Seutember, 1927
I N D U S T R I A L A N D ENGI.VEERI,VG CHEMISTRY
1021
The Electrical Resistance of Wood as a Measure of Its Moisture Content’ By Alfred J. Stamm U. S. FOREST PRODUCTS L~BORATO MADISUX, RY, 11-1s
This investigation indicates broadly the effects of moisture and certain associated experimental and natural factors on theielectrical resistance of wood, thus defining in greater detail than hitherto the limitations of resistance methods for determining average moisture contents and showing t h a t such methods would be of practical value in the lumber trade only in special cases. The experiments check Hasselblatt’s general findings that below the fiber saturation point the logarithm of the electrical resistance of the wood decreases directly as the moisture content increases. The variation is such t h a t the resistance increases 1.8 fold for a decrease of one per cent absolute moisture content. A t a given moisture content the ordinary resistance law for the dimensions of solid conductors (Rm;)
seems to hold.
The degree of
tightness with which the surface electrodes are clamped changes the observed resistance considerably, but with electrodes of proper type and with careful clamping, the
error in calculated moisture content introduced by slight differences in pressure may be neglected. In making moisture determinations by this method, the effects of species and density of wood, direction of flow of current through the wood (parallel or perpendicular to the fibers), temperature, ash content (within reasonable limits of variation), and extent of specimens beyond the electrode surfaces either are negligible or can be corrected for in many cases. Condensed moisture on surfaces parallel to the direction of flow of current and uneven distribution of moisture in the wood must be avoided. By the resistance method as here described, the moisture content of specimens below the fiber saturation point having a uniform distribution of moisture can be, in general, determined within one per cent absolute moisture content. For specimens of non-uniform moisture content, t h e use of pointed or blade electrodes which may be inserted into the board is suggested as a means of determining moisture content a t the center of the piece.
...... and other REVIOUS investigations on soil,2 hygroscopic materials4 of low conductivity show that their electrical resistance decreases very rapidly with an increase in moisture content. Hasselblatt’s data for birch indicate that the logarithm of the electrical resistance decreases in direct proportion to the increase of moisture content up to what appears to be the fiber saturation point. Above fiber saturation Hasselblatt found very little change in resistance with increased moisture content. A decrease to 56.8 per cent of the original resistance corresponded roughly to a moisture increment equal to one per cent of the weight of the dry wood. As any definite multiple change in the resistance corresponds t o the same absolute additive change in moisture content, all deviations considered in this paper will be presented on the basis of the absolute deviation in moisture content rather than on a percentage deviation basis. It must be borne in mind that the percentage deviations a t low moisture contents will be much greater than a t high moisture contents for the same absolute deviation. The findings of Kujirai and Akahira indicate a similar logarithmic relationship between the moisture content and the electrical resistance for the fibrous materials which they studied. This rapid change in resistance with moisture content below fiber saturation suggests the possibility of developing a quick and simple electrical method of determining the moisture content of wood. The accuracy of such a method and its adaptability to commercial requirements depends upon the effect of a number of factors associated with the testnamely, the amount of pressure applied and the type of contact between electrodes and wood, dimensions of specimens, extent of specimens beyond the electrode surfaces, direction of flow of current through the wood (parallel or perpendicular to the fibers), species and density of the wood; temperature,
P
Received April 25, 1927. Whitney, Gardner, and Briggs, Bur. Soils, Bull. 6 (1897). 8 Hiruma, Extracts Bull. Forest E r p t . Sfa.. Meguro (Tokyo). 69 (1915); Hasselblatt, 2. anorg. allgem. Chem., 154, 375 (1928). Curtis, Bur. Sfandards, Bull. 11, 359 (1915); Kujirai and Akahira. Sci. Pafiers, Ins;. Phys. Chem. Research ( T o k y o ) , 1, 96 (1923). 1
2
