It is very sensitive and can detect a large range of 

concentrations; however, this analytical method destroys the gas 

sample. The thermal conductivity detector-type gas chromatograph is 

used when gas concentrations are high. It is less sensitive, but it does 

not destroy the sample. Gas chromatography samples are normally 

obtained either by pulling a sample into a container that has been airevacuated or by forcing the sample into a container through an air 

pump. 

 A container often used by NIOSH personnel is the 10-to-20 ml airevacuated glass sampling tube. These are blood collection tubes that 

have been at least 95% air evacuated (Timko, 1991). Unfortunately, as 

they age, small quantities of CO have been detected in these tubes. 

Freedman et al. (1975) have reported that tube storage should not 

exceed one-to-two months when analyzing low levels of CO. 

 The Mine Safety and Health Administration (MSHA) prefers 

syringes that can be cleaned and reused. Many operators use Mylar®1

bags of varying volumes to sample gases. 

The easiest way to visualize the status of the atmosphere is to 

graph the collected data. Most gas concentrations and formulae 

presented in this work are commonly graphed in a semi-logarithmic 

format. This format is used when the data cover a very wide range or 

when one desires to transform a complex function into a linear function. 

An exception to the semi-log format is the Explosibility diagram (Figure 

3). In addition, Francart (2005) has reported that, in many instances, 

 

1

 Reference to specific manufacturers does not imply NIOSH 

endorsement. 
2 

linear plots are just as effective in determining the status of the 

atmosphere. 

 When analyzing for fire, one must obtain gas samples at the same 

locations at specific and frequent time intervals. Handheld and gas 

chromatography samples should be taken at the main fans and key air 

splits. If possible, sampling lines should be installed into return airways 

or areas that may become inaccessible. A simple, clear, color-coded 

mine map containing key air split readings should be generated. 

Estimate where the fire is located. 

 Track gas liberation rates using concentrations and ventilation 

readings. Many times only concentrations are examined. Since 

conditions often change relative to varying air quantities, looking at both 

gas concentration and ventilation will generate a more accurate 

determination of actual gas levels. 

 Once the decision is made to seal the area or the entire mine, 

additional rules of thumb apply. Adequate remote gas sampling 

locations must be established. Sample lines should be extended into the 

entry inby the proposed seal location. In addition, remember to install 

larger pipes in the seal for possible water or inert gas injection. 

 When obtaining a gas chromatography sample, it is important to 

determine that the sample line is out-gassing before taking a sample. 

This reduces the likelihood that atmospheric gases will enter the sample 

and dilute the result. Ensure sample accuracy by thoroughly purging 

the tubing before obtaining a sample. If a significant quantity of methane 

is being liberated, have 0-to-100% methane detection instruments 

available. 

 Sampling should be performed at regular around-the-clock 

intervals to ensure that diurnal pressure changes are not affecting the 

results. One must be especially careful in analyzing sample results 

whenever a steep atmospheric pressure gradient, caused by a strong 

cold front or deep low-pressure system, is affecting the area. A sealed 

mine typically has a regular diurnal pattern, based on daily temperature 

fluctuations (Francart, 1997). Government and private weather 

forecasting sources are available to predict barometric pressures. One 

should never take gas samples when the ambient or outside barometric 

pressure is higher than the pressure within a sealed area. This 

imbalance can cause air to in-gas or flow into the sealed volume which 

raises problems because the potential for sample dilution is high. Table 

1 shows gas samples taken from behind a seal in a main return. This 

purpose of this table is to show that, even at a location where outgassing is most common, dilution can still occur. 

Table 1. Out-gas vs. In-gas Sampling 

Sample Oxygen % Nitrogen % Carbon 

Dioxide % Methane % 

Out-gas 0.5 38.2 14.0 45.0 

In-gas 18.5 74.0 2.1 4.4 

 

 Be wary of large changes in individual gas samples. Although 

pressure changes across seals can be instantaneous, equalizing gas 

exchange takes time. The pressure within the entire sealed volume 

must equalize with the outby atmosphere before the potential for gas 

exchange across the seals stops. 

Gases 

For this work the gases are divided into two general categories: 

atmospheric gases and byproducts of oxidation or combustion. 

Atmospheric gases are those typically found in an ambient gas sample 

and include nitrogen, argon, oxygen, and carbon dioxide. Oxidation, or 

combustion byproducts, consist of carbon monoxide, hydrogen, and 

the alkane (CNH2N+2), alkene (CNH2N), and alkyne (CNH2N-2) series of 

hydrocarbon gases. 

 The progression of evolving gases is dependent on the 

temperature of oxidation. Typically, carbon dioxide is emitted first, 

followed by carbon monoxide and hydrogen. As temperatures 

increase, flaming combustion often occurs just after hydrogen is 

detected. Finally, hydrocarbons such as ethylene and propylene are 

routinely detected following the initiation of fire. 

