70 



THE INDIA RUBBER WORLD 



I November 1. 1914. 



metallic box (2) surrounded with a water jacket to maintain 

 constant temperature of the gas, provided with a Hanged top 

 (1), and ring and gasket arranged to hold the test piece. The 

 bottom of this box is fitted with a glass graduated tube (7) 

 which extends to the bottom of a concentric glass cylinder (8). 

 There are two connections in this graduated tube, one to pass 

 hydrogen into the box ami the other a "U" tube (4) to register 

 the gas pressure. Connected by a tube with pinchcock to the 

 bottom of the concentric cylinder is a bottle ((>> holding a 

 volume of water equal to that of the graduated tube. The 

 procedure is as follows: The fabric is lirst tested for im- 

 permeability to air bj closing the valve (5) maintaining a con- 

 stant pressure in the "U" tube by raising the bottle to the neces- 

 sary height and noting if the level of the water in the graduated 

 tube remains constant. If the fabric holds air it is then tested 

 for hydrogen diffusion. Hydrogen is passed through the box 

 under the fabric until the air is completely swept out (lest 

 burning of test tube of gas from "U" tube). The valve (5) is- 

 then closed and the loss of gas in a given time measured by the 

 rise of water in the graduated tube necessary to maintain the 

 stated pressure in the "U" tube. 



The principal disadvantage of these types of apparatus is that 

 their accuracy is dependent largely on the tightness of the drums 

 or bell jars ami connections, of which, due to necessary labora- 

 tory refinements, there are many; and that account must be 

 taken of the diffusion of air into the drum. 



The method used in the laboratories of the Goodyear Tire & 

 Rubber Co. is to maintain a constant pressure of hydrogen on 

 one side of a test piece oi fabric while air is passed by the other 

 side, taking with it the hydrogen which has diffused through 

 the fabr.ic, which hydrogen is burned to water in a combustion 

 furnace and weighed, giving with a simple calculation the leakage 

 i if the fabric over a definite time interval. With this apparatus 

 the only joint under appreciable pressure which must be kept 

 light is that between the fabric and the bottom half of the drum. 



The technique of the test is as follows: The two hemispheres 

 of the bronze diffusion drum (6) (Fig. 4) are opened and the 

 hallo. ,n fabric diaphragm (7) to lie tested is inserted, the two 

 halves of the drum being then bolted together so securely that 

 the joint is gas tight. The drum is then placed in a thermostatic 

 hath (not shown in diagram) and brought to the required tem- 

 perature, usually 20 deg. C. 



When the drum and its contents have attained the desired 

 temperature, hydrogen is allowed to pass from the cylinder ll) 

 through tin gas washing bottle (2) filled with strong sulphuric 

 acid for drying the hydrogen, thence through the preheating 

 furnace (3) whose purpose is to burn all organic gaseous im- 

 purities in the hydrogen and to combine any contained oxygen 

 with it to water ; the purified gas then passes through the gas 

 washing bottle (4) filled with caustic potash solution to remove 

 traces of carbon dioxide, etc.. and then through the bottle (5) 

 tilled with strong sulphuric acid to remove the last traces of 

 water; thenci into the under side of the diffusion drum (<>). 



The purified hydrogen gas is passed into the lower half of the 

 drum (6) and out through the tube (IS) until all the residual 

 air contained in this lower half has been displaced by hydrogen 

 and swept out. The tube (IS) is then closed and the pure 

 hydro-, ii nil-wed to diffuse upward through the diaphragm for 

 a definite time. 



The hydrogen diffusing through (7) into the upper half of 

 the drum (6) is swept out by a current of air admitted through 

 the tube (14). The hydrogen containing air passes through the 

 gas washing bottles (9 and 10) whose purpose it is to remove 

 aip. adventitious moisture, they being filled with strong sul- 

 phuric acid ; thence the gases pass into a heated combustion 

 tube (11) filled with platinized asbestos, where the hydrogen is 

 burned to water, which in turn is absorbed and weighed in the 

 Geissler bulbs (12 and 131. 



No. 8 is a pressure regulating device whose function is to 

 maintain a constant gas pressure in the lower half of the dif- 

 fusion drums, the pressure usually equaling a 2-inch column of 

 water. 



Knowing the exposed area of the diaphragm, the time of dif- 

 fusion, and the weight of the water obtained by the combustion 

 of the diffused hydrogen, a simple calculation enables us to 

 determine the diffusion per square yard in 24 hours, and results 

 are always reported as cubic feet of hydrogen diffused per 

 square yard in twenty-four hours at the given constant tempera- 

 ture and pressure. 



The apparatus of Professor Ueyn in use at the Imperial 

 Testing Laboratory Gross Lichterfelde (near Merlin) is essen- 

 tially similar to the Goodyear equipment, except that the test 

 piece is much smaller (about 4 inches against 9 inches diameter) 

 and no provision (unless this has been done recently) made 



f o r maintaining 

 uniform tempera- 

 ture of the gas and 

 fabric. It has been 

 shown with the 

 Goodyear and 

 other apparatus 

 that temperature 

 has a marked ef- 

 fect on the diffu- 

 sion rate. 



The apparatus 

 thus far described 

 in e a s u r e.s the 

 amount of hydro- 

 gen which has dif- 

 fused through the 

 fabric .d u r i n g a 

 given period. An 

 instrument on an 

 entirely different 

 principle has been 

 developed recently 

 in Germany with 

 which it is possible 

 to read directly 

 the rate of diffu- 

 sion. This is the 

 gas interferometer made by the firm of Carl Zeiss, Jena. 



The principle of the apparatus is briefly as follows: The upper 

 part of a pencil of light rays made parallel by a collimator 

 passes through the upper halves of two narrow vertical slits 

 in the objective of a telescope. Half the lower part of this pencil 

 passes through one of two gas chambers between -the collimator 

 and the telescope and the other half through, the other. These 

 two sets of rays from the gas chambers pass through the lower 

 part of the above mentioned slits in the. objective. If one of 

 the gas chambers is filled with pure air and the other with a 

 mixture of air and hydrogen, on, looking through the eye piece 

 of the telescope the lower interference spectrum will be displaced 

 with respect to the upper (whose rays do not pass through the 

 gas chambers). By turning, by a micrometer screw, a series 

 of prisms placed in the path of one of the lower sets of rays, 

 the principal dark lines in the two spectra can be brought 

 together. The number of divisions through which the screw is 

 turned is a measure of hydrogen content of the hydrogen and 

 air mixture. In practice air from above a test piece of fabric 

 (and therefore containing hydrogen diffused through the fabric) 

 is drawn at a known constant rate through the gas chamber of 

 the interferometer wdiile the other chamber is filled with purified 

 air. The test piece is held in a drum essentially similar to that 

 in the Goodyear apparatus. The instrument is calibrated by 



Fig. 3. Josse's Apparatus for Testing the 

 Permeability of Fabric to Gas. 