ash content, distribution of moisture in the specimens, and the presence of moisture on surfaces parallel to the flow of current. All these factors, as well as the fundamental relationship between resistance and moisture content were therefore examined in the experiments to be described. Only sufficient data were collected in each case to show in a semiquantitative way the general effect of each factor on the measurement~.~ Apparatus
The Megger insulation tester6 was used for the electrical measurements. Its principal parts are a hand-driven 1000volt, direct-current generator and a galvanometer, both operating between the same electromagnets. A constant voltage is assured when the generator crank is rotated at a speed above 120 r. p. m., as a friction clutch on the armature shaft automatically slips a t that speed so that the armature speed is not increased. The circuit containing the material whose resistance is t o be measured is connected in series with the generator and in parallel with the galvanometer, so that the greater the resi.stance of the specimen to be tested, the larger will be the proportion of the curren; flowing through the galvanometer. The instrument is calibrated to read directly in megohms resistance and has a range of 1 to 2000 megohms. Type of Contact between Electrodes and Wood
Three different kinds of surface electrodes were tried(1) plain copper plates 7.5 by 7.5 em. made from 1-mm. plate; (2) 7.5 by 7 . 5 heavy brass plates with routings 2 mm. wide and 2 mm. deep cut in both directions to give a rough cleated surface that could be pressed into the surface of a board; 8 Since this investigation was completed, a publication by Meyer and Rees. New York State College of Forestry, Tech. Bull. 2 6 , No, 19 (1926), has appeared on the determination of the fiber saturation point of wood by a n electrical resistance method. The authors suggest t h a t the method might be used for determining the moisture content of wood, but no examination of the various factors involved was attempted by them. 1 Thanks are due the Wisconsin Power & Light Company for the use of this instrument.
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I N D U S T R I A L A N D ENGINEERING CHEJfISTRY
(3) 5.0 by 5.0 cm. brass plates with soft lead faces which, under pressure, would conform to the surface of the board. Threefold deviations in the resistance of a one-inch mecimen were obtained when the plain copper-plate electrodes were held with a wooden screw clamp. The same order of deviations was noted by Butterfield' when using sheet-iron electrodes backed with fine sand. The other two types of electrodes, when set a t a reasonably firm pressure, gave results that generally checked within 20 per cent, the routed electrodes requiring the tighter clamping for equal constancy of readings. The lead-faced brass electrodes were therefore used in most, of the experiments. The wood surfaces were all plane, but were not specially dressed or surfaced before making the measurements. Dimensions of Specimen
The effect of the thickness of the specimen (distance between electrodes) for different degrees of tightness of electrodes was tested on a series of transverse sections of Douglas fir 5 cm. square. These had stood exposed to room conditions for months, equal moisture content and uniform moisture distribution being thus assured. The results (Figure 1) show that the resistance varies directly with the thickness, within experimental error, over the range tested. The three lines represent three different degrees of tightness of electrodes. When the clamping is as firm as can be made by hand without a special mechanical device, the contact resistance is negligible in comparison with the effect of other uncontrollable variables for all but very thin sections, as will be seen later. The effect of the Electrical Figure Resisfunce I - Thtcknerr c o n d u c t o r cross 1200 Douqias fir Pansverse ier'ionr 9 section was tested with two one-inch specimens of Sitka ) spruce of the same moisture content. one 5 cm. square and the other 7.5 cm. square. The first, fitted w i t h the 5 - c m . l e a d f a c e d electrodes, s h o w e d a resista n c e of 8 2 megohms. T h e sec8 Very fiqhf dornplng ond, fittedwith the 7.5-cm. plain elec, t r o d e s , gare 42 0 0.5 IO 1.5 2.6 megohms, and Thickness in cm. w i t h t h e routed brass e!ectrodes, 35 megohms. All three resistance figures are averaged from six determinations each. These results, calculated to unit cross section by the inverse law of areas. are 2050 megohms for the first specimen and 2360 and 1970 megohms, according to the electrodes, for the second. The second value is naturally high because of the impossibility of reducing the contact resistance of the plain brass electrodes t o a negligible value. The good agreement between the first and last values is considered as a close enough verification of the inverse law of areas for the purposes of this study.
