UNIVERSITY OF CALIFORNIA PUBLICATIONS COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA THE CALIBRATION OF THE LEAKAGE METER FUMIGATION STUDIES No. 8 BY C. W. WOODWORTH BULLETIN No. 264 Berkeley, CaL, January, 1916 UNIVERSITY OF CALIFORNIA PRESS BERKELEY 1916 Benjamin Ide Wheeler, President of the University. EXPERIMENT STATION STAFF HEADS OF DIVISIONS Thomas Forsyth Hunt, Director. Edward J. Wickson, Horticulture (Emeritus). Herbert J. Webber, Director Citrus Experiment Station; Plant Breeding. Hubert E. Van Norman, Vice-Director; Dairy Management. William A. Setchell, Botany. Myek E. Jaffa, Nutrition. Eobert H. Loughridge, Soil Chemistry and Physics (Emeritus). Charles W. Woodworth, Entomology. Ralph E. Smith, Plant Pathology. J. Eliot Coit, Citriculture. John W. Gilmore, Agronomy. Charles F. Shaw, Soil Technology. John W. Gregg, Landscape Gardening and Floriculture. Frederic T Bioletti, Viticulture and Enology. Warren T. Clarke, Agricultural Extension. John S. Burd, Agricultural Chemistry. Charles B. Lipman, Soil Chemistry and Bacteriology. Clarence M. Haring, Veterinary Science and Bacteriology. Ernest B. Babcock, Genetics. Gordon H. True, Animal Husbandry. James T. Barrett, Plant Pathology. Fritz W. Woll, Animal Nutrition. A. V. Stubenrauch, Pomology. Walter Mulford, Forestry. W. P. Kelley, Agricultural Chemistry. H. J. Quayle, Entomology. D. T. Mason, Forestry. J. B. Davidson, Agricultural Engineering. Elwood Mead, Rural Institutions. H. S. Reed, Plant Physiology. William G. Hummel, Agricultural Education. Leon M. Davis, Dairy Industry. John E. Dougherty, Poultry Husbandry. S. S. Rogers, Olericulture. Frank Adams, Irrigation Practice. David N. Morgan, Assistant to the Director. Mrs. D. L. Bunnell, Librarian. division of entomology C. W. Woodworth E. O. Essig W. B. Herms S. B. Freeborx E. C. Van Dyke E. P. Van Duzee Geo. A. Coleman M. R. Miller Geo. P. Gray E. R. de Ong THE CALIBRATION OF THE LEAKAGE METER By C. W. WOODWORTH Some of the leakage meters put on the market prove to have been incorrectly calibrated and indicate excessive leakages. This has not been serious as long at the instruments have been used only for the comparison of samples of tent materials, but they should be corrected to suit them, for use in connection with dosage tables. This is easily done by using a reamer on the small opening from the brass tube to to the small chamber behind the cloth under the clamp. It requires very skillful work, however, and it is wise to return to the manufacturer for adjustment any meters that do not register correctly. The present publication is intended to enable users of these instru- ments to determine approximately the correctness of this adjustment. The theory of the meter is that if two test plates were used to corre- spond with the two thicknesses of cloth to be tested then the diameters of the test plates would bear such a relation to the diameter of the sharp edge of the clamp that the areas of the smaller circle in the test plate should be between .20 and .25 per cent of the area of the larger circle of the clamp and precisely the amount shown by the test line on the meter. Instead, however, of using two test plates a single plate with a smaller hole of exactly the right size to make the same reading on the meter is furnished with the instrument. The smaller hole is just 84 per cent of the diameter that would give the right calculation of percentages of leakage through the doubled cloth, therefore for making calculations the diameters of the test plates must be increased. The testing of the meter consists in measuring the size of the ring of the clamp and of the hole in a test plate and finding whether or not the actual test corresponds with the percentages calculated from the measurements of these sizes. Accurate testing requires very careful work but it is very easy to make a test sufficiently accurate to determine whether the instrument shows gross errors. MEASURING THE CLAMP The best way to measure the clamp is to use a piece of cardboard with which to take an impression of the ring, putting in several thicknesses, if necessary, to obtain a clear, sharp mark. After obtain- [231] 232 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION ing the impression measure this carefully with a finely divided rule determining; the nearest 64th of an inch. The size will be found to be approximately % or 4 % 4 ths of an inch