Methods to determine the status 

of mine atmospheres -

an overview 

By Robert J. Timko, National Institute for Occupational Safety 

and Health, Pittsburgh, PA, R. Lincoln Derick, Twentymile 

Coal Company, 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 gas-sampling 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. Realtime instruments are designed to provide nearly instanta-
neous results. They range from hand-held sample tubes to 

more complex, larger, and less portable instruments. A 

number of different technologies perform real-time gas 

analyses. These include stain-tube chemical sensors, electro-chemical reactions, and infrared sensors. 

The second technique for analysing 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 ionisation 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 air-evacuated or by forcing the 

sample into a container through an air pump. 

A container often used by NIOSH personnel is the 10-to20 ml air-evacuated 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 analysing low levels of CO. 

The Mine Safety and Health Administration (MSHA) 

prefers syringes that can be cleaned and reused. Many 

operators use Mylar® bags of varying volumes to sample 

gases. 

The easiest way to visualise 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, linear plots are just as effective in determining the 

status of the atmosphere. 

When analysing 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 by 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-theclock intervals to ensure that diurnal pressure changes are 

not affecting the results. One must be especially careful in 

analysing 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 out-gassing is most common, dilution can still occur. 

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

Sample Oxygen Nitrogen Carbon Methane 

% % Dioxide % % 

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, equalising gas exchange takes time. The pressure 

within the entire sealed volume must equalise 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 by-products 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 emit­
ted 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 analysed. 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 analysed gases may not equal 

exactly 100%. When using less capable GCs, certain gases 

are not analysed. In this instance, all analyzed gases are 

summed and this value is subtracted from 100%. The 

remainder is the concentration of all gases not analysed. 

This mathematical process is called gas normalising. 

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 values2. 

2 In this paper, all N2 values imply N2 + Ar 

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

explosibility require that N2 values be measured, rather than 

normalised. 

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 oxygendepleted 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. 

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. 

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 

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 

micro-organisms oxididing coal or by mixing acidic mine 

water with calcium carbonate, 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 fire-related 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: 

COs R = 2Q ( ——— )
 coal air 1 x 106
 

where: 

Rcoal = rate of coal burning, lb/min 

Qair = Entry air quantity, ft3/min 

COS = CO concentration, ppm 

In Table 4 the sampled CO gas levels changed significantly while the ventilation rate remained essentially the 

same. 

Table 4. Approximate coal burning rate 

Sample Qair ft3/min COSppm Rcoallb/min 

1 6,700 7,490 100 

2 7,000 17,920 250 

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. 

Table 5. Density and flammability of alkanes 

Alkane Density relative Flammability 

to dry air 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 

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 analysed 

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. 

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 

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, 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 lcoke 

and 

Limestone + HEAT lCaO 

coke + CaO +1100EC lCaC2 + H2O + C2H2 

4CH4 + O2 . 820EC lC2H2 + 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: 

Cs CAF = –—————— 1-(02/20.95) 

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, CO AF 

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 analyse 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. 

CO 

ICO = ——————— 

0.265(N2) - O2 

where 

CO = carbon monoxide, ppm 

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

The ICO can signify increasing combustion if, after stabilising 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 

stabilise. 
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 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: 

(CO ICO = —————— 2 - 0.03)

2 (100 - O2) 

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: 

CO2 + (0.75CO)-(0.25H2)

JTR = ————————————
 0.265 (N2) - O2 

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. 

1-(0.0383 O2) Rl =( ———————) ICO N2 

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 reignition 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 

The concentration of CO in parts per million of air within 

the residual gas is: 

CO (CO)R S

g = ( —— ) x 100 Rg 

where 

COS = sampled CO, ppm 

From these values, the R-index (RI

) is created:
 

(CO)R R = ———— = g 

l Rg ( CO

——— S x 100 (R) 2

g ) 

If RI remains constant, one of two things is occurring: 

sustained low-temperature 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: 

-1⁄2 

R

(Rl

) =300 g

eq ( —— 

O )

2 

(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: 

(Rl)act - ) RATIO =——— = 1/3 CO 3/2 - 1⁄2

S (Rg (O (R 2) 

l

)eq
 

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

becomes: 

RATIO = 1/3 CO )-3/2 S(Rg (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% 

ppm 

CO 

ppm 

C2H6 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 
5

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

ambient temperatures have been reached. Equilibrium 

exists only if the RATIO stabilises at a value less than one. 

Litton (1986) recommends the minimum time for the RATIO 

to remain stabilised 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 stabilised fairly quickly and would 

not likely reignite when reentry was performed. 

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: 

1.01 (THC) - CH4 R1 = ( ————————— ) 1000 THC +C 

4Remember tha methane (CH4) should also be summed along 

with other hydrocarbons as part of the THC value 

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. 

Table 13. Calculating the Hydrocarbon Ratio, R1 

Days CH4 H THC 2 C2H6 R1 

ppm ppm ppm 

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 

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 analysed. 

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. 

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. 

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 stabilising, 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% O2% N2% CO2% EI

sample sample excess 

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 

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 

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 

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. 

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

CH

R = ———————— 4

(CO + CH4 + H2) 

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. 

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 ventila-
tion 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 analyse the atmosphere via the Explosibility diagram. 

Figure 4. Determining MAO 

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 analysed 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 re-entering 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. 

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 analyse 

trends. Through conscientious sampling and analysis, the 

status of the atmosphere can more accurately be determined. 

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