SILICONE VAPOR POISONING OF 

CATALYTIC METHANE SENSORS 

J. E. Chilton 

Research Chemist 

Pittsburgh Mining and Safety Research Center 

Bureau of Mines 

U.S. Department of the Interior 

Pittsburgh, Pa. 

J. N. Baran 

W. E. Thomas 

Electronics Technicians 

L. J. E. Hofer 

Chemist 

J. L. Snyder 

Chemical Engineer 

Mine Safety and Health Administration 

U.S. Department of Labor 

Pittsburgh, Pa. 

ABSTRACT 

Various silicone materials have been proposed for use in mining environments. The catalytic methane sensors now employed are poisoned by silicone 

vapors. Activated charcoal and carbon cloth adsorbents will increase the 

sensor lifetimes in an environment containing significant silicone vapor 

concentrations. The results of a study of catalytic methane sensor operation 

with adsorbent materials in hexamethyldisiloxane-air mixtures are reported. 

Sampling techniques developed for measuring silicone vapor content of mine 

atmospheres are described. 

INTRODUCTION 

The Code of Federal Regulations Title 30, section 75-300, administered 

by the Mining Safety and Health Administration (MSHA), Department of Labor, 

requires methane gas concentration monitoring in coal mines. Such monitoring 

is used both as a measure of mine atmosphere flammability and as a criterion 

for ventilation adequacy. Methane (CH4) action levels are set at less than 

0.25 % CH4 for use of air from nonworking areas, less than 1% CH4 for permissible work areas, greater than 1% CH4 for increase in ventilation, and 

greater than 2% CH4 for automatic deenergizing of machinery operating at 

the coal face. Thus accurate and reproducible methane sensors are mandated 

to insure productive and safe coal mine operation. 
Methane measurement by the use of catalytic methane sensors is 

used currently in most of the methane monitors and detectors underground. With this sensor methane is catalytically oxidized in air 

on finely divided platinum (Pt) or palladium (Pd) ceramic surfaces. 

The resulting heat of combustion increases the temperature of the 

surface and of an inner platinum wire spiral. The methane concentration in air is proportional to the resistance increase of the 

inner platinum wire. The catalytic methane sensor was developed in 

Great Britain by the Safety in Mining Research Establishment (SMRE) 

and has an excellent record of accuracy and stability for measuring 

combustible gases such as methane in mines as long as there is an 

adequate oxygen supply (greater than 10%) and absence of several 

specific poisons for the catalytic surface. 

The catalytic methane sensors using Pt or Pd catalysts are 

poisoned--suffer a loss in response--when brought in contact with 

several classes of chemicals. Manufacturers literature and earlier 

reported studies list several of these chemicals (table 1) . In general, compounds that react with Pt or Pd metals to form highly stable metal compounds--sulphides or phosphates--are poisons as well 

as silicones and lead oxide film formers. Organic halides are 

strongly adsorbed and are classed as temporary poisons. Although 

such poisons for catalytic methane sensors have been identified for 

some time, they did not become a problem during mining operations 

until lately when the commercial use of silicones or phosphate esters became more common. 

Silicone compounds may be used in the mining industry in many 

commercial preparations. These include grease formulations, silicone rubbers, hydraulic fluids, heat transfer fluids in transformers 

and capacitors, oils, and water emulsions for polishes, waxes, and 

aerosol sprays. 

Silicone compounds in these formulations consist mostly of 

linear dimethyl siloxane polymers. They are produced by three 

sources in the U.S.--Dow corning1 , General Electric, and Union 

Carbide. The poly dimethyl siloxane fluids have several unique 

properties: 

Wide fluid or liquid range, -50°C to 100°C 6 Viscosity range dependent on molecular weight, 0.6 cs to 10 cs 

Extreme stability to thermal degradation or oxidation at high 

temperatures, stable at 200°C in air 

High surface wettability. 

Selected physical properties of the linear dimethyl siloxane 

polymers with trimethyl siloxane terminal groups are summarized in 

table 2. The. number of silicon atoms in polymers may extend to 

Use of name brands is for identification only and does not imply 

endorsement by the Bureau of Mines. 
over several thousand for high-viscosity materials. Hexamethyldisiloxane 

is the lowest molecular weight species in the series and has the greatest 

vapor pressure. Silicone fluids have molecular weights from 300 to 1,000. 

