Mike1967
Contributor
You're right, I just reset defaults and it was 30/70 on Tec. I must have changed it when I first got it so the dive planning results would be the same on either mode.
Overshooting NDL and mandatory deco stops
- Thread starter NAUI Wowie
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seeker242
Contributor
PDCs don't bend divers: divers bend themselves. PDCs are as dangerous as the person using them. Suuntos are de-facto more conservative than any other group of PDCs out there.. While they might provide a somewhat "safer" output, the biggest problem facing the diver is still the person wearing it. PDCs can't force you to ascend slowly, do a full five minute safety stop, avoid riding your PDC's NDL, avoid bounce dives and otherwise dive conservatively. Only you can accomplish that so your safety is all up to you. Your PDC is only a tool that can be used or misused. It doesn't matter if you understand the underlying algorithms or not. Your safety is entirely up to you. PDCs don't bend divers: divers bend themselves.
Mike1967
Contributor
So that was copied from the Teric manual. The Perdix manual actually says this. Perhaps it has changed?
"
DECOMPRESSION AND GRADIENT FACTORS
The basic decompression algorithm used for the computer is Bühlmann ZHL-16C. It has been modified by the use of Gradient Factors that were developed by Erik Baker. We have used his ideas to create our own code to implement it. We would like to give credit to Erik for his work in education about decompression algorithms, but he is in no way responsible for the code we have written.
The computer implements Gradient Factors by using levels of conservatism. The levels of conservatism are pairs of number like 30/70. For a more detailed explanation of their meaning, please refer to Erik Baker’s excellent articles: Clearing Up The Confusion About “Deep Stops” and Understanding M-values� The articles are readily available on the web. You might also want to search for “Gradient Factors” on the web.
The default of the system is 30/70. The system provides several settings that are more aggressive than the default.
Don’t use the system until you understand how it works�
"
"
DECOMPRESSION AND GRADIENT FACTORS
The basic decompression algorithm used for the computer is Bühlmann ZHL-16C. It has been modified by the use of Gradient Factors that were developed by Erik Baker. We have used his ideas to create our own code to implement it. We would like to give credit to Erik for his work in education about decompression algorithms, but he is in no way responsible for the code we have written.
The computer implements Gradient Factors by using levels of conservatism. The levels of conservatism are pairs of number like 30/70. For a more detailed explanation of their meaning, please refer to Erik Baker’s excellent articles: Clearing Up The Confusion About “Deep Stops” and Understanding M-values� The articles are readily available on the web. You might also want to search for “Gradient Factors” on the web.
The default of the system is 30/70. The system provides several settings that are more aggressive than the default.
Don’t use the system until you understand how it works�
"
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For some scenarios (rapid ascent, missed SS, short SIs), Suunto's algorithm definitely is on the more conservative side. If you watch your ascent speed, complete your SS and make sure to take at least a 2-hour SI (all of which, IMNSHO, are good practices), Suuntos aren't particularly more conservative than many other computers.
That "Suuntos are so very conservative" legend is just that. A legend.
@ScubadriverDale
It has been stated that a reason for using the Perdix is that it has a superior and more intuitive user interface. I'm paraphrasing. I agree that is the case. I think it is important to note however that it's ease of use has a hidden danger and that is that it encourages some people to limit their understanding of the capabilities of the computer. It makes it possible to get it up and running without reading the manual. I'd like to outline why that is a bad idea based on my personal experience.
There are rather brilliant sets of menus within those two buttons. I have witnessed on a dive where a diver that didn't understand the gas definitions and selection menus started pushing the buttons underwater trying to get back to the basic dive screen and actually defined a gas as 41% nitrox, turned it on and switched to it in the middle of a dive. It is made easy to do this by this computer because it is assumed that you understand the computer and gives you the power to do it easily. She came to me at 100 feet and showed me a warning that she was exceeding her PPO limit even though she was diving air. She was having a hard time getting to a screen that made sense to her because she didn't really understand the computer beyond it's simple and easy to understand main screens. She was of course showing a much higher NDL at that point than she should have been because the computer thought she was on 41% nitrox.
