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Bureau of Mines Information Circular/1987 
 
 True Flammable Gas Detecting System 
 
 By J. E. Chilton and T. Kubala 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
t EaAJ**** *f ' / M^* ) 
 
 Information Circular 9163 
 
 True Flammable Gas Detecting System 
 
 By J. E. Chilton and T. Kubala 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 Donald Paul Hodel, Secretary 
 
 BUREAU OF MINES 
 
 David S. Brown, Acting Director 
 
YlOe 
 
 °fi 
 
 Library of Congress Cataloging in Publication Data: 
 
 ^ a tf 
 
 {\t> 
 
 H\ 
 
 Chilton, J. E. 
 
 True flammable gas detecting system. 
 
 (Information circular / United States Department of the Interior, Bureau of Mines ; 9163) 
 
 Supt. of Docs, no.: I 28:27: 9163. 
 
 1. Gas-detectors. 2. Mine gases. I. Kubala, T. E. (Theodore E.). II. Title. III. Series: In- 
 formation circular (United States. Bureau of Mines) ; 9163. 
 
 TN295.U4 
 
 [TN305] 
 
 622 s 
 
 [622'.8] 
 
 87-600229 
 
CONTENTS / 
 
 Page 
 
 Abstract 1 
 
 Introduction 2 
 
 Acknowledgment 3 
 
 Combustible gas measurement techniques 3 
 
 Designing the true flammable gas detecting system 7 
 
 Recommended maintenance and operating procedures 9 
 
 Summary and conclusions 10 
 
 Appendix 11 
 
 ILLUSTRATIONS 
 
 1. Explosive mixtures of CH4 and O2 in air 2 
 
 2. Rik.en-18 light refractance methanometer response to gases 4 
 
 3. Sound velocity measurements in gas/air mixtures 4 
 
 4. Thermal conductivity measurements in gas/air mixtures 4 
 
 5. Catalytic heat-of-combustion sensor response to CH 4 5 
 
 6. MX240 methanometer response to CH 4 -air mixture 5 
 
 7. MX240 response to 2.5 vol pet CH 4 with low O2 concentration 5 
 
 8. Methanometer response to H 2 mixtures 6 
 
 9. Model 502 response to CH 4 , H 2 , and CO 6 
 
 10. MX240 response to CH 4 , H 2 , and CO 6 
 
 11. Proposed gas dilution technique 7 
 
 12. Prototype gas diluter using two peristaltic pumps 7 
 
 13. Model 502 response to CH 4 using the gas dilutor 8 
 
 14. Model 502 response to H 2 using the gas dilutor 8 
 
 15. Lightweight true flammable gas detector 9 
 
 TABLE 
 
 1. Gas composition of sealed coal mines 3 
 

 UNIT OF MEASURE 
 
 ABBREVIATIONS USED 
 
 IN 
 
 THIS REPORT 
 
 in 
 
 inch 
 
 mL/rev 
 
 
 milliliter per revolution 
 
 L 
 
 liter 
 
 m/s 
 
 
 meter per second 
 
 L/min 
 
 liter per 
 minute 
 
 pet 
 psig 
 
 
 percent 
 
 pound per square 
 
 lb 
 
 pound 
 
 
 
 inch, gauge 
 
 mL 
 
 milliliter 
 
 vol pet 
 
 
 volume percent 
 
TRUE FLAMMABLE GAS DETECTING SYSTEM 
 
 By J. E. Chilton'and T. Kubala 2 
 
 ABSTRACT 
 
 The Bureau of Mines has developed a portable flammable gas detecting 
 system for estimating the flammable gas concentrations in a mine con- 
 taining one or more combustible gases at concentrations up to 100 vol 
 pet. With this portable detecting system, mine rescue or recovery per- 
 sonnel can quickly determine if they are working in a potentially explo- 
 sive gas atmosphere and either leave the area or make appropriate 
 changes in the fresh air ventilation of the mine to render the area 
 safe. The detecting system uses a commercial, intrinsically safe, flam- 
 mable gas detector with a range from to 5 vol pet methane (CH 4 ). The 
 flammable gas detector has a catalytic heat-of-combustion sensor that 
 can detect all flammable gases that would be encountered in a mine in- 
 cluding methane (CH 4 ), ethane (C 2 H 6 ), carbon monoxide (CO), and hydrogen 
 (H 2 ). To determine the mine gas concentration, the rescue worker di- 
 lutes a sample with fresh air carried within the detecting system. The 
 concentration of the diluted sample is then measured with the permis- 
 sable flammable gas detector. The actual mine gas concentration is cal- 
 culated from the product of the indicated diluted gas concentration and 
 the system dilution ratio using a pocket calculator or the nomograph on 
 the side of the unit. 
 