More viscous liquids used in antifoam preparations range from 40,000 to 

100,000 molecular weight units or 1,000 to 30,000 centistokes (cs) viscosity. 

Polishes and lubricants range from 100, to 5,000 cs viscosity, and gums 

used in rubber compounding are cross-linked to form the largest and stiffest compounds. The vapor pressure for the polymer system and the concentration of silicone vapor saturated in air is greater than 0.01 ppb for 

molecular weights of 977 or less. Commercial preparations of highmolecular-weight silicones are not pure compounds; molecular weights 

quoted are a number average, and some systems may contain small quantities 

of low-molecular-weight species which may be expected to volatilize during 

use. Commercial specifications for high-viscosity systems state less than 

2% loss in weight on volatilization at 200 OC for 48 hours. (?I* This 

weight loss may consist of volatile silicone species or some entrained 

moisture. Silicone compounds in commercial preparations may be composed 

of silicones other than the dimethyl siloxane system replacing some of 

the methyl groups with phenyl, hydrogen, or halogen atoms, resulting in 

compounds with greater volatility and increased reactivity. 

Silicone rubbers are composed of high-~nolecular-weight cross-linked 

systems and would be considered nonvolatile in finished form. However, 

if the rubber were to contain unreacted monomer or partially reacted 

short polymers, or if the rubber is of the room temperature volatilization (RTV) type and is still fluid and not completely set, then volatile 

silicone compounds will be present and in sufficient quantity could form 

poisons for catalytic sensors. RTV rubbers liberate acetic acid when 

reacting with atmospheric moisture to form reactive Si-OH groups. Systems 

in common use may involve reaction of methyl triacetoxy silane monomer with 

poly dimethyl siloxane a,w diols to form a cross-linked product. 

The boiling point of methyl triacetoxy silane is 90' at 9 rnm Hg pressure. An estimate of vapor pressure at room temperature is 1.8 mm Hg or 

2,400 ppm concentration in saturated air. Until RTV silicone is fully 

polymerized, this compound may poison catalytic methane sensors. 

Silicon plastics, resins, and two-component rubbers that have been 

heat-cured would be expected to have negligible volatile silicone concentrations. The vapor pressure of low-molecular-weight silicone linear 

compounds may be expressed by the following equation dependent on temperature and polymer length (number of silicon atoms): 

Log p = 6.59 + 0.47n - - 972 - 392.2n (10) 

T T 

where p = vapor pressure, mm Hg 

t = temperature absolute, OK 

and n = total number of silicon atoms in molecule. 

The constants were recalculated from more recent data. (8) 

* Numbers in parenthesis refer to References at the end of the paper. 
The saturated silicone concentration in air is calculated from the 

vapor pressure 

s 

C = rx 106 

where C = ppm silicone 

P, = silicone partial pressure, mm HG 

and P = total atmospheric 

EXPERIMENTAL WORK 

A summary of previous work on effects of poisons on catalytic 

methane sensors has been presented in a series of papers by Jack Firth 

and Allan Jones of SMRE, Great Britain. (1) (7) (14) Unprotected and 

charcoal-protected sensors were poisoned iitF hexamethyl disiloxane 

(HMDS) in air-CM4 mixtures, and loss of sensitivity of the palladium 

sensor to methane and other combustible gases was studied. The results 

of the test of poisoning of sensors by HMDS are summarized in table 3. 

The table includes poison effects noted for dimethylsiloxane polymer 

and triethyl phosphate as well. The times for poisoning reported were 

from 10 seconds to 10 minutes for losses in response from 12 to 50%. 

The shorter poisoning times were measured using direct impingement of 

reactive gases on the portable catalytic methane-sensing unit. The 

longer times noted were for poisoning effect of machine-mounted sensors 

which incorporate stainless steel screens and hezvy porous sintered 

metal filters for mechanical protection and flame safety (fig. 1); these 

times reflect the time for diffusion of silicone vapor through the diffusion barriers and adsorption of poison on the porous metal surface. 

The use of activated charcoal for adsorbing the poisons before contact 

with the sensors was extremely effective, increasing life of the sensors 

by 240 times. 

In this study two combustible gas detectors, one with palladium 

and one with platinum sensors, were tested for susceptibility to 

silicone poisoning with HMDS and with RTV silicone rubber fluids. 

