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View Full Version : Geo: Which to follow Bar graph or NDC time?



RockDoc
01-29-2009, 20:43
My wife and I both dive with Oceanic Geos.
On some dives, the tissue loading bar graph has, over the course of the dives, gone up to 4 (of 5) bars, and the Hi Nitrogen alarm has gone off (which we have set at 4 bars (5 is Deco). While the bar graph was showing 4 bars, the No Decompression Time display showed (depending on the dive) between 20 to 40 minutes of no deco time.
Question: which to believe? Is the bar right (i.e. go up at 4 bars, in spite of 30 minutes of NDT) or is the NDT the "hard" number and the bar basically an "idiot light" to alert you that you are getting close to Deco and to pay close attention to the time.

navyhmc
01-29-2009, 21:26
The better question is at what depth are you when this occurs? If your depth is fairly shallow, it way be that with respect to the NDC time, you are close to the high end of your N2 saturation. The two should correspond though.

Can you give an idea of your dive profiles that led to this reading?

I dive a Geo and have not noticed that problem.

cummings66
02-01-2009, 07:51
To be honest, you should not be making a habit of riding the NDL curve as far as it can take you. Keep a reserve for the just in case things that come up.

Look at the tables and get an idea of how long you can be down at a depth and then consider what the computer is telling you in relation to that, use that knowledge as a primary factor in diving. The computer is nice to keep you honest, but I don't think diving it to the edge every time is a good habit to get into.

The NDL time remaining to answer your question varies per dive, the graph display is a courser readout of the past dives and current dive. You will find that by the time it gets to deco the time display says the same thing. You'll notice that as you come up into shallow water that the display will drop down and the NDL time says you now have more time to deco. If you add some stops on the way up you'll notice that the N level drops a bit depending on the depth you chose to stop at.

But, I'll say it again, don't ride it on the edge without the knowledge of what to do if you fall off the edge on the other side.

in_cavediver
02-01-2009, 10:22
...But, I'll say it again, don't ride it on the edge without the knowledge of what to do if you fall off the edge on the other side.

One more point - the edge that is discussed isn't really an edge at all. Its a fuzzy grey area that moves. The computer is telling you that most of the time, you are likely going to be OK with a direct ascent to the surface. This is merely a generalization and it knows nothing about your dive and what is happening in you at the time. Many factors can influence nitrogen uptake and release. This is the reason the standing advice is to never push limits. If you stay further from that fuzzy area, you are far less likely to run into problems.

It is possible to get bent even if the computer/table says you won't!
(they are called undeserved hits and DAN publishes data on this every year)

IrishSquid
02-01-2009, 10:38
...But, I'll say it again, don't ride it on the edge without the knowledge of what to do if you fall off the edge on the other side.

One more point - the edge that is discussed isn't really an edge at all. Its a fuzzy grey area that moves. The computer is telling you that most of the time, you are likely going to be OK with a direct ascent to the surface. This is merely a generalization and it knows nothing about your dive and what is happening in you at the time. Many factors can influence nitrogen uptake and release. This is the reason the standing advice is to never push limits. If you stay further from that fuzzy area, you are far less likely to run into problems.

It is possible to get bent even if the computer/table says you won't!
(they are called undeserved hits and DAN publishes data on this every year)

Good reply.

CWSWine
02-01-2009, 16:28
My wife and I both dive with Oceanic Geos.
On some dives, the tissue loading bar graph has, over the course of the dives, gone up to 4 (of 5) bars, and the Hi Nitrogen alarm has gone off (which we have set at 4 bars (5 is Deco). While the bar graph was showing 4 bars, the No Decompression Time display showed (depending on the dive) between 20 to 40 minutes of no deco time.
Question: which to believe? Is the bar right (i.e. go up at 4 bars, in spite of 30 minutes of NDT) or is the NDT the "hard" number and the bar basically an "idiot light" to alert you that you are getting close to Deco and to pay close attention to the time.

I have a Manta which is the same thing as the Geo but sold by Aries. While I was in Mexico I had the same thing happen to me while on a dive between 70 and 80 feet for about 25 minutes my computer went into warning for Deco. I also carry a Wisdom II and it was fine and not showing Deco. Upon return to the boat I notice the elevation was set for 4000 feet but I was in the Gulf. So the theoretical depth for 80 foot would be 92 feet and the tables put me at Deco. I was with a group and my Manta was the only one giving a alert.