'7 if fc Med Slain lm$ Issin d U.S. DEPARTMENT OF AGRICULTURE, BUREAU OF PLANT INDUSTRY— Circular No. 20. B. T. GALLOWAY, Chief <>f Bureau. AN ELECTRICAL RESISTANCE METHOD FOE THE RAPID DETERMINATION OF THE MOISTURE CONTENT OF CHAIN. LYMAN J. BRIGGS, Physicist in Charge of Physical Laboratory. 98 I if. 20 08 ASHINGTO* : GOVEHNMEWT PRINTING OFFICE : 1*08 BUREAU OF PLANT INDUSTRY. Physiologist and Pathologist, and Chief of Bureau, Beverly T. Galloway. Physiologist and Pathologist, and Assistant Chief of Bureau, Albert F. Woods. Laboratory of Plant Pathology, Erwin F. Smith. Pathologist in Charge. Fruit Disease Investigations, Merton B. Waite, Pathologist in Charge. Laboratory of Forest Pathology, Haven Metealf. Pathologist in Charge. Cotton and Truck Diseases and Plant Disease Survey, William A. Orion. Pathologist in Charge. Pathological Collections and Inspection Work, Flora W. Patterson, Mycologist in Charge. Plant Life History Investigations, Walter T. Swingle, Physiologist in Charge. Cotton Breeding Investigations, Archibald D. Shamel and Daniel X. Shoemaker, Physiologists in Charge. Tobacco Investigations, Archibald D. Shamel, Wightman \V. Garner, and Ernest II. Mathewson, in Charge. Corn Investigations. Charles P. Hartley, Physiologist in Charge. Alkali and Drought Resistant Plant Breeding Investigations, Thomas IT. Kearney, Physiologist in I Soil Bacteriology and Water Purification Investigations, Karl F. Kellerman, Physiologist in Chargi . Bionomic Investigations of Tropical and Subtropical Plants, Orator F. Cook, Bionomist in Ch irge. Drug and Poisonous Plant and Tea Culture Investigations, Rodney II. True, Physiologist in Charge. Physical Laboratory, Lyman J. Briggs, Physicist in Chai Crop Technology and Fiber Plant Investigations, Nathan A. Cobb, Crop Technologist in Charge. Taxonomic and Range Investigations, Frederick V. Coville, Botanist in Charge. Farm Management. William .1. Spillman, Agriculturist in Clin Grain Investigations, Mark Alfred Carleton, Cerealist in Charge. Arlington Fiperirnental Farm and Horticultural Investigations. Lie C. Corbett, Horticulturist in Charge. Vegetable Testing Gardens, William W. Tracy, sr.. Superintendent. Sugar-Beet Investigations, Charles O. Townsend, Pathologist in Charge. Western Agricultural Extension. Carl S. Scofleld, Agriculturist in Charge. Dry-Land Agriculture Investigations, E. Channing Chilcott. Agriculturist in Charge. Pomological Collections, Gustavus B. Brackett, Pomologist in Charge. Field Investigations in Pomology. William A. Taylor and G. Harold Powell, Pomologists in Charge. Experimental Gardens and Grounds, Edward M. Byrnes, Superintendent. Foreign Seed and Plant Introduction, David Fairchild, Agricultural Explorer in Char.'. Forage Crop Investigations, Charles V. Piper, Agrostologist in Charge, Seed Laboratory. Edgar Rrown, •Botanist in Charge. Grain Standardization. John D. Shanahan, Crop Technologist in Chargi Subtropical Garden. Miami, Fla.. P. J. Wesler. in Charge. Plant Introduction Garden, Chico, Cal., W. W. Tracy, jr.. Assistant Botanist in Charge. South Teras Garden, Brownsville, Tex., Edward C. Green, Pomologist in Charge. Farmers' Cooperative Demonstration Work. Seaman \. ECnapp, Special Agent in Charge, Seed Distribution (directed by Chief of Bureau), Lisle Morrison. Assistant in General Charge. Editor, J. E. Rockwell. Chief Clerk. James I'.. Jones. [Cir. 20] 2 It. I . [.—422. A.\ ELECTRICAL RESISTANCE METHOD FOR THE RAPID DETERMINATION OF THE MOISTURE CONTENT OF GRAIN. INTRODUCTION. The shipping and storing qualities of grain arc so dependent upon it- moisture contenl that an accurate knowledge of the moisture in grain in storage and in transil is highly desirable. This subject lias been given special attention h\ Brown and Duvel," who have described a rapid method of making such moisture determinations. Their method consists in boiling the -rain in an oil having a flashing poinl much above the boiling poinl of water, condensing the water which distills off, and collecting and measuring it in a suitable graduate. Moisture determinations can by this method l>c made in about one-half hour, whereas determinations in the water oven require several days. This method is, however, suitable for labora- tory use only, necessitating the collecting of samples before the determinations can be made, and does not appear to be adapted to such grain products as meal and flour. Vi the request of the Office of Grain Standardization, the