Practical Application of Ratio Deco
I’d like to take a few moments to illustrate how standard gases and ratio deco have impacted my diving.
I posted a trip report of our first attempt to dive the SS Governor.
Here, I’d like to talk about the flexibility the UTD standard gases and decompression strategies. Keep in mind that ratio deco isn’t a decompression theory, it’s simply a strategy using existing models and research to help shape an ascent profile.
On our attempt on the Governor in 2009, we used 15/55 for backgas and EAN50 and O2 for deco. This allowed us a range of depths, generally in the 200’ to 240’ range. That afforded quite a bit of flexibility to explore the wreck, which was especially useful in this case as the wreck is sitting upright where the mudline is at 240’ and the deck is at ~ 200’...quite a variance in depths.
Our plan, however, was to try to keep max depth at 230’, and spend most of our time on the deck at 200’ to keep our average depth at 220’ or shallower, in turn keeping our deco time manageable.
Unfortunately, due to vessel traffic delaying our splash time, we weren’t able to do the Governor. We instead decided to do the Al-Ind-Esk-A-Sea, another large wreck that sank in 1982 after a fire broke out and numerous explosions occurred.
Though the profile of the “Al” is similar to the profile of the Governor, it really made me think about what would happen if we were to do another wreck with a completely different profile. In other words, what if we planned to do a wreck that was 240’, but due to conditions, we were forced to do a dive in 200’.
Well, for starters, we’d have to make sure our backgas was compatible with the profile of that dive. We had 15/55 in our tanks, which would allow us to go as deep as 240’, and would certainly work for 200’. Now that we know our backgas is ok, we need to figure out our decompression. There’s quite a difference in the deco obligation/profile for a 240’ dive and a 200’ dive. If we simply used the same profile for the 200’ dive as we do for the 240’ dive, our first stop will be much deeper than necessary and our deco time will be much longer than necessary. We could bring out the laptop if we had brought it and cut new tables, but we’d also have to print them and laminate them. We could just run our computers as well, which is fine, but if everyone has different computers running different algorithms, that can create some confusion as well.
This is what I like about Ratio Deco...”deco on the fly”, or “tailgate deco” as I sometimes call it, allows me to calculate an entirely new deco schedule in 2 minutes time on scratch paper. Instead of hurrying to pump out a new set of tables, or doing way more deco than necessary because I don’t have any other tables than the ones I made for the original profile, I just grab a pen, a napkin, scroll out my new schedule as a general plan, then relax and prepare mentally for the dive.
How can I do this? Standard gases all have the same deco shape. Sure, we may add extra deco gases, but the shape is the same. No new numbers to remember, no tables to cut every dive. The shape is always the same whether it's an mdl dive, Tech 1 dive, Tech 2 dive or a Trimix dive. This makes it very easy to remember and calculate.
How about the reverse? Had we a chosen a mix specific to the depth we planned to go to (or "best mix"), rather than a range and ended up going to a wreck deeper than the one we had originally planned, we wouldn’t be diving at all that day. All that time and effort preparing, the cost of the charter and the gas, and we can’t even dive!
While there’s more benefits to standard gases and ratio deco, these are 2 that I really appreciate and have really positively effected my diving. They allow me to actually explore. We can do our dive, with plenty of flexibility to adjust our profile as needed, then, when our dive is over, we figure out our deco on the ascent.
On our dive on the Al-Ind-Esk-A-Sea, we had no tables or deco schedules...nothing written on a wrist slate or in our wetnotes. We jumped in and did our dive...then, when our dive was done and we were ready to ascend, we figured out our deco. One quick signal of “average depth, 210’” and we both knew what the rest of our profile would look like.
The next 65 minutes we spent decompressing.
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