Monday, March 5, 2018
Average Depth Calculator (spreadsheet) Download and why your dive computer is just an expensive calculator. Decompression theory...
For those that haven't used tables in a while, you can watch my two videos on Dive Tables by clicking on the two links below if you want to re-familiarize with them. Even if you don't, the tables basically tell us how long we can spend at particular depth for any given first dive. This assumes that the diver is making a direct descent from the surface, reaching their planned depth (but not going past it), spending an amount of time there, and then making their way back to the surface. The dive times that one uses in the tables above accounts for the total time in the water (including descents and the ascent). A safety stop is recommended but not part of this number. The diver can consider the end of the dive at 15ft and do their safety stop for 3 minutes after which they would exit the water.
The beauty of the dive computer is that it will calculate how deep you are at any depth and at a particular interval (the intervals below are one minute in duration however there are computers that you can change this value to as little as every 5 seconds but it will use more battery power), then add the next depth to that interval and divide all of that by how long one has been in the water. You could do this manually but your entire dive would be doing math, not exploring the amazing dive site you're on! Once you get to your 2nd, 3rd, 4th dive, all but a few could do the math quick enough and it's more likely you'll start making mistakes. One depth and one time is easy to plan for.
This first example below is a typical dive to boat #1, approximately 45ft max depth starting from the shore, looking around, and then making one's way back to the shore. The ocean floor at Sunrise Motel makes a gradual slope to the boat and when one averages the time one spends at any particular depth and then averages it among all the others, you can see that even a 45ft max depth dive, is really a 25ft dive!
If you go back up to NAUI Dive Tables on the top of the page, you can see that a 45ft dive has a maximum dive time of 80 minutes. I don't know about you but 30-45 minutes is a pretty good dive in 50°F water. So if we are taking on have of the amount of nitrogen that is allowed, our risk of DCS is low. If we then go to the table and look for 25ft, we have to drop to 40ft which gives us 130 minutes. Again, a very conservative profile.
These values are from the entire dive starting at the surface, descending to the ocean floor (101ft), then making my way back to the shore. At one minute intervals, one could do the math, but why?
Here is the entire video: https://youtu.be/ZdgE_2emtPQ
Now, if we look at the dive I dive from the surface above the deep boat, as I average my depths together, even this 101ft dive averaged over a 30 minute total dive time to all those varied depths only gives me an average depth of 44ft. Wow! Not only is my 44ft dive for 30 minutes rather conservative, if I was to use the dive tables for this dive, I would only get a total of 15 minutes in the water. But wait! 3 minutes to descend, 3 minutes to ascend, and only 9 minutes of bottom time. That sucks! Who wants to do a 9 minute dive?
What does it look like on my dive computer when I finally got close to my max depth? Not too far off from dive tables. At 94ft, that would give me 22 minutes according to the dives tables, and when I finally got to 101ft, only 15 minutes.
So, when someone says that dive tables are SOOOO conservative, well... NOT really. We have to be conservative due to the limitation for most divers' (technical decompression divers using tables on a wreck, excluded) inability to calculate a bunch of math in their heads.
Now, here is the crux of my point about dive computers being expensive calculators. Is there any real difference between a $2,000 computer and a $50 used computer you bought on Craigslist? Not as much as you might think...
Don't get me wrong, the dive computer has other features that are great to have. They log your dives, they tell you how fast you are ascending, they tell you how long you have to wait to fly (without waiting the default 24-hours), they do all the math for you, etc.
When you have some time, download the calculator and play with it. Everything except the column should be locked so you don't change the calculations performed in each cell. The idea behind understanding what is happening to the nitrogen dissolving into your body is important! Tables or computers, it's up to you.
WARNING: This also doesn't mean you can blow your bottom time because you'll make it up in the shallows, either. You require specialized training in order to do decompression dives and there are considerations you have to prepare for, but if you want to learn how, I can teach you how to do it safely. This overview is part of the understanding you need to know about.