and the table below gives readings for possible variations for a sixteenth of an inch in either direction. A 64th of an inch variation modifies the reading by less than .02 of a per cent. MEASURING THE TEST PLATE The hole in the test plate is very much more difficult to measure ac- curately. The standard wire gauge known as the ' ' Brown and Sharpe ' ' gauge is to be had in every machine shop and is perhaps the most accurate measure for this purpose. Lay this on the test plate and examine it under a pocket lens. The hole should appear as shown in figure 1. Another method is to make a test plate by the use of an Fig. 1. — The way the hole in the test plate should appear under the wire gauze as seen through the microscope. ordinary sewing needle. This method will probably prove more accurate with most people than the use of a wire gauge because of the practical difficulty of making the comparisons under the microscope. The best test plates made in this manner are with soft metal, like copper, lead or zinc. A very thin sheet of these metals can be very easily perforated. Fair results can be secured by using a piece of good quality writing paper or cardboard. The fiber in the softer cardboard swells out and partially closes the hole and so gives low readings. If it is found that a needle fits tightly in a hole after withdrawing it a few minutes it is not suitable. A very good method is found to consist in perforating a sheet of celluloid in rather hot water, then placing the celluloid in cold water and leaving the needle in position until the celluloid is cool. The hole under these circumstances proved almost if not quite as good as in metal. THE CALIBRATION OF THE LEAKAGE METER 233 In all cases thrust the needle through to the middle so as to have the full width of the shaft. The use of the needle is so simple and convenient that it will probably be the most satisfactory method in practice to make a new test plate rather than to attempt to measure the one furnished with the instrument. MAKING THE TEST The test plate is clamped in the instrument and the reading care- fully made in the same manner as in testing tent material. Comparing this with the calculation given in the table in the column under the diameter of the hole in the test plate and the line of the diameter of the clamp ring enables one to determine the per cent of error. When the table values differ from the determined value the error is approximately 6 per cent for each line. The difference of adjacent columns is approximately four times as great. One degree on the meter between .15 and .20 corresponds with about six lines, between .20 and .25 four lines and from .25 to .35 about three lines. If the observed error is greater than this, the instru- ment should be sent to the maker for adjustment, and perhaps even for a lesser error, rather that to try to adjust by reading "strong"' or "weak." TABLE FOE TESTING LEAKAGE METERS Wire gauge No 20 21 Needle No 5 6 Diameter 032 .028 Augmented 038 .033 Diameter of clamp 36-64ths .564 in. 14.3 mm 35 37 .580 14.7 33 38 .595 15.1 31.5 39 .610 15.5 39.0 29.8 40 .625 15.9 37.0 28.5 41 .642 16.3 35.0 26.9 42 .658 16.7 33.3 25.6 43 .672 17.1 32.0 24.5 44 .688 17.5 32.5 23.5 22 23 24 25 7 8 9 10 .025 .022 .020 .018 .030 .026 .024 .021 ct meter readings X 100 28 21.7 17.9 14.5 26.4 20.4 16.9 13.6 25.0 19.5 16.0 13.0 23.8 18.5 15.3 12.4 22.7 17.6 14.6 11.7 21.5 16.7 13.8 11.2 20.5 15.9 13.1 10.6 19.6 15.2 12.6 10.1 18.7 14.5 12.0 CALCULATION OF THE TABLE While it is not necessary to make any calculations to test a meter it will be well to explain the calculation of the above table. 234 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The easiest method of doing this is by the use of a slide rule, the settings of which are as follows: C 21 _ C needle diameter C elamp diameter A v D 25 ~ D ~ ~^~ D ~ ~^~ = B t x = augmented needle diameter to allow for' doubled tents, y = correct meter readings of per cent of leakage. SUMMARY Some leakage meters are so inaccurate that they are unsuitable for use as guides to dosage. The accuracy can be easily tested by measuring the clamp ring and the hole of the test plate. Needles can be used to make test plates making their measurement unnecessary. The amount of the inaccuracy is determined by com- paring the reading with a table. If found inaccurate the instrument should be returned to the maker for adjustment.