 The configuration of the GC determines how many different gases 

are able to be analyzed. Some GCs are capable of analyzing a large 

number of gases. Because of number rounding and limited resolution 

of very low gas concentrations, the sums of these analyzed gases may 

not equal exactly 100%. When using less capable GCs, certain gases 

are not analyzed. In this instance, all analyzed gases are summed and 

this value is subtracted from 100%. The remainder is the 

concentration of all gases not analyzed. This mathematical process is 

called gas normalizing. 

Nitrogen 

Nitrogen gas (N2) is the most abundant gas, comprising 79% of 

the ambient atmosphere. N2 is inert; the number of moles of gas will 

remain the same even when the gas passes through an area 

undergoing combustion. Nitrogen gas density is 0.9671, compared to 

dry air. Analyses normally involve adding argon gas (Ar) to N2 values.2

 

Several equations dealing with fire existence, fuel, status, and potential 

explosibility require that N2 values be measured, rather than 

normalized. 

Blackdamp is a hazardous, low oxygen phenomenon that occurs 

in poorly ventilated areas. Since nitrogen makes up the greatest 

proportion of atmospheric gases, an atmosphere in which oxygen has 

been consumed will be composed of mainly nitrogen. In sealed 

underground oxygen-depleted atmospheres, the production of carbon 

dioxide will also result in blackdamp. 

 

Oxygen 

In a normal atmosphere, oxygen gas (O2) comprises slightly less 

than 21% by volume. Its density is 1.105 relative to dry air. The 

minimum O2 concentration allowed where miners normally work or 

travel in a coal mine is 19.5% (30 CFR, §75.321 (a)(1)). Oxygen is 

typically not considered a contaminant. As it passes through a mine, 

its concentration becomes less because of oxidation, including fire, 

with underground components. If a sufficient quantity of O2 is 

consumed, the atmosphere becomes oxygen deficient, represented by 

)O2. This deficiency can be calculated by: 

)O2 = 0.265N2 - O2

where: 

)O2 = oxygen deficiency, % 

N2 = measured nitrogen, % 

O2 = measured oxygen, % 

0.265 = ratio of ambient oxygen to nitrogen 

Mitchell (1990) stated that this calculation is valid only when air is 

flowing into the fire area and the O2-to-N2 ratio of this air is 0.265. If 

)O2 is increasing, the fire is consuming more fuel. Conversely, low 

)O2 typically signifies ambient conditions or low-level reactions taking 

place. 

Table 2. Examples of Oxygen Deficiency 

Sample O2% N2% )O2% 

1 20.94 79.02 0.00 

2 10.67 80.16 10.57 

3 20.62 79.06 0.33 

Sample 1 in Table 2 shows a ventilated entry under ambient 

conditions. In Sample 2, an active airway fire existed. Finally, Sample 

3 was obtained under the same conditions as Sample 2; however, the 

results were diluted by other air splits. A thorough understanding of 

variables associated with specific sampling locations is required. 

 

Carbon Dioxide 

Carbon dioxide (CO2) is considered a primary fire detection gas. 

Because its density (1.519) exceeds that of air by more than 50%, in 

low ventilation velocities it will most likely be found near the floor. The 

maximum time-weighted average permitted in bleeder entries traveled 

by miners is 0.5% and the short-term exposure limit is 3% (30 CFR, 

§75.321 (a)(1)). 

Carbon dioxide is a gas formed in a variety of ways. It is a major 

product of combustion. Normally, it is liberated by microorganisms 

oxidizing coal or by mixing acidic mine water with calcium carbonate, 

 

2

 In this paper, all N2 values imply N2 + Ar. 
3 

found either in surrounding strata or in rock dust. Gas samples can 

contain more CO2 than is actually being generated by combustion. On 

the other hand, CO2 is soluble in water; therefore, samples may 

contain less CO2 than is actually being evolved. Some mines liberate 

CO2 similarly to methane. It is important to determine, in advance, if 

this is occurring so that its influence on the fire equations is known. 

Carbon dioxide is produced abundantly in fires. In these 

situations, its concentration will most likely be greater than other 

gases. Mitchell (1990) states that there are three methods to assist in 

determining that increasing CO2 levels are fire-related: 

1. The trend of %N2 is relatively flat, 

2. Where the %CO2 trend rises, the %O2 trend should decrease 

at the same or a greater rate, and 

3. The sum of CO2 + CO + H2 (in percent) is less than the )O2, 

again in percent. 

 

While CO2 is an important indicator, other atmospheric and firerelated gases should simultaneously be examined. If an inert gas, such 

as CO2 or N2, is injected into a sealed area, the effects of this effort on 

the entire sealed atmosphere must be considered. Table 3 provides 

results of inerting a longwall gob atmosphere following an active fire 

within the gob. If 37% CO2 by volume was injected into the fire, the 

resulting N2 and O2 concentrations should have been approximately 

50% and 13%, respectively. Obviously the fire had some effect on the 

outcome. 