 ^Research chemist. 
 
 o . 
 
 ■^Research physicist. 
 Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
 
INTRODUCTION 
 
 In a mine emergency caused by fire, ex- 
 plosion, methane inundations, or massive 
 roof or ground disturbance, flammable 
 gases are released from coal or rock 
 strata or are produced from coal and wood 
 fires or CH 4 explosions. In a rescue 
 operation, the rescue teams must deter- 
 mine if a mine section that the teams are 
 entering contains flammable gases at a 
 concentration which is potentially explo- 
 sive. Rescue teams now carry portable 
 gas instruments (methanometers and oxygen 
 meters) that indicate combustible gas 
 concentrations from to 5 vol pet CH 4 
 and/or oxygen (0 2 ) concentrations from 
 to 25 vol pet in the mine atmosphere. 
 The methanometers, typically with cata- 
 lytic heat-of -combustion sensors, are not 
 adequate for detecting flammable gas con- 
 centrations over the entire explosive 
 range (from 5 to 15 vol pet CH 4 , for 
 example), or for measuring flammable gas 
 concentrations in gas mixtures with low 
 2 concentrations. When operating in 
 high concentrations of flammable gases or 
 in low 2 concentrations, the rescue 
 teams also take bottle samples of the 
 mine air for subsequent analysis at a 
 surface-sited gas chromatograph. Unfor- 
 tunately, this gas composition informa- 
 tion is not immediately available to the 
 rescue teams while they are working 
 within the potentially hazardous mine 
 site. 
 
 The explosive or flammable range for 
 mixtures of CH 4 with fresh air (20.9 vol 
 pet 2 ) at room temperature and pressure 
 is from 5 to 15 vol pet CH 4 . CH 4 con- 
 centrations above the upper explosive 
 limit can form flammable mixtures when 
 diluted with fresh air. The initial 2 
 concentration in the CH 4 mixture is also 
 a factor in determining which gas mix- 
 tures are flammable as indicated in fig- 
 ure 1. Nitrogen (N 2 ) mixtures within the 
 left-hand area of the graph are nonflam- 
 mable and cannot form an explosive mix- 
 ture if diluted with fresh air. CH 4 
 mixtures within the right-hand area of 
 the graph can form explosive mixtures 
 when diluted with fresh air. Special 
 precautions must be taken to ensure that 
 there are no sources of ignition in the 
 
 mine within the area being ventilated if 
 CH 4 concentrations should be in the ex- 
 plosive area or in the right-hand area of 
 the graph (fig. 1). Mine atmospheres with 
 CH 4 concentrations approaching 100 vol 
 pet have been encountered during rescue 
 or recovery efforts in coal mines that 
 had been sealed for several months. Ac- 
 curate measurement of CH 4 concentrations 
 in the range from 5 to 15 vol pet and 
 greater are necessary to avoid potential 
 mine explosion hazards when ventilating 
 the mine during the rescue and recovery 
 operations. 
 
 In coal mine fires and explosions, com- 
 bustible gases in addition to CH 4 may be 
 formed. The combustible gases found in 
 sealed mines include H 2> CO, and C 2 H 6 
 (table 1). The mine rescue teams clearly 
 need gas instrumentation that can ac- 
 curately measure the concentrations of 
 combustible gases found in mines. The 
 instrument should be able to measure 
 
 4 8 12 16 20 
 
 METHANE, vol pet 
 
 FIGURE 1.— Explosive mixtures of CH 4 and 2 in air. 
 
CH 4 above 5 vol pet concentration; to 
 measure CO, H 2 , and other hydrocarbons as 
 well; and the measurements should be 
 
 accurate in gas 
 concentration. 
 
 mixtures with any 2 
 
 TABLE 1. - Gas composition of sealed coal 
 mines, volume percent 
 
 Methane (CH 4 ) 
 
 Hydrogen (H 2 ) 
 
 Carbon monoxide (CO) 
 
 Ethane (C 2 H 6 ) 
 
 Carbon dioxide (C0 2 ) 
 Oxygen (0 2 ) 
 
 Mine A 
 
 10 days 45 days 
 
 19 
 .3 
 
 .4 
 
 .2 
 
 .4 
 
 15.4 
 
 95 
 .002 
 .001 
 1.4 
 .5 
 .4 
 
 Mine B, 
 39 days 
 
 0.5 
 
 1.6 
 
 1.7 
 
 
 
 8.9 
 
 4.3 
 
 Mine C, 
 2 days 
 
 26 
 2.5 
 4.2 
 
 .04 
 4.0 
 4.0 
 
 ACKNOWLEDGMENT 
 
 Recommandations of specific gas de- 
 tectors and discussions of system design 
 alternatives with George H. Schnakenberg, 
 
 supervisory physical scientist, Pitts- 
 burgh Research Center, Pittsburgh, PA, 
 were invaluable throughout this work. 
 
