Posted by MHK on May 01, 2001 at 15:31:39:
In Reply to: Some questions... posted by mattschechter on May 01, 2001 at 15:21:42:
When you are balancing your decompression gases you generally have 2 components. If you were using a 50% 02 and 50% N2, the component that drives the depth limitation is the 02 component. In this case you would use 50% 02 and using a 1.6 PP02 you would go no deeper than 70'.
As I noted I have been using a 50% 02 and 50% Helium based decompression gas, but once again, the driving force behind the depth limitation ( we call this the Maximum Operating Depth, a.k.a. MOD) is the 02 component. Again it's 50% so I wouldn't use that gas deeper than 70'.
For purposes of oxygen toxicity your main concern is the 02 content..
When we balance a gas we are trying to balance our mix so that we won't violate the MOD's, but by the same token the other component[s] will be necessary when figuring out the EAD or END..
We don't use greater than a PP02 of 1.4 for the working portion of our dive. We will however increase that to 1.6 while we are resting at deco. This helps us take advantage of the greater pressure gradients and allows us to off gas quicker, and thus shorten our deco time. Proper 02 windows suggest that we have stops, or gas switches at 20' ( 02), 70' ( 50%), 120 ( 35%), 190 ( 19%)...
SO in answer to your deep stop question we wouldn't use this gas deeper that the allowable MOD, in this case 70'...
Does that make it any clearer???? If not let me know and I'll expand a little...
Later