Vol. 19, No, 9
of the log), and the radial (perpendicular to the fibers and to the annual rings). The results are shown in Table I. T a b l e I-Variations
SPECIES Western red cedar Sitka spruce Alaska cedar Douglas fir
in the Electrical R e s i s t a n c e of W o o d i n Its Different Structural Directions ELECTRICAL RESISTANCE, R hlOISTURE LongituTanDENSITY CONTENT dinal Radial gential Per cent Megohms Megohms Megohms 9 22 24 0.336 14.0 0.417 15.7 10 18 20 0 547 15.6 18 27 27 0.584 15.3 11 21 23
The resistance in the tangential direction is only slightly greater than in the radial direction but about twice as great as in the longitudinal direction. The same sort of variation was found by Hiruma. The practical equality of radial and tangential resistance is of interest because it eliminates the "grain" factor in two directions as a variable in making resistance-moisture determinations. Moisture Content-Species-Density
Table I1 is a summary of experimental data on moisture content and electrical resistance of wood in a transverse direction a t 28" C. for several species of different densities and variable thickness of specimen. The lead-faced electrodes were used. To make the results comparable they have been converted to a specific resistance basis, which has been previously shown to be justifiable. The data for all the species plotted in Figure 2 show that within the moisture range examined the logarithm of the specific resistance has a linear relationship to moisture content. The data show no appreciable variation on account of the simultaneous variation of species and density. Hasselblatt found that variations in the electrical resistance were entirely within experimental error for the different specimens of birch of variable density which he used and also for changes from heartwood to sapwood It is thus very probable that variations in the species alone would have the same negligible effect. The slope of the average line through all the points indicates that the resistance decreases to 55.0 per cent of its original value for an increase of one per cent moisture content. This corresponds closely to the decrease to 56.8 per cent calculated from Hasselblatt's data between the same moisture-content limits. The calculated moisture contents for the different specimens taken from the curve are given in the last column of Table I1 and show the order of deviations from the straight-line relationship.
" " ' 7
T a b l e 11-Actual
7
Electrical Engineering Thesis, Purdue University, 1910.
___
M O. I. sT lrRR .. _ .
SPECIES Western red cedar
Sitkaspruce
Alaskacedar
Douglas fir
Direction af Current with Respect to Grain
The resistance of several cubes 5 cm. on a side were determined in each of the three structural directions of the woodnamely, the longitudinal (parallel to the length of the fibers), the tangential (perpendicular to the fibers and to the radius
a n d Calculated Moisture C o n t e n t s for Different Species
Basswood Western hemlock
MOISTURE CONTENT CONTENTTOTAL SPECIFICOBTAINED THICK- O B RESIST-RESIST- FROM DENSITY NESS SERVED A N C E , R AIVCE, R, CURVP % w t . Ohms X Ohms X %wt. Cm. dry znood 10-8 10-8 dry wood 2.0 14.4 40 5.0 13.0 0,336 1.2 15.4 14.0 24 5.0 0.336 222.0 7.0 6.3 2000 0,336 2.25 37.8 9.8 348 2.25 9.4 0.336 2.0 14.5 18 2.25 14.4 0.336 0.417 5.0 14.7 26 1.3 15.2 0.417 5.0 15.7 20 1.0 15.7 0,417 1.45 7.3 1600 276.0 6.7 0.473 2.7 10.45 320 29.6 10.3 0,547 5.0 14.3 35 1.75 14.7 L O 15.6 27 1.35 15.2 0,547 0,547 2.4 8.1 1600 156.0 7.8 0,547 2.4 6.9 2000 208.0 7.3 0,584 5.0 14.1 30 1.5 15.1 15 3 23 1.15 15.4 0.584 5 0 0.584 1.65 6.3 2200 334.0 6.3 0.584 1.65 6.8 1800 273.0 6.7 0 395 2 0 8 1 800 100 0 8 3 0 407
17
7 3
1800
265 0
6 9
It is the steep logarithmic relationship thus established that permits considerable variation in the resistance without
September, 1927
I,VDUSTRIAL A,VD EYGI.VEERI,VG CHEMISTRY
IargeIy affecting the calculated moisture content. The unimportant effect of variations in species and density, however surprising, is no doubt a consequence of the same relationship. The lightest of the woods tested, western red cedar, tends to give slightly lower resistance than the others, but practically the deviations are within the experimental error. If the measurements were so made that the current flowed longitudinally instead of transversely through the wood, the moisture content as determined from Figure 2 would be consistently one per cent too high; for, as has been shown, the longitudinal resistance is practically one-half of that in the transverse direction.