The poisoning of the catalytic methane sensors was monitored 

by observing the sensor response to methane during administration 

of a silicone-air mixture. The catalytic sensors were mounted in 

the top of an acrylic cylinder for initial tests and housed in an 

aluminum cylinder in later work. The test apparatus is represented 

in figure 2. The methane test gas was diluted to 2% (and 1%) concentration in a mixing tube; concentration was measured by periodic 

readings from a Riken methanometer by gas refractive index measurement. The methane concentration was intermittently varied from 2% 

concentration to 0%. concentration by control of a solenoid valve 

run by a mechanical timer. Pulse times ranging from 6 minutes air 

and 6 minutes 2% methane to 50 minutes air and 10 minutes 2% methane 

were used in testing. The gas samples were purified by passage through 

an activated charcoal filter before introduction into the silicone 

vapor generation chamber. 
Silicone vapor was generated by bubbling on air-CH4 mixture through 

a fritted disk immersed in HMDS cooled to O°C by ice in a Dewar vacuum 

container. The slicon vapor concentration was formed by using a fixed 

ratio of bypass air to airflow through the silicon bubbler and was calculated as follows: 

Ps Vsilicone flow-air lo6 

C Silicone in air-CH4 mix = p 'total flow 

where C = concentration in ppm silicone 

Ps = 8.6 mm Hg for HMDS at O°C 

P = total air pressure, 750 mm Hg normal 

'silicone = air-CH4 mix flow rate through silicone bubbler, ml/min 

and Vtotal flow = total air-CH mix flowrate, ml/min. 4 

Total flow rates were measured on a wet-test volume meter, noting 

volume measured in a given time period. Silicone vapor generator airflow 

was measured using a standard soap bubble meter measuring time of bubble 

transit through a calibrated volwne. The mixed air-silicone-CHq flows 

were introduced into the test chamber containing the catalytic methane 

sensors with methane response measured on a strip chart recorder. Determination of the silicone vapor concentration was initially performed by 

absorbing the silicone in 100 ml of methyl isobutyl ketone (MIBK) using 

two bubbler absorption columns in series. For runs of 30-minute duration, 

the second absorption column contained 1/7 the concentration of the first, 

and handling, cleaning, and refilling the scrubbers was time consuming. 

Activated-charcoal tubes containing CAL Calgon charcoal were used to 

monitor silicone vapor composition in test gas in later experiments. 

All of the silicone adsorbed was confined to the first 1-cm bed tube; 

HMDS concentration was 54 ppm, and the solid tubes were convenient to use. 

Adsorption experiments were conduct,ed with very low concentrations 

of HMDS in air by the use of diffusion tubes (fig. 3). I-IMDS contained in 

a 200-ml flask was connected to an upper flask with glass tubing of fixed 

diameter and length. The upper flask had inlet and outlet tubes for 

diluent airflow. HMDS vapor moved from the lower flask to the upper flask 

through the tube by diffusion. The vapor pressure of the HMDS was determined by holding the flask at a fixed temperature, either O°C or -63O~, 

by external refrigeration of fluid in a Dewar vacuum container.(l5) It 

was found that gas flow through the upper flask must be constantsince 

oscillation of pressure caused by air from a diaphragm pump increased 

the rate of material transport by convection many times over the rate 

calculated for diffusion. The concentration of silicone in air produced 

by use of the diffusion tube was monitored by adsorbing silicone on 

activated charcoal in a tube, and analysis for adsorbed silicone was 

conducted after the test. 
Analysis of the MIBK absorption liquid or of the activated charcoal 

tubes after extraction of adsorbed silicone (HMDS) by MIBK was performed 

using atomic absorption (AA) spectrometric measurement. The silicone 

solution in MIBK was aspirated into a nitrous oxide-acetylene flame 

at high flow rate, and the silicon optical absorption was determined 

at 2516 A0 using a 2 A0 slit width on a Perkin-Elmer Model 503 unit. 

Standard solutions were made by dilution of a weighed HMDS solution 

in MIBK. Standard analyses were also made with a solid octaphenyl 

cyclotetra siloxane weighed sample dissolved in MIBK. Standard 

curves constructed with experimental optical density plotted versus 

weight-percent silicone were linear. 

Hexamethyl disiloxane was chosen as catalyst poison for this test since 

it could be obtained in pure form; it is very volatile, boiling at 100°C, 

and since it is a small molecule, it is not as easily adsorbed on charcoal 

as larger molecules. In addition to experiments with HMDS, tests with 

both Dow Corning and General Electric RTV silicone rubber fluids were 

conducted. 