 

Table 3. Sealed Longwall panel before and after CO2 injection 

 O2% N2% CO2% 

Before 19.80 79.73 0.87 

After 1.27 57.46 38.09 

Carbon Monoxide 

Carbon monoxide (CO) is used as a fire detection gas more 

frequently than any other gas. Its density is only slightly less than dry 

air (0.97 versus 1.00), so it will readily diffuse with surrounding gases. 

McPherson (1993) explains that, when heated, CO can be found with 

smoke in layers along the roof. 

The formation of CO is evidence of incomplete combustion of 

carbon-based material. CO readily burns, having a flammability range 

of from 12.5% to 74.2% in air. Its most explosive concentration occurs 

at 29%. Concentrations in excess of 15% (150,000 ppm) are possible 

during a mine fire. 

Mitchell (1990) provides a rule of thumb that, for every pound of 

coal burned, 0.5 cubic foot of CO is created. Knowing the CO 

concentration and the air quantity flowing through the fire area, the rate 

of burning coal can be determined: 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

= 6 101

S CO

air 2Q coal R

where: 

Rcoal = rate of coal burning, lb/min 

Qair = Entry air quantity, ft3

/min 

COS = CO concentration, ppm 

Table 4. Approximate coal burning rate 

Sample Qair 

ft3/min 

COS 

ppm 

Rcoal 

lb/min 

1 6,700 7,490 100 

2 7,000 17,920 250 

In Table 4 the sampled CO gas levels changed significantly while the 

ventilation rate remained essentially the same. The resulting rate of 

coal burning more than doubled. This could indicate the change from 

an oxygen-rich fire to a fuel-rich fire. In any event, these results show 

that conditions are becoming more serious. 

 In other CO-related research, Timko and Derick (1995) have 

shown that changes in CO concentrations are directly dependent on 

ventilation air quantities and corresponding ventilation pressures at a 

particular location. When the CO level is decreasing, this could be due 

either to additional ventilation flowing over the affected area, or the 

heating is actually slowing. In spontaneous combustion-prone mines, 

once a heating begins, it rarely stops without ventilation changes or 

other controls being implemented. 

Hydrogen 

Hydrogen gas (H2) is another key component in determining if fire 

exists. Hydrogen gas is most often generated as a byproduct of 

battery charging. It has been found in limited instances as strata gas. 

Hydrogen can also be liberated in elevated coal oxidation situations. It 

is detected at temperatures just below, and in high levels following, 

combustion. Hydrogen can also be created by an interaction between 

burning coal and water (so-called water gas reaction), at times in very 

high concentrations (approaching 15%), and can actually exceed 

carbon monoxide levels. Because of its low (0.070) density relative to 

air, if adequate mixing is not taking place, H2 will most likely be found 

at the roof. 

Hydrogen is highly explosive. It is flammable in concentrations 

from 4% to 74% in air. Low H2 levels begin to be detected at 

temperatures just below those required for active coal flaming. 

Alkanes 

Alkanes (Table 5) are commonly known as the methane series of 

gases. These are the products of the anaerobic decay of plants and 

are sometimes referred to as paraffins. Alkanes are saturated aliphatic 

hydrocarbons. A hydrogen atom unites with each available carbon 

atomic bond, and thus all carbon bonds are single bonds. In aliphatic 

compounds, the carbon atoms form open-chain molecules, rather than 

rings. Alkanes that are typically analyzed through gas chromatography 

include methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), 

and pentane (C5H12). 

As methane concentrations increase, so do other alkanes. These 

are usually in descending concentrations relative to increasing density. 

When methane concentrations are very low, ethane may not be 

detected. When it is noted in possible fire samples, many confuse it as 

the fire gas ethylene. Alkane quantities tend to increase as 

temperatures rise. 

Table 5. Density and flammability of alkanes 

Alkane 

Density 

relative to 

dry air 

Flammability 

in air (%) 

CH4 Methane 0.55 5.0 to 15.0 

C2H6 Ethane 1.05 3.0 to 12.4 

C3H8 Propane 1.56 2.1 to 10.1 

C4H10 Butane 2.07 1.8 to 8.4 

C5H12 Pentane 2.49 1.5 to 7.8 

Alkenes 

Alkenes (Table 6) are unsaturated aliphatic hydrocarbons. These 

gases contain less than the maximum quantity of hydrogen atoms. 

Here carbon atoms are double-bonded, while all hydrogen atoms 

surrounding the carbon atoms are single bonded. The alkenes 

measured were ethylene or ethene (C2H4) and propylene or propene 

(C3H6). 