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were heated while clamped in place and then allowed t o stand until the temperature of the wood and that of the electrodes differed by less than 1" C. The electrical resistance of the wood was then determined. The results, shown in Table 111, indicate a falling off of resistance with increasing temperature, a phenomenon contrary to the behavior of ordinary solid conductors but similar to that of electrolytic conduc13.0 I
Extent of Specimen beyond Electrodes
If the method is to be of any practical value, it is highly desirable that the resistance measurements be made upon specimens the cross sections of which are infinitely large in comparison with the area of the electrodes, thus eliminating the necessity of cutting specimens from the boards to be tested. A few experiments were therefore made to investigate the decrease of resistance when the wood extended appreciably beyond the electrodes. A basswood specimen 21.5 by 19.5 cm. and 2.7 cm. thick gave 450 megohms with the 5 cm. square lead-faced electrodes, whereas a section 5 by 5 cm. cut from the same piece gave 800 megohms. Similarly, a Sitka spruce specimen 10 by 15 by 2.7 cm. gave 240megohms, whereas a 5 by 5 cm. section of the same thickness gave 350 megohms. I n the first case increasing the cross section of the specimen 16.4 times caused a decrease in resistance of 44 per cent; in the second a sixfold increase in cross section caused a decrease in resistance of 32 per cent. This shows that the rate of decrease in resistance diminishes rather rapidly with an increase in the total cross section. The thinner the section the less important becomes the decrease in resistance upon extension. Sections 5 cm. square and 18 by 24 cm. cut from 0.2 cm. thick veneer gave 1800 and 1600 megohms, respectively, showing a decrease in resistance of only 11 per cent for a surface 17.3 times as great as the original. From the theoretical considerations it would be expected that the tendency of current density to become uniform over the whole specimen, rather than to drop off abruptly beyond the part immediately between the electrodes, would increase with an increase in conductivity-in wood, specifically, an increase in moisture content-and an increase in the thickness of the specimen. Thus, in pieces of large area and considerable thickness the resistance would become an exceedingly complex function of the moisture content, so that a complete tabulation of the moisture-resistance relation for large-size stock between zero moisture content and fiber saturation could best be worked out empirically. Nevertheless, within a very common range for practical determinations, the correction of moisture value necessitated by extent of wood surface is small, and in some cases can be evaluated with sufficient accuracy or be neglected altogether. Thus for reasonably dry veneer, no correction for extent of specimens need be made, while for reasonably dry 1 l/,-inch stock it appears that the correction would not exceed a subtractive correction of more than one per cent absolute moisture content when making calculations from Figure 2. It would be preferable in general practice, however, to use a standard computation curve obtained upon the type of stock under consideration. Temperature
The effect of temperature change was tested on a Sitka spruce specimen 2.7 cm. thick, fitted with the brass-routed electrodes. A hole was bored in the end of the piece 2 cm. deep and a mercury thermometer inserted. The electrodes
\
a Douafas f i r Re2 Cedur c, Alaska Cedar n western Hemiock
4
'?
8.0
Sitka Spruce
Q
e
7.0
Basswood
-
6.0
I
50
I
I
I
I
1
I
1
I
Butterfield found that the resistance doubled when the temperature was lowered by 11" C., a result which checks the present findings fairly well. Thus it appears that for measurements made a t 16" C., one per cent would have to be added to the moisture content, and for measurements a t 40" C. one per cent would have to be subtracted from the moisture content obtained from Figure 3, which was determined a t 28" C.
tion.
