280 



WET SUITS 



low in toxicity, are not flammable or explosive, and do not conduct electricity. The Freons 

 listed in Table 33 cause less than 1 percent elongation of natural rubber due to swelling. 



Table 33 

 PROPERTIES OF FREON GASES [8] 



*Group 6, Underwriters' Laboratories Classification of Comparative Life Hazard of Gases and 

 Vapors: "Gases or vapors which in concentrations up to at least about 20 percent by volume 

 for durations of exposure of the order of 2 hours do not appear to produce injury." 



Nitrogen was also considered, because it has better thermal insulating properties than 

 helium and, in small percentages, can be present in a high-pressure breathing gas with no del- 

 eterious effects. But it is not as insulative as either CO 2 or Freon gas. Compressed air was 

 ruled out because it will support combustion and would represent a fire hazard in an electri- 

 cally heated suit. 



Purging the helium-oxygen mixture from the suits with insulating gases was ruled out in 

 the Sealab n habitat. This procedure was considered unacceptable because of the need for 

 stringent atmosphere control. 



Thermal conductivity is not the only aspect to consider for selection of an insulating gas. 

 Heat transfer by convection is also a participant in the total heat transfer across the insulating 

 sandwich. Radiation will be small because of the relatively low absolute temperature differen- 

 tial. No attempt was made to calculate the effect of convective cooling or to measure the total 

 heat transfer coefficient of the insulating sandwich. Such study would not fit into the time 

 frame of the research program. 



The effect of convective cooling in cellular materials is currently a topic of study at the 

 U.S. Army Research Institute of Environmental Medicine at Natick, Massachusetts. 



Low Compressibility Insulation— Materials other than open-cell foams were evaluated for 

 the insulating laminate. The use of microballoons was rigorously examined but was found to be 

 impractical. The writer suggested incorporating microballoons into a diving-suit insulating 

 material during 1963. Subsequent in-house research was conducted by U.S. Rubber Company to 

 study the thermal and mechanical characteristics of microballoons encapsulated in a polymeric 

 matrix. The matrix material was chosen to be a polyvinyl chloride plastisol for convenience. 

 Its specific heat is very nearly the same as other rubber-like organic materials. 



The results of the thermal- conductivity measurements were not very promising, and the 

 material was deemed too rigid for use in diver's suits. But, while there are not written reports 

 offering data, there was a claim by an overseas investigator that such materials exhibit a frac- 

 tion of the thermal conductivity of closed-cell neoprene at depth. The details of the foreign 

 measurements were not made known, but much interest was generated in the United States. 



To settle the question, a second set of measurements was made by U.S. Rubber independent 

 of the first ones. The results were not vastly different, although the sample preparations and 

 measurements were performed by different investigators in independent studies. The results 

 are given in Table 34. The difference between the two control samples amounts to 9.5 percent. 

 This difference is probably due to different degrees of cure, degassing of the material, and 

 measuring procedures. 