 COMBUSTIBLE GAS MEASUREMENT TECHNIQUES 
 
 Combustible gases may be detected by 
 physical measurements such as light re- 
 fraction, sound velocity, and thermal 
 conductivity; and by chemical measure- 
 ments such as the heat of combustion pro- 
 duced by the reaction of flammable gases 
 with 2 . Each of these methods has de- 
 monstrated a response to specific combus- 
 tible gases; however, the physical 
 methods may also respond to noncombus- 
 tible or interferent gases such as C0 2 , 
 which are often present in mine gas mix- 
 tures. Interference to the responses of 
 the combustible gases may be additive, 
 which may lead to an indication of a con- 
 centration greater than actually present, 
 or subtractive, leading to an indication 
 of a combustible gas concentration lower 
 than actually present. Both of these 
 conditions are undesirable, in particu- 
 lar, gas measurements with negative 
 interferences may cause unsafe mine work 
 operations in potentially explosive gas 
 mixtures. 
 
 Infrared (IR) measurement techniques 
 have not been considered for this ap- 
 plication because not all combustible 
 gases, for example H 2 , can be detected by 
 an IR technique; furthermore, many IR 
 instruments are not portable, and IR 
 
 technology is two to six times more 
 expensive than the other techniques under 
 consideration. 
 
 The typical response of a gas detector 
 (Riken model 18^) based on measurement of 
 the index of refraction of light to CH4, 
 C 2 H 6 , and H 2 gas mixtures in air is shown 
 in figure 2. The instrument response to 
 H 2 is negative. Mixtures of combustible 
 gases including H 2 would cause negative 
 error in the indication owing to the sum- 
 ming of the negative and positive re- 
 sponses of the individual gases. The re- 
 sponse to C0 2 (not shown) is equal to 
 that of CH4, therefore in mixtures with 
 significant amounts of C0 2 the indicated 
 concentration will be incorrectly high. 
 
 The velocity of sound through mixtures 
 of two gases, CH 4 and C0 2 , with air is 
 given in figure 3. H 2 response, not 
 shown, is six times greater than the CH 4 
 response. Gas mixtures with both H 2 and 
 CH 4 would thus cause an incorrectly high 
 response. On the other hand, since the 
 response to C0 2 is roughly the negative 
 of that of CH 4 , mixtures of gas with 
 
 •^Reference to specific 
 not imply endorsement by 
 Mines. 
 
 products does 
 the Bureau of 
 
o 
 
 CL 
 
 ~o 
 
 > 
 
 UJ 
 
 < 
 
 UJ 
 
 CO 
 
 < 
 
 UJ 
 CO 
 
 o 
 
 Q_ 
 CO 
 UJ 
 
 cc 
 
 00 
 
 I 
 
 UJ 
 
 a: 
 
 -2 
 
 Hydrogen 
 
 12 3 
 
 GAS CONCENTRATION 
 IN AIR, vol pet 
 
 FIGURE 2.— Riken-18 light refractance methanometer 
 response to gases. 
 
 10 vol pet of both CH 4 and C0 2 would 
 indicate a zero amount of CH4. 
 
 The gas response of a thermal conduc- 
 tivity detector with respect to air is 
 indicated in figure 4. As is the case 
 for the velocity of sound measurements, 
 the response to H 2 is much greater than 
 that of CH4. For example, the MSA model 
 510B, 4 portable methane detector, has a 
 response to H 2 , 5.4 times the response to 
 CH 4 so the measurement of mixtures of H 2 
 
 440 
 
 420 
 
 400 
 
 t 380 
 
 o 
 
 o 
 
 UJ 
 
 > 
 
 360 
 
 o 
 
 CO 
 
 340 
 
 320 
 
 300 
 
 ^x Carbon dioxide 
 
 _L 
 
 20 40 60 80 
 
 GAS IN AIR, vol pet 
 
 100 
 
 FIGURE 3.— Sound velocity measurements in gas air- 
 mixture. 
 
 20 40 60 80 
 
 GAS IN AIR, vol pet 
 
 See footnote 3. 
 
 FIGURE 4.— Thermal conductivity measurements in gas air- 
 mixtures. 
 
and CH4 will result in positive error. 
 In addition, CO2 may cause negative er- 
 rors in the CH4 determinations. The de- 
 tector response to CO is close to that 
 for air, and flammable gas mixtures 
 with percentage quantities of CO would 
 not be detected. 
 