T a b l e 111-Effect of T e m p e r a t u r e o n Electrical Resistance TEMPERATURE DROP TO CORRESPONDING DETERMINATIONTEMPERATURB R DOUBLING OF R c. Megohms c. 1
28.5 32 5 40.0
2
28.2 32.0 39.2
;;
40 130 100 80
12.3 12.8
Ash Content
As stock of considerably variant ash content was unavailable, the ash content was varied by the absorption of sodium chloride from solution. A specimen which had absorbed 1.1 per cent of the salt showed a resistance, corrected t o the original moisture content, of 120 megohms, as compared with an original resistance of 490 megohms. Thus the re;istance is reduced to one-fourth its original value by an increase of about one per cent sodium chloride. or (apparently) halved by each increase of about 0.5 per cent. Only in exceptional cases is the variation in ash content as great as this; furthermore, as a large part of the natural ash in wood is insoluble, it is probable that the variations in calculated moisture content introduced by variations in the natural ash content are much less than one per cent. Butterfield investigated the effect of zinc chloride on the resistance. He found that the resistance, a t several different moisture contents, of sections containing 1.5 per cent zinc chloride is about one-fifth that of the untreated sections. I n this case, also, the resistance seems to vary inversely with the concentration of the salt in the wood, so that a t a given moisture content the resistance would be halved for an increase of about 0.6 per cent zinc chloride.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1024
Surface Moisture
DEPTH
Cm.
The effect of a film of surface moisture was examined. A dry specimen that offered a resistance of 2000 megohms was moistened in a streak from electrode to electrode with a wet cloth. The instrument was short-circuited, indicating a resistance of less than one megohm and thus showing the extremely high conductivity of the surface film in comparison with that of the bulk wood. Tests were made to see if a similar conductive film would be formed on the surface by absorption of moisture a t a relative humidity above the dew point. The resistance of specimens in equilibrium with room humidity and temperature, 72 per cent and 28” C., respectively, was halved by exhaling the breath on the surface. In the same manner the resistance of a somewhat drier specimen was reduced to one-fourth its original Talue. It is thus evident that measurements made under humidity conditions that would give a higher equilibrium moisture content for the wood than that which it possesses, tend to give low resistances because of the conductivity of the surface film. Calculated moisture content results might be in error by a considerable percentage for relatively dry pieces of wood when the humidity is high a t the time the measurements are made. Moreover. if the temperature of the surface is below the dew point, a short circuit will result and no measurements can be made. The resistance method is thus completely eliminated for boards whose surface has recently been wet, and it becomes uncertain under high humidity conditions. Unequal Moisture Distribution
Measurements were made to determine the effect of variations in the moisture distribution through a section. Four pieces of veneer 0.2 cm. thick a t moisture contents of 15.0, 27.7, 6.4, and fi.2 per cent, respectively, and averaging 13.8 per cent, were clamped together between the electrodes. With the first and second outside, the resistance was 450 megohms; with the first and second inside the resistance was 560 megohms. These results shorn a relatively inconsiderable effect of the arrangement of the layers, greater moisture content on the surface giving only a slightly lower resistance when it is not continuous between electrodes. Under normal conditions there would be a moisture gradient in all three directions in a piece of wood rather than only (as in this experiment) in one direction. This in itself would tend to produce a considerable difference in electrical resistance between specimens having a drying or an absorbing moisture gradient, even though the moisture contents were equal. Surface conductivity is not the worst part of an uneven distribution, however, as will be seen from the moisture content, 7.8 per cent and 7.4 per cent, respectively, calculated from the above resistances. The reason for this large deviation horn 13.8 per cent, the actual average moisture content, can be readily understood when the individual resist1 ances of the sections as measured separately-namely, megohm, less than 1 megohm, 400 megohms, and 400 megohms; total, 801 megohms-are added together, giving a calculated moisture content of 6.9 per cent. It is because of the logarithmic relationship between resistances and moisture content that it is impossible to get an nrithmetric average of moisture contents of the laminae. This effect is further illustrated by resistance measurements on a 76 by 10 by 5.4-cm piece of Sitka spruce that was coated on the ends and edges with shellac and tinfoil while green and then dried for 18 hours at 160” F. (71” C.) and 15 per cent relative humidity. I t s moisture condition, as calculated by TuttleJ8was then as follows: 8
J . Franklin I n s t . , 203, 609 (1925).