Table 6. Density and flammability of alkenes 

Alkenes 

Density 

relative to 

dry air 

Flammability 

in air (%) 

C2H4 Ethylene 0.97 2.7 to 36.0 

C3H6 Propylene 1.45 2.0 to 11.1 

C4H8 Butylene 2.00 1.6 to 10.0 

C5H10 Pentene3 2.40 1.4 to 8.7 

Ethylene and propylene are byproducts of fire that are detected at 

approximately the same temperature. Normally, flaming combustion 

will take place prior to the detection of either of these gases. In fires, 

 

3 Pentene is a liquid below 30EC. 
4 

alkenes can form from alkanes through the loss of hydrogen atoms by 

pyrolysis: 

Large alkanes + HEAT ÷ Smaller alkanes + alkenes + H2 

Alkynes 

Alkynes are highly unsaturated aliphatic hydrocarbons. In these 

molecules, the carbon atoms are triple bonded, while the hydrogen 

atoms surrounding the carbon atoms are again single bonded. The 

only alkyne that normally is analyzed is acetylene or ethyne (C2H2). 

The density of acetylene is 0.906, making it slightly less dense than air. 

The flammability range of acetylene in air is from 2.2% to 100%. 

Other than using this gas for oxygen/acetylene cutting and 

welding underground, the only way this gas should be found 

underground is as a byproduct of combustion. Two more common 

ways to produce acetylene in coal mine fires are as follows: 

Coal + HEAT ÷coke 

and 

Limestone + HEAT ÷CaO 

coke + CaO +1100EC÷CaC2 + H2O + C2H2

4CH4 + O2 . 820EC ÷ C2H2 + 2CO + H2 

In the first reaction, coal and rock dust are heated to initially produce 

coke and calcium oxide (CaO). A higher temperature then causes 

these to react and form calcium carbide (CaC2) and water, which finally 

combine to form acetylene. In the second reaction, methane and 

oxygen are heated to form acetylene, carbon monoxide, and hydrogen 

gas. Both reactions involve high temperatures. These will most likely 

occur in areas that are undergoing combustion and continuing to be 

ventilated because sufficient oxygen is necessary to permit 

temperatures to reach the required values. In mine fire situations, the 

acetylene concentration will usually be lower than alkene series 

concentrations. 

Air-free Gases 

 To eliminate the effects of sample dilution, one can determine 

changes in gas concentrations without the influence of oxygen. This 

can be accomplished with the following equation: 

( ) 20.95O1

C C

2

S

AF − =

where 

CAF = Air-free gas concentration, ppm or % 

CS = Sampled gas concentration, ppm or % 

O2 = Oxygen concentration, % 

 Air-free carbon monoxide levels found in Table 7 were obtained at 

a sealed mine immediately following a fire. These samples were taken 

at 24-hr intervals over six days. Air-free and sampled results closely 

parallel each other. Only when oxygen values approach ambient will 

air-free gas concentrations greatly differ from the actual sampled gas 

value because the denominator in the equation approaches zero when 

oxygen values move toward ambient. 

Table 7. Air-free Carbon Monoxide, COAF

Sample Day O2% COS ppm COAF ppm 

1 1.84 19,971 21,894 

2 1.15 23,267 24,618 

3 0.42 11,575 11,812 

4 0.57 8,816 9,063 

5 1.08 8,857 9,338 

6 1.88 4,863 5,342 

Fire-related Equations 

Several equations have been developed to aid in establishing the 

status of a mine atmosphere. These predictive models can be used to 

assist in determining: 

1. If a fire exists, 

2. What is burning, 

3. If the fire is continuing to burn following sealing the 

affected area or mine, and 

4. If the sealed atmosphere is presently explosive or will 

become explosive when ventilation commences. 

 In this section, a brief description of each equation is provided. 

Remember that these models are designed primarily to analyze trends 

and not to reach conclusions regarding the results of individual values. 

Only through long-term analysis can a realistic picture of the mine 

atmosphere be created. 

These equations are most useful when specific parameters 

regarding sampling location and ventilation adequacy are known. 

Is there a fire? 

Index of Carbon Monoxide 

The index of carbon monoxide (ICO), or Graham’s index (1914), 

is a dependable indicator of conditions underground because it 

eliminates the effects of sample dilution. The ICO is temperature 

dependent; i.e., its concentration rises with increasing temperature. 

2 O) 2 0.265(N

CO

ICO

−

=

where 

CO = carbon monoxide, ppm 

0.265(N2) – O2 = Oxygen deficiency ()O2), % 

 

 The ICO can signify increasing combustion if, after stabilizing for 

a time, the values begin to rise. ICO levels will increase with 

increasing temperatures until an ignition occurs. Table 8 data were 

obtained at a bleeder evaluation point over several days. ICO values 

are increasing; gas sampling frequency should be increased and 

additional caution must be exercised. Once active combustion occurs, 

the rate of increase begins to slow and, in many cases, will stabilize. 