Sensor poison tests with RTV silicone rubber fluid (before solid 

formation) were performed by extruding l-l/4-inch lengths of silicone 

at each end of a polyethylene 3/4-inch-OD tube. The air-/% CH~ mixture was 

passed through the tube and over the sensor on test. Vapor pressure 

of silicone in RTV rubber fluid was measured by flowing a known volume 

of air through sample tubes with l-1/4-inch lengths of RTV silicone 

rubber fluid and adsorbing the vaporized silicone onto a CAL charcoal 

bed 1 cm thick in another tube. Thirty-minute runs were conducted at 

flow rates from 1 to 1.1 l/min with fresh RTV silicone added at 15-minute 

intervals. The charcoal test samples were analyzed for silicone content 

after extraction of adsorbed silicone with MIBK by the previously described 

AA technique. 

The ability of silicone compounds to poison catalytic methane sensors 

depends on the volatility of the silicone and on transport of the siliconeair mixture by air currents to the sensor. Because of ventilation requirements in coal mines--3,000 cfm airflow minimum into the working area--formation of 5 ppm silicone vapor concentration would require vaporization 

and mixing of 3 grams of silicone compound per minute. Low-molecular-weight 

silicone compounds in fluids or as impurities in high-molecular-weight 

viscous systems may yield significant silicone volatile species from 

commercial products or by depolymerization during use. Only by tests 

of the poisoning ability of commercial mixtures can the actual volatile 

silicone content or ability to poison a sensor be ascertained. 
Initial experiments in poisoning of catalytic methane sensors were 

performed with HMDS as poisoning agent at 54 ppm concentration in air and 

in air-2% CH4 mixtures. Platinum and palladium sensor methane monitors 

were tested with and without CAL charcoal over the sensor to remove silicone vapor by adsorption. The time response for methane indication during 

poisoning is presented in figure 4. A 10% loss in response was obtained 

for both sensors in 15 minutes without protection. With a 2-mm thickness 

of CAI, charcoal protecting the sensor, runs were obtained for 10% loss in 

response in 42 hours and 144 hours for palladium- and platinum-type sensors, 

respectively. Although the silicone concentration in the air inlet to the 

test chamber was 54 ppm, initial values of 18 to 22 ppm silicone obtained 

from the chamber exit air show significant adsorption by the chamber contents: four sensors, two coatings of CAL charcoal, four sintered-metal 

flame suppressors, and the chamber walls. 

Experiments on platinum sensor methane monitors were conducted at very 

low levels of HMDS generated by a diffusion tube held at -63'~ by mechanical 

refrigeration. The average assault concentration of HMDS during test was 

5.5 ppb. The concentration of HMDS ranged from 16 ppb initially to 3.77 ppb 

in the final test, and 4,000- to 7,900-minute tests were made to obtain 

enough HMDS on each series of CAL charcoal sample tubes for AA analysis. 

The response of the monitors was measured in series for the HMDS measurement, 

and breakthrough of the low concentration of HMDS vapor was noted on two 

tests for these very long trapping times. 

Silicone vapor poisoning of catalytic methane sensors may be delayed by 

adsorption on activated charcoal surrounding the sensor elements. The 

thicker the layer of charcoal, the longer the sensor can operate without 

being poisoned. The thicker layers of charcoal cause a longer sensor 

response time. Practical methane sensors require rapid response as well as 

protection from poison, so there is a limit to the thickness or adsorbent 

material that can be used. For a response time of 1 minute or less the 

maximum adsorption layer thickness allowed is 2mm for this sensor. 

Tests previously conducted used times of sensor response loss in methane 

as a measure of protection offered by a given adsorbent. Possible protection 

effect may also be assessed for a material by measuring the amount of adsorption of silicone vapor on a known amount of absorbent. The adsorption isotherms for HMDS on various charcoal substitutes were measured by equilibrating 

the test substrate with known concentrations of HMDS in air, and the amount of 

HMDS adsorbed was measured after extraction into MIBK with analysis by AA 

spectrometry. 

An adsorption isotherm for HMDS on CAL charcoal is presented in figure 5. 