At surface 0.63 1.26 1.89 2.52 Average
Vol. 19, No. 9 MOISTURE Per cent 2.8 13.0 20.0 25.5 25.6
19.7
This piece of wood had a resistance across the gradient of 1700 megohms, giving an apparent moisture content of 7.7 per cent as compared with an actual moisture content of nearly 20 per cent. This result again shows that when the resistances of the various laminae in series are added together, it is the high values that are most effective in determining the calculated moisture content. Electrolysis and Electroendosmose X o effort was made t o study the effect of electrolytic action and the extent of electroendosmose caused by the use of direct current a t moisture contents below the fiber saturation point, as has been done in the case of saturated sections.9 An unpublished Forest Products Laboratory report by Taylor10 shows that the d. e. resistance may build up to twice the a. c. resistance after a considerable time. Taylor attributes the effect entirely to polarization, but a t least a part of it may be due to the building up of an uneven moisture distribution by the electroendosmotic motion of the water.11 For the purpose of the present study, however, such relatively gradual changes in resistance are unimportant as compared with the immediate and large effect of moisture content.
Conclusions
The results of this investigation show that the determination of the electrical resistance of wood as a means of measuring its average moisture content using surface electrodes is practical only within narrow limits. For stock in which the moisture content is below the fiber saturation point and approaches an equilibrium condition (does not possess a steep moisture gradient), and which does not have an appreciable surface film of condensed moisture tending to shortcircuit the electrodes, the average moisture content can be determined by the resistance method to an accuracy of about one per cent absolute content moisture. Factors such as contact resistance, species and density of wood, direction of flow of current, temperature, ash content (within ordinary limits of variation), and extent of specimen beyond the electrodes are either negligible or corrections can, in many cases, be made for them. Although the limitations to which it is subject eliminate resistance measurement as a method of determining the average moisture content of wood under widely variable conditions, it may possibly be applied in special cases as the measurement of the moisture content of thin veneers, which soon attain moisture equilibrium with their surroundings and are not likely to retain steep moisture gradients. It could be used for any wood, such as furniture stock, that had been thoroughly seasoned and had been kept for a considerable time under fairly constant humidity conditions. The method is not applicable for following the seasoning of stock in a dry kiln, or for determining the moisture content of ordinary lumber shipments which may have been only partially seasoned or may have been exposed to extreme humidity changes in transit. 0 Stamm, “Electroendosmose through Wood Membranes” (Saturated Sections), Colloid Symposium Monograph, Vol. I V (1926). 10 “Effect of the Electrolysis of Maple Wood on Its Electrical Conductivity.” 1 1 Electroendosmose has been regarded by Evershead, J . I n s t . Elcc. Eng. (London), 69, 51 (1914),as the cause of variation in resistance of moist porous insulators with change in applied d. c. voltage.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
September, 1927
These experimental results suggest, however, that the method could be used for determining center moisture content of stock, over a sufficiently small thickness range so that the moisture gradient effect would be small by using side-insulated point or knife-blade electrodes which can be pressed into the interior of the wood. Experiments along
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this line are now being carried on by the author a t the*Forest Products Laboratory. Though the method is obviously limited in its application, it is hoped that it can be uqed to meet several special needs perhaps better than the over-all resistance method using surface electrodes as described in this paper.