writer undertook the development of an electrical resistance method for measuring the moisture content of grain adapted to measurements in a car or elevator, as well as in a laboratory, and requiring only two or three minutes for a determination. The measurements so far have been confined to wheat. The results obtained are so promising that a brief preliminary description of the method is given. Corresponding measurements will he made for other -rain-, a- well a- for Hour and corn meal. A portable apparatus suitable for measurements in car- and elevators i- also being constructed. DESCRIPTION OF THE ELECTRICAL RESISTANCE METHOD FOR MEASURING THE MOISTURE CONTENT OF GRAIN. Tic method developed consists essentially in the measurement of the resistance offered to the passage of an electric current through the grain from one metallic rod or electrode to another. The el Bullel .111 of Plant Industry . 1907. [fir. .'II] 4 DETERMINATION OF MOISTURE CONTENT OF GRAIN. trical resistance decreases rapidly as the moisture content of the grain increases. The electrical resistance of wheat containing 13 per cent of moisture is seven times that of wheat containing 14 per cent and fifty times that of wheat containing 15 per cent of moisture. This method, therefore, gives a very open scale, and a considerable variation in resistance can take place without seriously affecting the accuracy of the moisture determinations. The relation between the electrical resistance and the moisture content of wheat is shown graphically in figure 1. The moisture percentages in this figure are plotted as ordinates and the natural logarithms of the corresponding resistances are plotted as abscissas. Five widely differing types of wheat — soft red winter, hard red winter, Xo. 1 hard spring, durum, and a badly mixed wheat con- taining many weed seeds — were used in these determina- tions. The close- ness with which the different points on the diagram ap- proach the straight line drawn through them illustrates the accuracy with which moisture determi- nations can be made by this method. The logarithms of the resistances in- stead of the resistances themselves are plotted in order to condense the diagram and to bring out the straight line relation between the two variables as shown. /e X r r 1 * « V? -OG- Fig. 1.— Chart showing the relation between the moisture eontent and the electrical resistance of wheat. Measurements made at 75° F. For description of electrodes, see text. Resistances expressed in megohms. Moisture percentages based on weight of moist grain. RELATION OF ELECTRICAL RESISTANCE TO TEMPERATURE. The electrical resistance of wheat is also dependent upon the tem- perature of the grain. In fact, the rapidity with which the resist- ance decreases as the temperature increases i> quite remarkable and greatly exceeds lha! occurring in most substances. The manner in which the electrical resistance of wheat varies with the temperature is shown graphically in figure 2, in which temperatures are plotted as onlinates and electrical resistances as abscissas. The resistance at 4° C. (39° F.) is seen to be IS times the resistance at 24° C. (75° F.). LClr. 201 DETERMINATION OF MOISTURE CONTENT 0] GRAIN. 5 This curve is based upon 34 groups of measurements made upon hard red winter, sofl red winter, hard red spring, durum, and a mixed wheat. Dots on the diagram refer to one sample, crosses to another, and so on. In order to construct a mean temperature resistance curve, the resistances corresponding to the differenl sam- ples were all increased or decreased tn an am i corresponding to the mean of the ratios of the resistances to the corresponding resist- ances of one curve taken as a standard. In making these determi- nations, the wheat, after being cooled in an ice chest, was allowed to approach the temperature of the room and a series of resistance measurements were made as the temperature increased. The grain ^ l \.