The amount of HMDS adsorbed can be determined from this figure for known concentrations of silicone.vapor in air. Although more HMDS is adsorbed at high 

concentration, sufficient HMDS is still adsorbed at low concentrations to 

protect sensors. Saturated (77OF) values of HMDS adsorption for various carbon materials can be used to screen possible adsorbents, and equilibrium 

values are listed in table 4 for dry air-HMDS systems. In terms of total 

quantity of HMDS adsorbed, the following preference in substrate is indicated: 

CAL, PCB, MDA cloth, CDE cloth (20 cal/gm heat of wetting with benzene). 
CONCLUSIONS 

Catalytic methane sensors are poisoned rapidly by volatile silicone 

materials, confirming early work performed by SMRE, Great Britain, and manufacturers' literature. Once a catalytic methane sensor in use in a mine 

is poisoned by silicones or related poisons, the instrument may incorrectly 

indicate safe methane concentrations when possibly high concentrations, in 

explosive range, are present. Silicone compounds may be incorporated in 

transformer coolants, hydraulic or brake fluids, lubricants, sealants, 

waxes, polishes, and other mixtures available or intended for use in 

mines. 

Volatile silicone compounds or other poisonous agents should not 

be introduced into mine atmospheres when methane detectors or monitors 

are in use, unless appropriate steps are taken to assure that harmful 

monitor or detector poisoning does not occur. Should it be suspected 

that such a silicone has been introduced into a mine immediate steps 

should be taken to test each methane detector o monitor to ascertain 

performance accuracy using known methane-air (12f - mixtures. Siliconecontaining materials may be identified by labeled contents listing, 

or by chemical analysis for silicon by wet chemical, emission spectroscopic, or AA spectrometric techniques. The presence of volatile silicone compounds in a mine atmosphere may be tested by adsorption of 

the silicone vapor on a CAL activated charcoal 1-cm-thick trap using 

a permissible air sample pump to deliver a 1- to 4-l/min flow rate. 

The adsorbed silicone can be eluted into MIBK and analyzed for silicone 

content by the AA technique described. 

Protection of catalytic methane sensors by adsorption of poison before 

contact with the sensor element is practical for low concentration levels 

of silicones. Thin layers of adsorbent on diffusion heads are necessary 

for protection so that minimum effect will be obtained on response times. 

The Mine Safety and Health Commission of the European communities 

has proposed the use of a layer of activated charcoal cloth covering the 

sensor element for protection of handheld methane detectors. The use of 

such protection may become mandatory in Great Britain, France, and Germany, 

for example, requiring demonstration of protection of handheld methane 

detectors against 5 ppm HMDS concentrations in air-CH mixtures for a 4 test time of 1 to 2 hours without loss in response greater than 10%. 

Regular maintenance of such units is recommended, including replacement 

of the charcoal cloth during maintenance checks every 2 to 4 weeks. 

It is recommended that manufacturers of catalytic methane sensors, 

recognizing the increased importance of protection from poisoning 

with the proposed use of new mining compounds, would market such protective devices to prevent poisoning of the sensors by attachment of an 

adsorbent to the present diffusion-type methane monitor or detector. 

Manufacturers should recommend also a maintenance program with periodic 

replacement of adsorbing materials to assure device reliability. 

This study has focused attention only on the use of adsorbents to 

remove poisons from contact with catalytic methane sensors. It is also 

possible that suitable oxidation catalysts may be developed to react 

with poisons to form nonpoisonous products that will give a longer 

protective life and require less frequent replacement. 
ACKNOWLEDGEMENTS 

We wish to acknowledge the guidance given by Robert W. Dalzell, 

Chief, Division of Health Technology, MSHA, and the work by Lewis D. 

Raymond, Analytical Chemist, MSHA Technical Support, Pittsburgh, for 

silicon analyses. 

REFERENCES 

1. Firth, J. G., A. Jones, T. A. Jones. Flammable Gas Detectors. 

Proceedings of Symposium on Chemical Process Hazards V, Institute of 

Chemical Engineers Symposium Series 39 (a), pp 307-21, 1974. 

2. Cooper, L. R. General Considerations of the Design of Instruments 

for Use in Coal Mines. Institute of Physics Conference Series 34, 

Bristol 1976, London, pp 27-38, 1977. 

3. Bacharach Instrument Company, 2300 Leghorn St., Mountain View, CA 94043 

Instruction Manual, Model CD 800 Combustible Gas Alarm System. 

4. General Monitor Inc., 3019 Enterprise St., Costa Mesa, CA. 

Instruction Manual Model 180 Combustible Gas Monitor. 

