Mailing List Archive Mailing List Archive
>I was under the impression that the Prism was a semi (rather than a fully) - >closed system. Am I wrong?? No this is not wrong. However the Prism and the BMD/SCR-4 operate using completely different principles. The Prism operates using a constant mass flow of gas regardless of depth. The final FO2 of the inspired gas is related to the rate of flow into the system and the rate that O2 is removed from the system via metabolism. This can create (at least) 2 problems. If the rates of O2 consumption and addition are not matched then the PO2 may enter a range incompatible with life. In order to calculate the final FO2 you would have to know your O2 consumption rate. The second problem is scrubber failure which can result in PCO2 levels incompatible with life. I don't know how likely either of these are in practice. The BMD/SCR-4 operates by replacing a constant volume of gas ( 25% ) with each breath. The harder you breath the more gas is replaced by the unit. In theory you can calculate the final FO2 for a given depth and supply mix. BMD claims the FO2 can be calculated by subtracting 4% from the supply FO2 and then adding 1% for each atmosphere of water over your head (note that this means that FO2 rises with depth which results in a more restricted operation range for a given mix). The problem of CO2 poisoning if the scrubber fails is solved by venting 25% of the respired gas. I haven't gone through the math but presumably the end point PCO2 if the scrubber fails is not fatal. These aren't the only 2 issues involved in selecting a rebreather. However they are the 2 that BMD picked for thier sales pitch. Barrie Kovish Vancouver, Canada bmk@ds*.bc*.ca*
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]