 

Table 8. Index of Carbon Monoxide, ICO 

Sample O2% N2% CO ppm ICO 

1 17.16 78.99 49 12.99 

2 17.54 79.35 45 12.90 

3 17.71 79.32 44 13.29 

4 18.51 79.01 51 21.01 

5 18.56 79.07 55 22.98 

In a like manner, once a mine area is sealed, ICO values will 

increase and will remain elevated until temperatures fall to near 

ambient levels. When the mine atmosphere approaches ambient 

levels, other indicators such as RATIO or the Hydrocarbon Ratio 

should be monitored along with the ICO because very small quantities 

of CO can be produced in normal oxidation and mineral desorption. 

This is common as the sealed atmosphere goes from smoldering 

combustion at slightly elevated temperature to ambient temperature 

oxidation. No minimum ICO value has been established to assist in 

determining when the atmosphere is safe. 

Other problems exist for the ICO. When CO levels decrease and 

O2 values remain the same or increase, values in the denominator are 

forced to zero because O2 is no longer being consumed. This usually 

occurs when combustion has slowed. If corresponding CO levels are 

decreasing at a slower rate, the ICO may actually increase, indicating 

continued combustion. Fortunately, the ICO errs on the conservative 

side, suggesting that conditions are worse than actual. Another 

problem is that there is no single CO value, ratio, or index that can be 

applied uniformly to all mines. In some, small levels of CO will 
5 

continue for extended periods. Conversely, Dougherty (1969) states 

that, once the area is again ventilated, the possibility of re-ignition does 

exist even though CO levels are low. 

Index of Carbon Dioxide 

Graham (1917) also developed a carbon dioxide index (ICO2) to 

monitor elevated CO2 levels: 

( )

( ) 2 O100

0.03 2 CO

2 ICO −

− =

This equation, based on gas percentages, indicates elevated CO2

values in an oxygen-free atmosphere. In a normal atmosphere, where 

only ambient CO2 is generated, the ICO2 provides the proportion of CO2

to total inert gas and should approach zero. In a mine fire atmosphere, 

elevated CO2 would be compared to total inert and gaseous 

byproducts of combustion. 

 If CO2 is a product of combustion, an ICO2 versus temperature 

plot will closely parallel an ICO versus temperature plot. Where no 

temperature information exists, an ICO/ICO2 ratio can be developed. 

Once ignition occurs, ICO2 values increase and continue to 

increase as long as the temperature or fuel quantity increases. In 

sealed areas having no combustion, the ICO2 value will also rise, but 

then stabilize with time. Table 9, using the same samples as shown at 

Table 8 data, shows increasing ICO2 values, reinforcing the concern 

raised by the ICO results. 

Table 9. Index of Carbon Dioxide, ICO2

Sample O2% CO2% ICO2x10-2

1 17.16 1.41 1.70 

2 17.54 1.35 1.60 

3 17.71 1.40 1.66 

4 18.51 1.43 1.72 

5 18.56 1.47 1.77 

What is burning? 

Jones-Trickett Ratio 

The Jones-Trickett Ratio (JTR), as described in Jones, Trickett 

(1954) determines sample reliability. It is based on the principle that 

the number of molecules of matter consumed in a fire is proportional to 

the number of molecules of gas produced, which is proportional to the 

volume of gas, assuming constant temperatures. It was originally 

developed to help researchers differentiate between gas or coal dust 

explosions. It has been adopted to mine fire situations. 

The JTR equation, based on percent gas concentrations, is: 

2 O) 2 0.265(N

) 2 (0.75CO) (0.25H 2 CO

JTR −

+ −

=

 

It varies with the type of fire, depending on the fuel. If the JTR is less 

than 0.4, no combustion exists. When the JTR is between 0.4 and 0.5, 

methane is the fuel. JTR values between 0.5 and 0.9 indicate that the 

fuel is coal, oil, conveyor belt, insulation, or urethane foam. If the JTR 

is between 0.9 and 1.6, wood is burning. Values of more than 1.6 

generally occur only under controlled laboratory conditions. When JTR 

results above 1.6 are encountered during a possible mine fire, the gas 

chromatograph or sampling device should be examined for calibration 

or mathematical errors. 

 Table 10 continues the evaluation of the ICO and ICO2 results 

previously shown. These five analyses should stimulate the operator 

to begin a more aggressive evaluation of possible heating sources. 

Table 10. The Jones-Trickett Ratio, JTR 

Sample O2% N2% CO ppm CO2% JTR 

1 17.16 78.99 49 1.41 0.37 

2 17.54 79.35 45 1.35 0.39 

3 17.71 79.32 44 1.40 0.42 

4 18.51 79.01 51 1.43 0.59 

5 18.56 79.07 55 1.47 0.62 

 

Following sealing, what is the fire status? 