 The commercial flammable or combustible 
 gas detectors used in mines have cata- 
 lytic heat-of-combustion sensors. The 
 gas that is being measured is reacted 
 with O2 from air on the catalytic surface 
 of the sensor and the heat generated by 
 this oxidation causes both the tempera- 
 ture and the resistance of the sensor to 
 rise. This increase in resistance is 
 converted to a voltage change propor- 
 tional to the gas concentration for dis- 
 play on an analog or digital meter by the 
 electronic circuit within the detector. 
 
 The response of a typical catalytic 
 heat-of-combustion sensor to CH4 is given 
 in figure 5 as a function of the CH4 con- 
 centration. The response is linear below 
 the maximum response value 10 vol pet CH4 
 where stoichiometric quantities (2 mole- 
 cules of O2 for each molecule of CH4 ) of 
 CH4 and O2 react. The response falls for 
 CH4 concentrations greater than 10 vol 
 pet because insufficient O2 is available 
 for complete oxidation of the CH4 . The 
 
 20 40 60 80 
 
 METHANE CONCENTRATION, vol pet 
 
 100 
 
 normal range for commercial CH4 detectors 
 using the catalytic heat-of-combustion 
 sensor is from to 5 vol pet CH4. The 
 linear response in this range for a 
 typical methanometer (MX240) is shown in 
 figure 6. 
 
 The effect of low O2 concentrations on 
 the methanometer response is given in 
 figure 7 for measurements conducted in 
 
 12 3 4 5 
 
 METHANE CONCENTRATION, vol pet 
 
 FIGURE 6,— MX240 methanometer response to CH 4 -air mix- 
 ture. 
 
 3.0 
 
 2.5- 
 
 o 
 co 
 
 UJ 
 
 cc 
 
 UJ 
 
 z 
 < 
 
 X 
 
 2.0 
 
 1.5- 
 
 1.0 
 
 
 
 1 1 
 
 1 
 
 1 1 
 
 r 
 
 30CFR _ 
 tolerance 
 
 - 
 
 
 
 
 
 X 
 
 
 
 L 
 
 
 1 
 
 / 
 
 ( 
 
 > 
 
 
 
 
 
 I 1 
 
 1 
 
 1 1 
 
 5 10 15 20 25 
 
 OXYGEN CONCENTRATION, vol pet 
 
 30 
 
 FIGURE 5.— Catalytic heat of combustion sensor response 
 to CH 4 . 
 
 FIGURE 7.— MX240 response to 2.5 vol pet CH 4 with low 2 
 concentration. 
 
2. 5 vol pet CH 4 mixtures. The response of 
 this methanometer to the 2. 5 vol pet CH 4 
 concentration was within the tolerence 
 required by 30 CFR 22. 7d(2) in O2 concen- 
 trations from 20.9 vol pet (fresh air) to 
 7 vol pet. The lower limit for the O2 
 concentration of a given CH 4 mixture for 
 an accurate CH 4 response may be expected 
 at an O2 concentration equal to twice the 
 CH 4 concentration, that is 5 vol pet O2 
 for 2.5 vol pet CH 4 . The 2:1 ratio of 2 
 to CH 4 is required for complete reaction 
 to form C0 2 and water (H 2 0). The low re- 
 sponse to CH 4 below 7 vol pet O2 may be 
 from significant CH 4 reaction to form CO 
 with a low heat of reaction. In gas mix- 
 tures with the O2 concentration approach- 
 ing zero, the catalytic sensor will not 
 respond to CH 4 or any other flammable 
 gas. 
 
 The methanometers with catalytic heat- 
 of-combustion sensors will respond to all 
 combustible gases. For example, in fig- 
 ure 8 the response to H2 mixtures is 
 shown for the MX240 and M502 methano- 
 meters. The responses indicated in vol- 
 ume percent CH 4 were measured with the 
 H 2 concentrations expressed in volume 
 
 o 
 
 Q. 
 