° f* * • ^ • ° -- Q 1 — cW fO so so /oo /?o /to /GO /so ?oo /=r£\5~/^~7--^/^scr£'—/w£r&o>-"^*s~ • ol temperature upon the electrical resistance of wheat. was in each case stirred to obtain as uniform a temperature distri- bution as possible before each sel of measurements. Temperatures above thai of the room were obtained in a similar manner by heating the grain and measuring the resistance as ii cooled. It is difficult to determine the true temperature of grain while it is being wanned or cooled in this way, which account- for the father wide departure of some of the points from the mean curve. THE DETERMINATION OF THE MOISTURE CONTENT OF WHEAT AT DIFFERENT TEMPERATURES. \\\ combining the data shown in figures 1 and 2 we can construct a charl showing the moisture content of a -ample of wheat corre- sponding to a given electrical resistance at anj temperature within leu- 6 DETERMINATION OF MOISTURE CONTENT OF GRAIN. the range of the experiments. Such a chart is presented as figure 3. Tliis chart is similar to that shown as figure 1, except that we have here lines showing the relation between moisture content and resist- ance not only for a single temperature, as in figure 1, but for tempera- ture intervals of 5 degrees from 80° to 40° F. In this chart the moisture contents are plotted as ordinates and the logarithms of the electrical resistances as abscissas. To facilitate the use of the chart, resistances are written in place of the corresponding logarithms. To illustrate the use of the chart, suppose that a resistance of 55 megohms was observed in a given sample of wheat at a temperature of 75° F. Referring to the chart, it will be seen that the imaginary line corresponding to 55 megohms crosses the 75° F. line at a point corresponding to 13.95 per cent of moisture. This statement assumes, of course, that the measurements were made with elec- trodes of standard size, to which tins chart is only applicable. APPARATUS FOR MEASURING ELECTRICAL RESISTANCE OF GRAIN. Unless the grain is very wet, its specific electrical resistance is very high. The resistance, while electrolytic in character, is so great that polarization is not troublesome and measurements can be made with direct currents. The electrical apparatus required fur such measurements is therefore similar to that used tor testing the insulation of cables. The measurements described were made principally with a Wheatstone bridge, using a fairly sensitive gal- vanometer and an electromotive force of 17 volts. In the driest samples (below 12 per cent) the resistance was so high that it could not be measured by this method. For these samples the direct deflection method was used, the galvanometer and grain resistance being connected in series with a battery having an electromotive force of 10 volts. In all the measurements described, the electrodes used consisted of two parallel l-inch round brass rods, lj inches between centers and 12 inches long. These rods were kepi parallel and insulated from each other by being supported in a hard-rubber block at their upper ends. Connecting wires with extra heavy rubber insula- tion were soldered to the two upper ends of the electrodes. The grain during measurements was held in glass battery jars 5 inches in diameter and 11 inches high. The height of the *grain, inside measurement, was 10 inches. The lower ends of the electrodes rested upon the bottom of the jar. The temperature was measured with a mercurial thermometer having a cylindrical bulb, which could We readily forced into the grain. [CIr. 20] DETERMINATION OF MOISTURE CONTEN RAIN. °o «fc °o °o °S * s to K > /oooo 9000 8000 7000 sooo sooo ^o so 60 70 Fig. 3. < rminlng them' otent of wheat when the electi adtem- perature are known. El dimensions [escribed in the text must « itli this (hurt. [Clr. 20] 8 DETBEMINATION OF MOISTURE CONTENT OF GKAIN. ^ Before each measurement the electrodes were removed and the | 1 oaeked by iarring the bottom of the container agatnai some |j fohd Zct It "portent that .his precaution in packing be oj ob r^Tf satisfactory results are to be obtained Tins wUl not be ., X in measurements made in cars since the settlmg of the . ™w, in transit will hace reduced it to a stable conditwn. »i g O 1 er t\me oTelectrodes and containers designed for usmg smaller |; ^SX. hace been tried, but the most sahsfac.ory results s . aeo in the Division of Soils for the measurement of the moisture con Zti 'frr t' ittrot rnot encountered m the me— not been indicated in the samples so far examined. SUMMARY. This paper deals with an electrical -f^.™^ 1 ^2S determination of the moisture content ^-* « „ f haye so far been co nfined to wheat^ The^te ct^ ^ wheat contammg 13 per cent of u ^ b™ c , bable moisture content ean be determined **■£££ ^ p „ror not exceeding 0.3 j« r « The paratua is ■ J S3Ea"^S o^her grained gr products U now being investigated. Approved: James Wilson, . Secretary of Agriculture- Washington, D. C, October 2P, J«W. | (if. 20] q ECO = 00 ■c\j