5. Mine Safety P.ppliances Company, Braddock, Thomas and Meade Streets, 

Pittsburgh, PA. 15208. Combustible Gas Detector Model 510 Instruction 

Manual. 

6. Fire Surveyor. Gas Detection-Flammable Gas Detectors. pp 26-39, 

April 1977. 

7. Firth, J. G., A. Jones, T. A. Jones. The Use of Catalytic Sensing 

Elements for the Detection of Gases. Ann. Occup. Hyg. 15, 321-6, 1972. 

8. Data from General Electric Co., Silicone Product Division, 1978. 

9. McGregor, R. R. Silicones and Their Uses. McGraw Hill, 1954. 

10. Encyclopedia of Polymer Science and Technology, vol. 12, Interscience, 

1970. 

11. Noll, W. Chemistry and Technology of Silicones, Academic Press, 1968. 

12. Barrett, R. E. Memorandum to all mine operators, January 17, 1978. 

Introduction of possible poisons of catalytic mixtures sensors into 

mine atmospheres. 

13. Mulford, C. Perkin-Elmer Atomic Absorption Applicability Study #225, 

1967. 

14. Gentry, S. A., A. Jones. Poison Resistant Flammable Gas 

Sensors. International Symposium, The Monitoring of Hazardous 

Gases in the Working Environment. The City University, London, 

December 1977. 

15. Analytical Instrwnent Development, Inc., Avondale, Pa., 19311 Application Information Data AN 204A, Diffusion Tubes. 
Table 1.--Catalytic methane sensor poison classes 

Conpound Structure Reference 

Organic Halides ........ R- X 1, 4, 5, 6, 7 

X = C1, Br, I 

Phosphate .............. R-OP- (0 R)3 1, 6 

Metal............. ..... Pb - (C2Hs)4 1, 3, 4, 5, 6 

Sulfide ................ R2 -S 4, 5, 6 

Other .................. (mist) 

TABLE 3.--Catalytic methane sensor poisoning effects 

Material Life Life, charcoal % Response Reference 

protected 

MS 200/10 

silicone oil. ..... 

Triethyl 

phosphate ......... 

Hexamethyl 

siloxane (HMDS) , 26 pprn............ 

HMDS, 40 ppm ........ 

HMDS, 54 ppm. ....... 

5 min 20 hr 

5 min 20 hr 

26 sec 22 hr 

10,30 sec - 

10 min* 

8 min** 

167 hr* 

99 hr** 

20/2% CH4 SMRE data 

15/2% CH4 do. 

33/1% CH4 do. 

12 Present 

12 work 

25 

5G 

* Pd sensor ** Pt sensor 
OON 

0 \D In b N 

boo 

moo n 

0 0 0 

0 0 0 

000 

n n n 

OOM 

m 0 b 4 4 
TABLE 4.--Equilibrium adsorption of HMDS by 

charcoal substrates. 25 to 27 OC 

Substrate HMDS adsorbed per 

g charcoal. mg 

Granular : 

CAL. Calgon ......................... BPL. Calgon ......................... PCB. Calgon ......................... 

WVG Nuchar. Westvaco ................ h'VH Nuchar. Westvaco ................ PC1333. Barneby-Cheney .............. 

Foam carbon: 

RVC 4x5 80A. Chemotronics Ir~tl ...... 33 

Cloth: 

CC.2. MDA Scientific ................ 265. 271 

Chemical Defense Establishment. 

Great Britain: 
AI,O, coating with 

Pt or Pd catalyst 

Pt wire 

2 mil 

Catalytic sensor 

Porou s 

metal - 

- 

Pt sensor head 

steel 

en 

Porous 

metal 

Pd sensor head 

WY' 

FIGURE 1.--Catal,ytic Methane Sensors 

19-1 3 
Bypass rotometer n , 

Bypass 

r---------- 

I 

Charcoal trap I HMDS rotometer 

I J 1 Test chamber 1 

Dewa r 

CaI charcoal tu

FIGURE 2.--Catalytic Sensor Test Apparatus 
FIGURE 3.--Diffusion Tube Apparatus. 
FIGURE 4.--54 ppm Hexamethyldisiloxane Poisoning of 

Catalytic Methane Sensors 
6/6w '1~03~~~3 N1 SaWH a388osav 

FIGURE 5.--Adsorption Isotherm Hexamethyldisiloxane. 77 OF