[elworthy-murray] balloon FABRICS 41 



(a) Purity of Gases 



In these experiments the hydrogen used was supplied by the 

 British Oxygen Co., Ltd., and was guaranteed to contain less than 

 0-5% impurities (nitrogen and carbon monoxide). The helium, 

 separated from natural gas by a liquefaction process, had been purified 

 by a continuous-flow charcoal-absorption method^ and was prac- 

 tically 100% pure. 



(&) Temperature Control 



As permeability has a considerable temperature coefficient, 

 the permeameter and coils enclosed in dotted lines in Fig. 1 were 

 placed in an electrically controlled thermostat maintained at a con- 

 stant temperature (15-5°C. for the majority of the tests). The 

 variation of the thermostat temperature was about 0-l°C. In some 

 preliminary tests when the temperature was not at 15 •5°C, the hydro- 

 gen permeability was corrected to the temperature of the helium 

 permeability, using a temperature coefficient of 5%. This correction 

 (for less than TC.) was applied to results IIA, IIIA, IVA, VA and 

 VIA in Table III given below. 



(c) Rate of Air Flow 



The rate of air flow was varied from about 2 to 15 litres per hour 

 according to the type of fabric under examination. 



(d) Pressure Control 



The gas and air drums of the permeameter were connected by a 

 dififerential gauge and the pressure on the gas side controlled so as to 

 give an excess gas pressure over air of about 3 cms. water. Extreme 

 accuracy of control was not aimed at, but the pressure difference was 

 substantially the same for the same fabric for hydrogen and for helium. 



(e) Time for Equilibrium 



After adjustment of the gas and air rates, readings of the gas out- 

 let Katharometer were taken until it was ascertained that the gas 

 chamber was completely filled with gas — except for the small amount 

 of air leaking through the fabric. The diffusion of gas into the air 

 chamber was followed by Katharometer observations. It was found 

 that a period of an hour sufficed for equilibrium conditions to be 

 established. 



1 Edwards and Elworthy. Proc. Roy. Soc. of Canada, 1919. 



Sec. Ill, Sig. 4 