 "o 
 
 > 
 
 uJ 
 
 CO 
 
 z 
 o 
 
 Q_ 
 CO 
 UJ 
 
 cr 
 
 UJ 
 
 z 
 < 
 
 X 
 
 I- 
 
 UJ 
 
 12 3 4 
 
 HYDROGEN CONCENTRATION, vol pet 
 
 FIGURE 8.— Methanometer response to H 2 mixtures. 
 
 percent. If the flammable gas concentra- 
 tion is expressed as percent lower ex- 
 plosive limit (LEL) and the measuring 
 instrument response is given in percent 
 LEL, the determination of total combus- 
 tibility of a gas mixture can be made 
 with these methanometers. Figures 9 and 
 10 show the results of measurements with 
 CO, CH 4 , and H2 combustible mixtures for 
 both the MX240 and M502 methanometers. 
 They are read as percent LEL by multiply- 
 ing the reading in volume percent CH 4 by 
 20; e.g. , 1 vol pet CH 4 in air is a 20- 
 pct LEL mixture. The responses of the 
 
 CO 
 
 O 
 Q- 
 CO 
 
 o 
 
 o 
 
 UJ. 
 
 UJ 
 
 CM 
 O 
 
 < 
 
 co 
 
 uu 
 
 1 
 
 
 1 1 1 r\ 1 
 
 
 
 
 / 
 
 80 
 
 - 
 
 
 / Ideal 
 / response 
 / 
 
 60 
 
 
 
 
 40 
 
 
 
 / cv^ 
 / /" 
 
 X _s^ 
 
 
 
 / 
 / 
 
 * J^ KEY 
 
 
 / 
 
 ^^ H 2 
 
 20 
 
 / 
 
 
 S^ x CH4 
 
 
 / 
 
 
 a CO 
 
 
 / o" 
 
 
 
 c 
 
 / 
 
 
 1 1 1 1 1 
 
 20 40 60 80 100 120 140 
 
 GAS CONCENTRATION, pet LEL 
 
 FIGURE 9.— Model 502 response to CH 4 , H 2 , and CO. 
 
 20 40 60 80 
 
 GAS CONCENTRATION, pet LEL 
 
 FIGURE 10.— MX240 response to CH 4 , H 2 , and CO. 
 
 00 
 
M502 methanometer to H 2 is less than the 
 responses to CH 4 or CO. The low H 2 re- 
 sponse for the M502 may be caused by par- 
 tial reaction of H 2 on the reference or 
 
 inactive element in the sensor. The re- 
 sponses of the MX240 are approximately 
 equal for CH4, H2, or CO when all concen- 
 trations are expressed in percent LEL. 
 
 DESIGNING THE TRUE FLAMMABLE GAS DETECTING SYSTEM 
 
 The proposed true flammable gas detect- 
 ing system should have certain character- 
 istics in order to be used in postdiaster 
 operations in gassy mines. The methano- 
 meter must be intrinsically safe and ap- 
 proved for use in CH 4 mixtures with air 
 per Mine Safety and Health Administration 
 (MSHA) specifications. Any future need 
 for certification of electrical equipment 
 for operation in CH4-H 2 -air mixtures 
 should be addressed by MSHA. The meth- 
 anometer should have a catalytic heat- 
 of-combustion sensor so that the meter 
 will respond to all combustible gases 
 found in mines; for example, CH 4 , C 2 H 6 , 
 H 2 and CO. In order for the methanometer 
 to work properly in mine atmospheres with 
 minimum 2 , the mine sample must be di- 
 luted with fresh air to supply enough 2 
 for total combustion and to reduce the 
 combustible gas concentration to a value 
 to be measured by the methanometer with a 
 0- to 5-vol pet CH 4 range. The system 
 should be small size and light weight for 
 carrying over the shoulder while travel- 
 ing in the mine. 
 
 Syringe 
 
 10 mL 
 
 3-way 
 stopcocks 
 
 Methanometer 
 
 One gas dilution system considered for 
 use with this concept is shown in figure 
 11. This system used a 10-mL syringe for 
 taking a gas sample and a 50-mL syringe 
 for diluting and mixing the gas sample 
 with fresh air. The sample was then 
 transferred to the methanometer for mea- 
 surement of the combustible gas content. 
 This system required the manipulation of 
 two three-way stopcocks for operation. 
 The proper sequencing of the valves would 
 be difficult to accomplish during mine 
 rescue operations; for this reason, this 
 dilution system was rejected. 
 
 The dilution system finally selected is 
 shown in figure 12. This system uses two 
 peristaltic hand-driven pumps for the 
 dilution and transfer of the gas mixture. 
 The mine gas is drawn into one pump con- 
 taining tubing with a small sample volume 
 and the fresh air sample is drawn into 
 the other pump containing tubing with a 
 large sample volume. The two pumps are 
 connected by a common shaft which is 
 hand-cranked. By this coupling of the 
 pumps, a given volume of mine gas and 
 fresh air is pumped for each turn of the 
 handle and the dilution volume is con- 
 stant, dependent only on the diameters of 
 each of the tubes. 
 
 Valve 
 
 Mine air 
 
 FIGURE 11.— Proposed gas dilution technique. 
 