Influence of Diffusion of Oxygen on the Rate of Combustion of Solid Carbon' By J. T. Ward,z and J. B. Hamblen FUEL AND GAS ENGINEERING, I v ~ A S S A C H U S E T T S INSTITUTE
R O N gas analyses and other data taken during the operation of a hand-fired furnace a t firing rates of 19.7 to 185 pounds of fuel per square foot per hour, the U. S. Bureau of Mines3 showed that the oxygen disappeared 3 to 6 inches above the grate, that the carbon dioxide reached a maximum a t about 4 inches from t'he grate, and that the composition of the gases was practically independent of the rate of air supply. W. K. Lewis4 averaged these gas analyses and from the shape of the resulting smooth curves concluded that the rate O2 = Conis limited only by the rate of of the reaction C diffusion of oxygen through the gas film around each particle of carbon. This conclusion was based on the following facts: (1) The carbon dioxide content of the gases rises rapidly a t the start, and its initial slope varies but little with the air velocity; (2) the oxygen content falls sharply a t the start practically independent of air velocity; ( 3 ) in the initial stages the carbon dioxide curve is approximately the reverse of the oxygen curve. Although in the Bureau of Mines tests the air velocity was increased approximately tenfold, the oxygen disappeared a t nearly the same distance above the grate in every case. Since the increase in air velocity gives decreased time of contact, this may be explained only on the basis tJhat diffusion is the controlling factor or that the specific reaction rate increases in proportion to the velocity. The data show that the increase in fuel bed temperature is not sufficient t o account for such a large increase in the specific r e d o n rate. However, this oxygen disappearance can be easily explained on the basis that diffusion is the controlling factor. It is known that the thickness of the film on the surface of a solid particle is decreased approximately in proportion to the increase in gas velocity. Furthermore, Rhead and Wheelers have shown that the rate of this reaction increased 400 per cent, even between 350' t o 500' C., and hence it may be concluded that the rate of chemical reaction of carbon and oxygen is substantially instantaneous a t fuel bed temperat'ures. Therefore, the rate of combustion must be determined by the rate a t which oxygen can diffuse through the film to the surface of the carbon. Further data on gas composition a t several points in fuel beds of various thicknesses, which are in accord with those of the Bureau of Mines, have
F
+
Presented before the Division of Gas and Fuel Chemistry a t the 73rd Meeting of the American Chemical Society, Richmond, Va., April 11 to 16, 1927. * Assistant professor of chemical engineering a t Massachusetts Institute of Technology. a Kreisinger, Ovitz, and Augustine, Brrr. Mines, Bull. 137 (1914). 'THIS JOURNAL,15, 502 (1923). J . Chem. Soc., 101, 831, 846 (1912).
OF
TECHNOLOGY, CAMBRIDGE,
hfnSS
recently been obtained by Haslam and collaboratorsG and by recent work of the Bureau of ?\lines.7 The reaction between carbon and oxygen under fuel bed conditions is so important in the field of combustion that i t seems desirable to secure, if possible, direct experimental verification of the validity of the diffusion theory. A method of attack which consisted in determining the concentrations of the gases on the carbon side of the gas film surrounding a burning carbon particle was suggested by W. K. Lewis. Figure 1 is a diagrammatic representation of the concentration gradients in such a system. The concentration of oxygen on the air side of the film is 21 per cent but decreases to zero on the carbon side.
% OXYt€N
Figure 1 - D i a g r a m m a t i c Representation of Oxygen C o n c e n t r a t i o n i n Gas F i l m
The diffusion velocities of gases are inversely proportional to the square root of their respective molecular weights. When combustion is continuous the oxygen must diffuse in and the resultant carbon dioxide must diffuse out a t the same rates. Since the specific diffusion rate constants of oxygen and carbon dioxide are inversely proportional to fiand 6, respectively, the concentration of carbon dioxide on the carbon side of the film must be greater than that of oxygen on the air side in order to create a driving force equal t o that of the oxygen. If the gases on the carbon side of the film 6 Haslam, Entwistle, and Gladding, THISJOWRNAL, 17, 586 (1925); Haslam, Mackie, and Reed, Ibid., 19, 119 (1927); Haslam, Ward, and Mackie. Ibid., 19, 141 (1927). 7 American Gas Association, Report of Carbonization Committee, 1926.