Please consider taking some time to become more familiar with the concepts of enriched air diving and it's consequences, decompression algorithms and gradient factors and please read the manual a few times. The learning curve is rather steep at first and there is not a big rush. Take your time. The reason for my post is just to let you know that there are good reasons to learn beyond the minimums related to your current level of diving and there are really good reasons to completely understand the Perdix, expecially the concepts around gas selection and gradient factors.
It has been stated that a reason for using the Perdix is that it has a superior and more intuitive user interface. I'm paraphrasing. I agree that is the case. I think it is important to note however that it's ease of use has a hidden danger and that is that it encourages some people to limit their understanding of the capabilities of the computer. It makes it possible to get it up and running without reading the manual. I'd like to outline why that is a bad idea based on my personal experience.
There are rather brilliant sets of menus within those two buttons. I have witnessed on a dive where a diver that didn't understand the gas definitions and selection menus started pushing the buttons underwater trying to get back to the basic dive screen and actually defined a gas as 41% nitrox, turned it on and switched to it in the middle of a dive. It is made easy to do this by this computer because it is assumed that you understand the computer and gives you the power to do it easily. She came to me at 100 feet and showed me a warning that she was exceeding her PPO limit even though she was diving air. She was having a hard time getting to a screen that made sense to her because she didn't really understand the computer beyond it's simple and easy to understand main screens. She was of course showing a much higher NDL at that point than she should have been because the computer thought she was on 41% nitrox.
Please consider taking some time to become more familiar with the concepts of enriched air diving and it's consequences, decompression algorithms and gradient factors and please read the manual a few times. The learning curve is rather steep at first and there is not a big rush. Take your time. The reason for my post is just to let you know that there are good reasons to learn beyond the minimums related to your current level of diving and there are really good reasons to completely understand the Perdix, expecially the concepts around gas selection and gradient factors.
Do you understand what the Gradient Factor does?
Do you understand why there are different recommended GF settings?
Do you understand that the GF settings change with different gases?
You where asked what GF settings you currently have, you didn't know, that means your computer could be at 100/100, which means you are diving pure Bhulmann?
As an example. if you where using 18/45 Trimix and pure oxygen. For a 50 meter 20 minute dive
at GF100/100 total time is 40 minutes
for GF20/70 total time is 67 minutes.
That's one hell of a difference in the decompression profile!
Now I have dived with those who run one computer 100/100 and the second set for the real dive profile e.g 18/45.
If it all goes to ratshit, you follow the 100/100, running the risk/likelyhood of a helicopter ride to the nearest chamber.
Addendum
I am pretty certain Shearwater used the ZHL16C which is more conservative than the B and A variants. Which according to both the Bhulmann table and the Suunto table I have, on a 30m dive is within a minute or so of the Suunto algorithm, for a 30meter air dive, so not quite as bad as I thought it was going to be.
I know the VR3 used the Bhulmann algorithm, but not which variant, probably C. I know that if you didn't increase the safety factor (which assumed a higher nitrogen load rather than gradient factors,) they where too chamber friendly. Especially when compared to the Suunto. But, then the VR3 was not really used for no stop dives.
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If you don't want to put in the effort required to learn some basic deco science, I'd recommend just following what they taught when I took my PADI OW.
If you overstay your NDL by less than 5 minutes, stay at safety stop depth for 8 minutes and avoid diving for at least 6 hours. If you overstay your NDL by more than 5 minutes, stay at safety stop depth for at least 15 minutes if you have enough gas and avoid diving for at least 24 hours.
(Translated back to English from my OW manual)
Personally, if I overstayed my NDL by more than 5 minutes, I'd spend the rest of my tank at safety stop depth, surface with some 10-ish bar (~150 psi) left and then ask for the O2 bottle when I got back on the boat. And I'd be looking out for any DCI symptoms.
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