Relative Intensity 

Power industry engineers use the relative intensity (RI) equation 

to determine boiler and fuel efficiency, oxygen excess or deficiency, 

water vapor production, and heat losses (Mitchell, 1984). RI measures 

the quantity of air available to burn a unit mass of fuel (11.6 lb of air 

are required to completely consume 1 lb of coal), the percentage of 

oxygen consumed, and the effect of combustion on temperature. 

 

( ) ICO

2 N

2 O0.03831

RI ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − =

Adapting the RI to mine fire analysis, it is used to determine the 

proximity of the sampling location to the fire. It performs best when 

many sampling locations, such a boreholes, are spread throughout the 

mine. 

All gas concentrations used in the equation are in percent. The 

RI equation is designed to show the location of the sampling point 

relative to the fire. If the RI increases, the fire is either growing or 

moving closer to the sampling location. Conversely, if the RI is 

decreasing, the fire is being throttled or moving away from the 

sampling location. 

Table 11 data were remotely obtained over a six-hour period from 

a return entry. The mine was undergoing an uncontrolled spontaneous 

combustion event that had to be fought from the surface with high 

expansion foam. Relative to the fire itself, these results are 

inconclusive. 

Table 11. Relative Intensity, RI 

Sample O2% N2% CO 

ppm 

ICO RI 

1 13.91 79.72 9540 1322 7.75 

2 14.02 79.49 9460 1343 7.82 

3 13.83 79.69 9550 1310 8.71 

4 13.95 79.71 9610 1340 7.83 

5 14.10 79.54 9540 1367 7.91 

6 14.00 79.59 9690 1366 8.03 

RATIO 

Litton (1986) developed RATIO to monitor a sealed mine 

atmosphere. It was designed to reduce the potential for re-ignition 

when a mine or section is reopened and oxygen is reintroduced. Only 

four gases are monitored: oxygen, carbon monoxide, methane, and 

ethane. The atmosphere is divided into three parts: air, methane and 

ethane, and residual gas. The only caveat is that this initial derivation 

of RATIO requires the oxygen concentration to be greater than 1%. 

Air = a(O2) 

where 

a = (100.000/20.946) = 4.774 

Residual gas (Rg) is the volume percent of gas within a sealed 

coal mine after air and methane and ethane have been subtracted 

from the sample. Rg contains excess nitrogen (not part of air), 

products of combustion (CO, CO2, H2, and hydrocarbons), along with 

products formed by chemical processes not related to combustion. 

Rg = 100.0 – Air – CH4 – C2H6
6 

The concentration of CO in parts per million of air within the residual 

gas is: 

( ) 100

Rg

S CO

Rg

CO = ×

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

where 

COS = sampled CO, ppm 

From these values, the R-index (RI

) is created: 

( )

( )

100 2 R g

S CO

Rg

Rg

CO

I R = = ×

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎝

⎛

If RI

 remains constant, one of two things is occurring: sustained lowtemperature combustion or a sustained low rate of ambient CO 

production. If there is combustion, the average RI value will be higher 

than if ambient CO production is occurring because CO production 

increases with temperature. 

 Actual measured RI

 is called (RI)act. A second RI, the maximum 

equilibrium value, (RI

)eq, is obtained by: 

( ) 2

1

2 O

Rg 300 I eq

R

−

= ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

(RI

)eq represents the maximum equilibrium value of the R-index for a 

gas sample, if only ambient temperature processes are responsible for 

CO production. 

 A RATIO equation has been created to determine the state of a 

sealed area: 

( )

( ) ( ) ( ) 2

1

2 2 O

3

CO31 S Rg

I eq

R

I act R

RATIO −− = =

If the oxygen content is less than 1% the RATIO equation becomes: 

RATIO = 1/3 COS(Rg)

-3/2(O2) 

 

If RATIO is greater than one, ambient temperatures have not been 

reached; smoldering combustion or above-ambient temperature 

oxidation is occurring. This condition is considered unsafe for reentry. 

Table 12 data was obtained remotely from a panel that was sealed 

because of an uncontrolled fire. Samples were obtained every ten 

days to develop a long-term trend. 

Table 12. Determining RATIO 

Days O2% CH4% CO ppm C2H6 

ppm 

RATIO 

0 13.89 0.38 9,540 740 4.45 

10 12.69 2.61 8,750 700 3.68 

20 10.80 3.41 5,440 640 1.83 

30 9.45 3.93 3,970 660 1.19 

40 8.83 4.20 1,950 580 0.56 

50 6.69 5.10 820 490 0.21 

60 5.81 5.23 635 440 0.16 

70 5.34 5.49 352 410 0.09 

80 2.25 5.42 140 410 0.04 

 If RATIO is less than, or equal to, unity, it is possible that ambient 

temperatures have been reached. Equilibrium exists only if the RATIO 

stabilizes at a value less than one. Litton (1986) recommends the 

minimum time for the RATIO to remain stabilized at less than one is 30 

days. If the RATIO remains below one for a time, then rises to a 

number greater than one, the 30-day period must begin anew. If the 

fire originated from spontaneous combustion, then consideration must 

include possible re-ignition once oxygen is reintroduced during 

recovery, even though the ratio indicates it is “out.” Based on the fact 

that the RATIO has remained below one for forty days (Figure 1), 

these data show that the panel had stabilized fairly quickly and would 

not likely reignite when reentry was performed. 

