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GRAVIMETRIC

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Originally appearing in Volume V03, Page 279 of the 1911 Encyclopedia Britannica.
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GRAVIMETRIC See also:

VOLUME In the heading of the 6-in. range table we find the description of the See also:charge. Charge: See also:weight 13 lb 4 oz.; gravimetric See also:density 55.01/0.504; nature, See also:cordite, See also:size 30. So that P=13.25, the G. D.=0•504, the upper figure 55.01 de-noting the specific volume of the charge measured in cubic inches per lb, filling the chamber in a See also:state of See also:gas, the product of the two See also:numbers 55.01 and 0.504 being 27.73; and the chamber capacity C =13.25X55.01 =730 cub. in., See also:equivalent to 25.8 in. or 2.15 ft. length of See also:bore, now called the equivalent length of the chamber (E.L.C.). If the shot was not See also:free to move, the closed chamber pressure due to the See also:explosion of the charge at this G.D. (=0.5) would be nearly 49 tons per sq. in., much too See also:great to be safe. But the shot advances during the See also:combustion of the cordite, and the See also:chief problem in interior See also:ballistics is to adjust the G.D. of the charge to the' weight of the shot so that the advance of the shot during the combustion of the charge should prevent the maximum pressure from exceeding a safe limit, as shown by the maximum See also:ordinate of the pressure See also:curve CPD in fig. 3. Suppose this limit is fixed at 16 tons per sq. in., corresponding in Table I. to a G.D., 0.2 ; the See also:powder-gas will now occupy a volume b=: ;C=1825 cub. in., corresponding to an advance of the shot X2.15=3.225 ft. Assuming an See also:average pressure of 8 tons per sq. in., the shot will have acquired See also:energy 8 X 47rd2X 3.225 = 730 See also:foot-tons, and a velocity about v =1020 f/s, so that the See also:time over the 3.225 ft. at an average velocity 510 f/s is about o•oo63 sec. Comparing this time with the experimental value of the time occupied by the cordite in burning, a start is made for a fresh estimate and a closer approximation. Assuming, however, that the agreement is See also:close enough for See also:practical requirement, the combustion of the cordite may be considered See also:complete at this See also:stage P, and in the subsequent expansion it is assumed that the gas obeys an adiabatic See also:law in which the pressure varies inversely as some m°i See also:power of the volume.

The See also:

work done in expanding to infinity from p tons per sq. in., V 24-22 20 - 22 20 18 IS See also:rc p Cl N N IC W 0 16 I6 14- 12 °4 12 10 10 8 6 ...._ter... ..... ~.._ ... 4 2 2 00 1 2. 8 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 200 at volume b cub. in. is then pb/(m—I) See also:inch-tons, or to any volume 13 cub. in. is (9) mpb1 [ (b\m-IJ J J It is found experimentally that m=1.2 is a See also:good average value to take for cordite; so now supposing the combustion of the charge of the 6-in. is complete in 0.0063 sec., when p=16 tons per sq. in., b=1825 cub. in., and that the gas expands adiabatically up to the muzzle, where (so) B 216+25'8=3'75, b 2.5X25.8 we find the work realized by expansion is 2826 foot-tons, sufficient to increase the velocity from 1020 to 2250 f/s at the muzzle. This muzzle velocity is about 5 % greater than the 2150 f/s of the range table, so on these considerations we may suppose about lo % of work is lost by See also:friction in the bore; this is expressed by saying that the See also:factor of effect is f =0.9. The experimental determination of the time of burning under the See also:influence of the varying pressure and density, and the size of the See also:grain, is thus of great practical importance, as thereby it is possible to estimate close limits to the maximum pressure that will be reached in the bore of a See also:gun, and to See also:design the chamber so that the G.D. of the charge may be suitable for the weight and See also:acceleration of the shot. Empirical formulas based on practical experience are employed for an approximation to the result. A great See also:change has come over interior ballistics in See also:recent years, as the old See also:black See also:gunpowder has been abandoned in See also:artillery after holding the See also:field for six See also:hundred years. It is replaced by See also:modern See also:explosives such as those indicated on fig. 4, capable of giving off a very much larger volume of gas at a greater temperature and pressure, more than threefold as seen on fig. 8, so that the charge may be reduced in proportion, and possessing the military See also:advantage of being nearly smokeless.

End of Article: GRAVIMETRIC

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