Post by Al Johnston on Nov 19, 2005 9:24:39 GMT -4
To be fair to certain contributors (however undeserving) vacuum systems do leak.
When I worked making Titanium, we used vacuum arc furnaces to melt the metal. The vacuum seal essentially consisted of a pair of flat steel rings pressed together with a single rubber O-ring between them: not the most perfect system, but good enough, particularly as there were pumps to maintain the pressure differential. However, during the course of a three-hour melt, introducing too much air would oxidise the titanium to an unacceptable degree, so one of the start-up tests was a check of the leak rate: if it was more than a few mmHg per minute, the melt was delayed while the cause was investigated (usually dirt in the seal or a duff O-ring).
Part of the extensive development and testing process for the Apollo spacesuits would have included a check of the leak rate, keeping it to a level where the pressure in the suit could be maintained within the specified range for the full anticipated EVA duration. A quick and dirty check that the Astronauts could perform before depressurising the spacecraft would have been part of their suiting up checklist.
As an aside, the vacuum-arc process involved melting Titanium (MP 1941K) into a crucible made of Copper (MP 1357K). This was made possible because the heat of fusion was abstracted by cooling the crucible using either flowing water or liquid metal (NaK, a sodium-potassium mix, MP 298K). The cooling flow of water was dependent on the correct assembly of furnace, crucible and crucible liner: on one occasion when the incorrect liner was used, we had a dramatic demonstration of exactly why each water-cooled furnace was enclosed in its own individual blast pen...
When I worked making Titanium, we used vacuum arc furnaces to melt the metal. The vacuum seal essentially consisted of a pair of flat steel rings pressed together with a single rubber O-ring between them: not the most perfect system, but good enough, particularly as there were pumps to maintain the pressure differential. However, during the course of a three-hour melt, introducing too much air would oxidise the titanium to an unacceptable degree, so one of the start-up tests was a check of the leak rate: if it was more than a few mmHg per minute, the melt was delayed while the cause was investigated (usually dirt in the seal or a duff O-ring).
Part of the extensive development and testing process for the Apollo spacesuits would have included a check of the leak rate, keeping it to a level where the pressure in the suit could be maintained within the specified range for the full anticipated EVA duration. A quick and dirty check that the Astronauts could perform before depressurising the spacecraft would have been part of their suiting up checklist.
As an aside, the vacuum-arc process involved melting Titanium (MP 1941K) into a crucible made of Copper (MP 1357K). This was made possible because the heat of fusion was abstracted by cooling the crucible using either flowing water or liquid metal (NaK, a sodium-potassium mix, MP 298K). The cooling flow of water was dependent on the correct assembly of furnace, crucible and crucible liner: on one occasion when the incorrect liner was used, we had a dramatic demonstration of exactly why each water-cooled furnace was enclosed in its own individual blast pen...