 

0.01

0.1

1

10

0 20 40 60 80

Days

RATIO

Figure 1. RATIO values over 80 days. 

Hydrocarbon Ratio 

 Justin and Kim (1988) developed the hydrocarbon ratio after 

finding that low molecular weight hydrocarbon gases desorbed from 

coal in direct proportion to increasing temperature. Methane is readily 

liberated at ambient temperatures. The progression of gas desorption 

due to heating is carbon dioxide, carbon monoxide, hydrogen, 

ethylene, propylene, and acetylene. In effect, hydrocarbon 

concentrations increase with rising temperatures. 

 Knowing the desorption relationship resulted in development of 

the Hydrocarbon Ratio (R1). It is defined as: 

( ) 1000

cTHC

4 CHTHC1.01

1 R ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

− =

where 

THC = total hydrocarbon concentration, ppm4

CH4 = methane concentration, ppm 

c = constant, 0.01 ppm 

 

 R1 is designed to equal zero when no hydrocarbons are detected, 

10 when methane is the only measured hydrocarbon, and about 1,010 

at the upper limit. The only limiting factor is that methane 

concentration must be greater than 20 ppm (0.002%). Table 13 is 

taken from the same data shown in Table 12; samples were taken 

every ten days. 

 For bituminous coal, R1 values are closely dependent on 

temperature. Because of this, limits have been derived to aid in 

determining the temperature of coal being analyzed. R1 values from 0 

to 50 indicate that normal oxidation is occurring. Values from 50 to 

100 indicate possible elevated temperatures. Above 100, coal is 

undergoing elevated temperature oxidation. 

 

4

 Remember that methane (CH4) should also be summed along with 

other hydrocarbons as part of the THC value. 
7 

Table 13. Calculating the Hydrocarbon Ratio, R1

Days CH4 ppm H2 ppm 

C2H6 

ppm 

THC R1

0 3,800 5,300 740 9,840 624 

10 26,100 3,840 700 30,640 158 

20 34,100 1,950 640 36,690 81 

30 39,300 670 660 40,630 43 

40 42,000 80 580 42,660 25 

50 51,000 5 490 51,495 20 

60 52,300 440 52,740 18 

70 54,900 410 55,310 17 

80 54,200 410 54,610 18 

 Figure 2 shows the results of the same data presented in Figure 

1. It is essentially reaching the same conclusion as the RATIO; i.e. 

over time the atmosphere is becoming more stable and active heating 

is less likely. It is still important to understand that, while active 

combustion within the section is slowing, sampling should continue at 

an enhanced rate. 

10

100

1000

0 20 40 60 80

Days

R1

Figure 2. Using R1 to determine atmospheric status. 

Is the sealed atmosphere explosive; could it become explosive 

once ventilation is restarted? 

Explosibility 

 Ideally, once an area is sealed,combustion begins to slow. Gas 

concentrations then begin to change. It is important to determine 

whether the atmosphere is explosive or has the potential to become 

explosive. Understand that during recovery, if combustion restarts and 

the atmosphere is explosive when mixed with air, an explosion will 

likely result. This type of problem is most often encountered when the 

mine is being recovered and air is being reintroduced. 

 Zabetakis et al. (1959) created a diagram based on H.F. 

Coward’s 1928 work that examined explosibility by comparing methane 

and oxygen. The Zabetakis diagram monitors explosibility of sealed 

atmospheres having a maximum hydrogen gas concentration of 

50,000 ppm (5%) and a 30,000 ppm (3%) maximum carbon monoxide 

concentration. The values of all gases obtained for the following 

equations should be in percent. The diagram has three major 

elements: effective inert gases (EI), effective combustible gases (EC), 

and the ratio of methane to total combustibles (R): 

EI = (N2)excess + (1.5 H CO2) 

where 

(N2)excess = (N2)sampled ! (3.8 H O2 sampled) 

 The data for Tables 14 through 16 were the same as those 

collected for Tables 12 and 13. While the atmosphere based on the 

two previous equations appeared to be stabilizing, the explosibility 

values will enable one to predict the likelihood of an explosion once 

fresh ventilation air is again introduced into the area. 

