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12 6 AIR-TO-WATER PERMEABILITY RATIO 41-3 PERMEABILITY OF SOIL TO AIR 527 <br /> ' o w in which <br /> to N E <br /> permeability with air, cm.2, <br /> a o [ L = length of soil column, cm., <br /> ° 3 cr W G M V = volume of tank, cm.3, <br /> a r <br /> W J <br /> a g Z 0� 0 N 7 = viscosity of air at the temperature at which determination was made, <br /> a W <br /> Z N w 0 ti `� dyne second cm.-2 (poises) (Values of viscosity +1 may be obtained <br /> u t 0 C <br /> N W > from the Handbook of Physics and Chemistry or other such hand- <br /> Z — <br /> ° ; ;'; books.), <br /> ° <br /> A = cross sectional area of sample, cm."-, <br /> 0 a o o P = atmospheric pressure, dynes per cm.2, <br /> v � S = slope of log y vs. time curve = (loglo yj — logio y2)/At(the constant <br /> > N052.30 in equation.[12] is consistent with the use of logarithms to the <br /> 1 3 a n base 10 in calculating S; for natural logarithms, the value of the <br /> • ' constant is 1), <br /> y =E a a displacement of the water surface in one arm of the manometer,cm., <br /> >,a � P <br /> °= <br /> '= U At = time interval in seconds for the water surface in the manometer to <br /> C 0° Z .N..� N•yn <br /> m as� drop from yl to y.�. <br /> 3 a> <br /> ° as 0M <br /> U. ir N mom, a The units of air permeability k'a as calculated above are cm? Because <br /> z w w CDt a� ti <br /> w J m X E•'• of the extremely small number for soils, it is convenient to express results <br /> m <br /> a ca <br /> J N c Q in square microns µ2 or milli square microns m(µ"-) as follows: <br /> —wp C <br /> € o 0 0 °N •° I µ'2 = I X 10-11 cm.2 <br /> W <br /> I m(µ2) = I X 10 "cm.2 <br /> ( \ z m C,C C iC <br /> F \ a a Air permeability values encountered for soils as determined by this method <br /> c have ran ped from about 0.10 to 18 or from 100 to 18,000 m/ <br /> 0 0-- 3 <br /> 41-3.4 Comments <br /> g " E g 2 The air-flow line and all valves and fittings in the line of the pressure <br /> KI N O a E G7y Y>¢• apparatus should be 1/2 inch in diameter or larger to reduce frictional losses <br /> y <br /> J h in the flow line to a negligible level. <br /> ® 3 N a a°v The size of tank can be varied to allow a suitable rate of pressure drop, <br /> to W m C-a b depending upon the permeability of the soil. Two tanks, V = 200 liters <br /> �, Y <br /> $ a H W N = (approximately) and V = 25 liters, have been used to cover the range <br /> o of k'a = 0.1 to 20µ2. For the size of sample described above and for <br /> 3 w 4 � manometer readings, yl = 20 cm. and y2 = 10 cm., the time interval has <br /> ranged approximately as follows: <br /> w At <br /> w c 4== (liters) (µ-) (sec.) <br /> Y M ~ <br /> o 0 V 200 1 — 20 20 — 400 <br /> v3° o 25 0.1 - 1.0 50 - 500 <br /> E L) <br /> w" The approximate volume required can be calculated from the equation <br />