 FIGURE 12.— Prototype gas diluter using two peristaltic 
 pumps. 
 
The gases are mixed and transferred to 
 the methanometer for measurement of the 
 diluted combustible gas concentrations. 
 In operation, a sample of gas is fed into 
 the sensor cavity, the sensor is then 
 electrically powered, and the gas sample 
 is completely reacted. The M502 analog 
 meter response rises to a maximum value, 
 then falls as the combustible gas in the 
 cavity is consumed. The methanometer 
 selected for this system was the model 
 502 because it requires a gas sample of 
 only about 10 mL volume for a gas mea- 
 surement. The M502 methanometer is cali- 
 brated so that the maximum value noted on 
 the meter is equal to the measured gas 
 concentration. 
 
 Tests were also run using the MX240 
 methanometer and over 300 cranks of the 
 pump handle were required to give a con- 
 stant response. This unit is continually 
 powered and requires a large gas sample 
 volume for an indication. The MX240 unit 
 cannot be used with this dilution system 
 since the system contains only a limited 
 volume of fresh air for making the gas 
 dilutions. 
 
 The first prototype system was assem- 
 bled in an aluminum box containing a 
 fresh air bag, the methanometer, the two 
 peristaltic pumps, a rubber squeeze bulb 
 for filling the fresh air bag, and inter- 
 connecting tubing. This box was approxi- 
 mately 6 by 8 by 9 in and weighed 7.5 lb. 
 The dilution system was tested using CH 4 - 
 air mixtures (fig. 13). About seven 
 cranks of the handle driving the pumps 
 
 were needed to deliver sufficient gas for 
 measurement. The average gas dilution 
 ratio was 28.3 for the tests at different 
 CH 4 concentrations. Gas concentrations 
 from 100 to 10 vol pet CH 4 can be mea- 
 sured with this system. System response 
 to various H 2 -air mixtures using the 
 model 502 detector are shown in figure 
 14. For these measurements, the poly- 
 vinylchloride (PVC) tubing in both pumps 
 was replaced because of wear, and a new 
 dilution ratio of 26. 7 was obtained for 
 100 vol pet H 2 , and then the same ratio 
 was measured for the dilution of CH 4 . 
 
 The first prototype unit was demon- 
 strated to the MSHA mine rescue teams. 
 The teams recommended that the unit be 
 made smaller and lighter in weight in 
 view of the considerable weight and vol- 
 ume of other equipment that must also be 
 carried in mine rescue missions. They 
 also recommended that the dilution ratio 
 be reduced to more accurately measure the 
 explosive region of CH 4 , 5 to 15 vol pet 
 in air. A second gas detector unit has 
 been constructed which is 8 by 6 by 5 in 
 and weighs 5 lb. The following design 
 changes were implemented: 
 
 • The fixed length crank handle has 
 been replaced with a folding handle. 
 
 • Two of the pumps were changed to a 
 different sized tubing to obtain a 14:1 
 dilution ratio. 
 
 • Silicone rubber tubing was used in 
 place of the PVC plastic tubing in the 
 pumps to improve tubing life and thus 
 obtain a more constant dilution ratio. 
 
 5 10 15 
 
 PUMP CRANK, turns 
 
 20 
 
 Pump 
 
 dilution 
 
 ratio 
 
 28.6 
 
 28.5 
 
 27.7 
 27.6 
 
 28.7 
 28.5 
 
 FIGURE 13.— Model 502 response to CH 4 using the gas 
 dilutor. 
 
 _ 4 
 
 w 3 
 in 
 
 z 
 o 
 
 CL 
 CO 
 Ul 
 
 cc 
 
 Ld 
 
 < 
 
 X 
 
 CM 
 O 
 IT) 
 
 - 
 
 I 
 
 - — • 
 
 1 
 
 • 
 
 1 
 
 1 
 
 — 
 
 / / 
 // 
 
 ■ 
 
 =a 
 
 — 
 
 / /• 
 
 f / 
 
 
 _ 
 
 
 1 
 
 1 
 
 1 
 
 5 10 15 
 
 PUMP CRANK, turns 
 
 H2, 
 vol pet 
 
 Pump 
 
 dilution 
 
 ratio 
 
 100 
 
 26.7 
 
 75 
 
 26.3 
 
 50 
 
 27.0 
 
 25 
 
 20 
 
 27.8 
 
 FIGURE 14.— Model 502 response to H 2 using the gas 
 dilutor. 
 
Four of the lightweight prototype units 
 have been designed and fabricated. Fig- 
 ure 15 shows the lightweight flammable 
 
 gas detector. The air pumps and the 
 fresh air bag are placed in the space 
 under the permissible methanometer. 
 