Table 14. Effective Inert, EI 

Day N2% 

sample 

O2% 

sample 

N2% 

excess CO2% EI 

0 79.62 13.89 26.84 4.31 33.31 

10 80.13 12.69 31.91 3.20 36.71 

20 80.42 10.80 39.38 4.85 46.66 

30 81.09 9.45 45.18 4.95 52.61 

40 81.53 8.83 47.98 5.10 55.63 

50 82.37 6.69 56.95 5.90 65.80 

60 82.92 5.81 60.84 5.90 69.69 

70 83.08 5.34 62.79 6.00 71.79 

80 83.18 5.25 63.23 6.00 72.23 

EC = CH4 + (1.25 H H2) + (0.4 H CO) 

 In the EC equation above, note that the values 1.25 and 0.4 are 

the lower explosive limit ratios of CH4 to H2 and CO, respectively. 

Table 15. Effective Combustible, EC 

Day CH4% H2% CO% EC 

0 0.38 0.5300 0.9840 1.44 

10 2.61 0.3840 0.8750 3.44 

20 3.41 0.1950 0.5440 3.87 

30 3.93 0.0670 0.3970 4.17 

40 4.20 0.0080 0.1950 4.29 

50 5.10 0.0005 0.0820 5.13 

60 5.23 0.0635 5.26 

70 5.49 0.0352 5.50 

80 5.42 0.0140 5.43 

( ) 2 H 4 CHCO

4 CH

R

++ =

Table 16. Ratio of Methane to Total Combustible, R

Day CH4% CO% H2% R

0 0.38 0.9840 0.5300 0.2 

10 2.61 0.8750 0.3840 0.7 

20 3.41 0.5440 0.1950 0.8 

30 3.93 0.3970 0.0670 0.9 

40 4.20 0.1950 0.0080 1.0 

50 5.10 0.0820 0.0005 1.0 

60 5.23 0.0635 1.0 

70 5.49 0.0352 1.0 

80 5.42 0.0140 1.0 

 The EI, EC, and R-values from Tables 14 through 16 can be 

plotted on the diagram shown in Figure 3. EI is plotted on the x-axis 

and EC on the y-axis. R-values, found in Table 16, vary between 0 

and 1. Resulting data will fit into one of three areas: explosive, 

explosive when mixed with air, and nonexplosive. The specific area 

into which the sample will fit depends on R. In fact, when applying the 

R values to the Figure 3 diagram, it becomes obvious that, as the R

values increase from 0 to 1, the area under the curve becomes smaller 

and the likelihood that specific gas concentrations will result in 

explosive mixtures becomes less. 
8 

Figure 3. Explosibility diagram 

 When the obtained data were plotted, they all fell into the 

“Nonexplosive Mixture” category. If one plotted the results 

chronologically, it became easier to determine if the atmosphere was 

moving toward or away from becoming explosive. In this case the 

atmosphere appeared to be moving away from potential explosibility. 

Maximum Allowable Oxygen 

 Another alternative to determining if the atmosphere is inert or will 

become explosive upon introduction of ventilation air is the Maximum 

Allowable Oxygen (MAO) analysis. If the measured oxygen 

concentration is below a specific value when compared to R, 

determined by the equation above, the air is either nonexplosive or 

may become explosive when mixed with air. 

 To determine the status of the atmosphere, calculate the R-value, 

refer to Figure 4, proceed horizontally across the chart from that 

number until intersecting the diagonal, and then continue vertically 

downward to find the corresponding MAO value. If the measured 

oxygen concentration is less than the MAO, the atmosphere cannot be 

ignited in its present state. Using only the MAO, care must be 

exercised when reintroducing air to the section because of the 

potential for the oxygen concentration to exceed the MAO. If the 

measured oxygen value is greater than the MAO, it is necessary to 

analyze the atmosphere via the Explosibility diagram. 

Discussion 

 Mine personnel should obtain gas samples at predetermined 

underground locations on a regular basis. This paper provides an 

overview of gas sampling equipment that should normally be on-hand 

and methodologies for taking samples. It gives instructions into 

various gases and gas-related equations that should be analyzed and 

relates their changes over time to assist personnel in determining if a 

heating event is taking place.

 Atmospheric status equations have either been developed 

specifically for the mining industry of adapted from other industries. 

These are tools to help personnel determine the condition of 

underground atmospheres. Some equations provide realistic analyses 

of atmospheric conditions as the mine or section continues to be 

ventilated, other equations are used once the mine or section has 

been sealed. 

 Prior to reentering a sealed area, equations exist that can enable 

personnel to determine if the fire will rekindle once fresh air is 

reintroduced. In addition to the fire reigniting, an equation is available 

that assists personnel in determining if the atmosphere could become 

explosive when reintroducing fresh air. 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

5 6 7 8 9 10 11 12

MAO

R

Figure 4. Determining MAO 

 In conclusion, mine personnel must understand that, except in 

specific instances, individual gas samples or equation results are less 

important than repetitive samples. It is much more important to group 

samples and analyze trends. Through conscientious sampling and 

analysis, the status of the atmosphere can more accurately be 

determined. 

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9 

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