 RECOMMENDED MANTENANCE AND OPERATING PROCEDURES 
 
 While in storage, the flammable gas de- 
 tecting system should be placed at a 
 central location with the manufacturer's 
 charger connected to the permissible 
 methanometer when required to keep the 
 batteries charged. In operation, the 
 flammable gas detecting system and a 
 calibration kit would be carried to the 
 mine site. The calibration kit contains 
 a CH 4 -air mixture to calibrate the 
 methanometer and a CH 4 or CH4-N2 mixture 
 to measure the dilution ratio. The air 
 bag should be filled by operating the 
 hand squeeze bulb while the unit is in 
 fresh air. The air bag position under 
 
 f h. 
 
 FIGURE 15.— Lightweight true flammable gas detector. 
 
 the methanometer air is visible through 
 the acrylic cover of the unit. 
 
 Calibrate the methanometer with the 2. 5 
 vol pet CH 4 standard gas mixture, and 
 measure the gas dilution ratio using a 
 100-vol pet or a 50-vol pet CH4 mixture 
 depending on the unit's recorded dilution 
 ratio. Record the methanometer calibra- 
 tion data in a permanent log book along 
 with the record of the dilution ratio and 
 also post it on the dilution system 
 case. 
 
 A nomograph for direct conversion of 
 the indicated gas concentration for the 
 diluted sample to the calculated value 
 for the original mine gas concentration 
 is placed on the side of each unit. The 
 nomograph was initially constructed using 
 the originally measured gas dilution 
 ratio, but if subsequent measurements are 
 sufficiently different, the nomograph 
 should be redrawn to fit the new dilution 
 ratio value. 
 
 In the mine, operate the unit by turn- 
 ing the handle 7 to 10 turns and taking a 
 reading. If possible, repeat the mea- 
 surement to verify the accuracy of the 
 initial reading. The original mine gas 
 concentration can be determined from the 
 meter indication by use of the nomograph 
 on the side of the case. 
 
 The air bag has a sufficient volume for 
 over 30 measurements, and may be refilled 
 in the mine at a verified fresh air site. 
 Verify mine fresh air sites by measuring 
 the CH 4 gas concentration directly with 
 the methanometer and verifying that the 
 CH 4 concentration is zero. In addition, 
 use a permissible O2 meter to verify that 
 the 2 concentration is 20.9 vol pet. 
 
 Since the PVC plastic or silicone rub- 
 ber tubing loses its resiliency with con- 
 tinued use, replace the pump tubing after 
 prolonged use or when a significant 
 change is found in the dilution ratio. 
 New tubing may have a slightly different 
 inner diameter even if it comes from the 
 same tubing lot. Measure the dilution 
 ratio before each use by testing the 
 
10 
 
 dilution system with a suitable calibra- 
 tion gas. For pump combinations with the 
 dilution ratio of 20 or greater, a cali- 
 bration gas of 100 vol pet CH 4 can be 
 used. For example, if 100 vol pet CH 4 is 
 diluted and a methanometer reading of 3. 5 
 vol pet CH 4 is obtained, the dilution 
 ratio is 100/3.5 or 28.6. 
 
 This system was named a true flammable 
 gas detecting system because the cata- 
 lytic heat-of-combustion sensor will re- 
 spond to all flammable gases in the 
 diluted sample. The accuracy of the re- 
 sponse from the M502 methanometer is high 
 for CH 4 or CO, but the response for H 2 is 
 only 60 vol pet of the true H2 concen- 
 tration. Thus, mixtures of flammable 
 gases with H2 will have a response less 
 than the true response. For example, a 
 
 mixture of 5 vol pet CH 4 , 4 vol pet CO, 
 and 4 vol pet H 2 run in a true flammable 
 gas detection system with a dilution 
 ratio of 28.6 would read 0.34 vol pet CH 4 
 on the M502 meter. 
 
 The value of 0. 34 vol pet CH 4 for the 
 diluted gas corresponds to an undiluted 
 gas sample of 9.6 vol pet CH 4 or 192 pet 
 LEL for the sum of the flammable gas 
 LEL's (5 vol pet CH 4 = 100 pet LEL). 
 This calculated value of 192 pet LEL for 
 the M502 measured sum of the gas constit- 
 uent LEL is 17 pet less than the true sum 
 of the LEL's for this mixture, which, is 
 232 pet LEL. 
 
 A list of parts for construction of the 
 true flammable gas detection system is 
 given in the appendix. 
 
 SUMMARY AND CONCLUSIONS 
 
 A true flammable gas detecting system 
 was developed that uses a catalytic heat- 
 of-combustion combustible gas detector to 
 measure the combustible gas concentration 
 of a mine gas sample diluted by fresh 
 air. The dilution equipment consists of 
 two peristaltic hand-operated pumps that 
 supply a fixed ratio of fresh air to sam- 
 ple gas. The dilution ratio is set by 
 the volumes of the flexible tubing within 
 the pumps. Two units were prepared with 
 approximate dilution ratios of 14:1 for 
 accurate measurement of CH 4 within the 
 5- to 15-vol pet explosive range. The 
 dilution ratio for these units is deter- 
 mined by use of a standard 50 vol pet 
 CH 4 -N2 gas mixture. In addition, two 
 units were fabricated with approximate 
 dilution ratios of 28:1, which can be 
 used to measure gas mixtures containing 
 up to 100 vol pet CH 4 concentrations. 
 The dilution ratios for these units is 
 determined by calibration with a standard 
 gas of 100 vol pet CH 4 . For all units, 
 the dilution ratios must be periodically 
 measured and, if found to vary from ini- 
 tial values, new flexible tubing should 
 be placed in the air pumps. The fresh 
 air is contained within a flexible plas- 
 tic bag and the bag is filled before en- 
 tering the mine. The 1-L bag contains 
 
 enough fresh air for over 30 gas con- 
 centration determinations. The catalytic 
 heat-of-combustion sensor was chosen for 
 measuring the flammable gases because it 
 responds to all combustible gases includ- 
 ing H 2 , CO, CH 4 , and C 2 H 6 , that could be 
 encountered in mines or in mine fires or 
 explosions. Other sensors using thermal 
 conductivity, infrared absorption, and 
 velocity of sound systems are available 
 to measure CH 4 in concentrations up to 
 100 vol pet. These sensors could be used 
 in mine gas detectors if only CH 4 gas in 
 air were to be measured. In coal mine 
 fires or explosions, CH 4 is found togeth- 
 er with other combustible gases such as 
 H 2> C 2 H6, and CO, and the catalytic heat- 
 of-combustion sensor is of more general 
 application. This flammable gas detect- 
 ing system may be especially useful for 
 quickly providing combustible gas infor- 
 mation in rescue or recovery operations. 
 The use of theses units in mine rescue 
 operations by mine workers or mine rescue 
 personnel from MSHA or the Deep Mine 
 Safety Division of the Pennsylvania 
 Department of Environmental Resources, 
 Uniontown, PA, is expected to contribute 
 to the safety of the mine rescue work and 
 to aid the mine recovery efforts. 
 
11 
 
 APPENDIX 
 
 A parts list for the critical compo- 
 nents of the true flammable gas detecting 
 system is given. 
 
 • Instrument case . — rectangular box, 
 aluminum, 5-in width, 8-in length, and 
 6-in height, Zero Corp., 288 Main St., 
 Monson, MA 01057. 
 
 • Hand-operated peristaltic pumps . — 
 Masterflex pump head R 7013-20 (0.006 
 mL/rev) and Masterflex pump head 
 R 70015-00 (long shaft) (1.67 mL/rev). 
 Cole-Palmer Instrument Co. , 7425 North 
 Oak Park Ave., Chicago, IL 60648. 
 
 • Silicone flexible tubing, selected 
 dimensions. — Cole-Palmer Instrument Co. 
 
 • Permissible methanometer . — Model 
 M502 (Auer) range from to 2 vol pet and 
 from 2 to 5 vol pet CH 4 , part No. 460139, 
 Mine Safety Appliances , 600 Penn Center 
 Blvd., Pittsburgh, PA 15206. 
 
 • Calibration equipment . — Model SGP 
 plastic case, disposable steel cylinder 
 PN559--2.5 vol pet CH 4 in air (103 L at 
 1,000 psig), flow regulator— PN7 15 (0.31 
 L/min flow), Alphagaz, Woods Road, P.O. 
 Box 149, Cambridge, MD 21613. 
 
 • Specialty gas . — 100 vol pet CH4 or 
 50 vol pet CH 4 in N, 14 L at 240 psig, 
 regulator for 1-to 25-psig delivery pres- 
 sure, Scott Specialty Gases, Route 611, 
 Plumsteadville, PA 18949. 
 
 US GOVERNMENT PRINTING OFFICE: 1987 - 605017/60117 
 
 INT.-BU.0F MINES,PGH.,PA. 28583 
 
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 Burea u of Mines— Prod, end Distr. 
 Cochrane Mill Roed 
 P.O. Box 18070 
 Pittsburgh. Pa. 15236 
 
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