Dive Computers, Deco Planning, and Bailout: A Beginner's Guide

Louby Lou

Hello Again

Rob Infante has kindly said we can publish his article Dive Computers, Deco Planning, and Bailout: A Beginner's Guide which can be found in the library HERE

If you would like to ask Rob any questions, leave any comments or feeback please drop them below on this thread. If you would like to leave reputation, please use the article.

Thanks again Rob

Cheers

Miss Lou

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bgpartri

Rob,

I'm glad that you're pleased with the computer, but I am concerned about your results. Although stops aren't always exactly to the minute, this is by design. If you clear to the next ceiling after 10 seconds, it lets you go. It also takes a finite amount of time to calculate the ceiling, so it's possible for a stop to take a few seconds extra while the algorithm is calculating.

But for a stop to take twice as long, there is something else going on. I have done hundreds of dives in the water and in the chamber, and the TTS prediction is usually within a minute. With 5 hours of deco, it's still within a few minutes.

I hope you don't mind me taking this opportunity to expand on a few things that obviously aren't covered well enough in the manual.

First, a bit of an overview of how the computer does this stuff.

There are two things going on all the time.

First, the compartment loading function uses the actual PPO2 and the actual depth to calculate the changes in compartment gas loading.

Second, the TTS function predicts the length of time at the current stop and the total TTS. It does this by using the current PPO2 and the profile. It always assumes that you are going to ascend smoothly at 30 feet per minute to the current ceiling, and then stay at that ceiling until it clears. If you have more than one CC gas programmed, it assumes that you are going to switch to that gas at an appropriate depth.

Anything that you do on the dive that is off profile will change the TTS and the time at the current stop prediction.

Some things have very little effect. Small changes in PPO2 on the bottom have very little effect. Small changes in PPO2 on shallow stops make a big difference.

Being a few feet off the stop at deep stops makes little difference. Being 10 feet off the stop at shallow stops makes a big difference.

Here are a few things that could cause that sort of problem.

1) Programming a CC gas and then not switching to it.
The gas will flash in that case. For example, I got a call last week about a 1 minute stop that took 4 minutes. He had 99% programmed as a CC gas. So here's what the computer assumes. You have told it you are going to switch by programming the gas in CC. If you were using it as bail out, you would have programmed it as an OC gas. The computer assumes that you are going to switch to O2 in the next second.

But you don't switch, so the compartment loading routines continue to offgas with your actual PPO2. The next TTS calculation, again assumes that you are going to switch to O2 in the next second but you don't switch and on and on....

Here's an example. I put in 99% as a CC gas. The computer assumes that I am going to switch to a 99% diluent at 20 feet. So the prediction function uses a PPO2 of 1.6, while the compartment loading functions uses the actual PPO2 of say 1.2. The prediction will underestimate the remaining time at the stop.

2) Following the profile of another computer.
For example, if the GF is showing a 100 foot ceiling, and you start doing stops at 150, the GF will add deco time. For the TTS to match, the profile has to match.

3) Not being at the stop.
If you have a 10 foot stop of 20 minutes and you stop at 20 feet, it will take significantly longer. This is similar to number 1. The computer assumes that in the next second you are going to ascend at 30 feet per minute to 10 feet and complete your deco there.

However that compartment loading function is always calculating with your actual depth. So the prediction is always wrong.

By the way, there's nothing wrong with doing that. I will often do my 10 foot stop at 15 feet. The computer won't penalize you. But the time at stop estimate will always be too short for the above reasons.

Thanks for the opportunity to clarify these issues.

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Bobmac

Great article. Thanks for putting it together.

Mark Chase

Nice article.

Strangly I have a similar problem with the Hammer Head GF deco. It will say somthing like 1min at 15m and the VR3 says 4. Three mins later it still says 1min at 15m and the VR3 is now in full agreement

Running 25/85 the GF computer is getting me out about 10mins faster than the VR3 on 0 safety folowing 30-40mins at 65m.

ATB

Mark Chase

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decoweenie

Which profile do you actually follow ?

Or do you follow both (stop whenever a stop from either profile calls for it) ?

I always get funny feelings when reading people quoting that they compared 2 different algorithms by following both profiles on the same dive. Then the profile is no longer what it should be, thus the comparison is invalid (theoretically).

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Mark Chase

On day one the VR3 so i dont bend it. On the last day the HH.

Its not that hard basicly i have to add the very very deep stop asked for by the VR3 and usualy insert a couple of stops between 18m and 9m to keep the GF happy then extend the last stop to clear the VR3. Nothing i wasn't doing already.

ATB

Mark Chase

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Is it supposed to make that noise ?

I took my unit to the dive shop and demanded they bolt on every thing that would fit.

Join my elite diving teem and get a Tshirt

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decoweenie

I am not inquiring what you do because I think it is rocket science... I just don't like comparisons that are not apple-to-apple since the result will not a correct.

For example, inserting deeper stop on a GF profile will offset its normal on-offgas sequence when compare to an exact GF profile without insertion. And inserting intermediate stops also do the same.

The only valid thing anyone could try to do when doing a Lousiana-Gumbo like that is finding a combination of settings that would give you the same RT...

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bgpartri

Not all computers do a multiple gas TTS. On the original Meg Shearwater, it does a single gas TTS. It assumes that you are going to stay on the current gas and the current set point for the rest of the dive. That works fine for CC in any but the most extreme dives. Most people don't switch diluent in current practise. I believe the HH is the same. (Correct me if I'm wrong about that.)

The Shearwater GF does multiple gas TTS and time at stop. It's not so important for CC, but if you go to OC bailout, and you are planning to use a 12/35 and 50, your TTS would be way off if it assumed you were going to ride the 12/35 to the surface. Since in a bailout situation, it is important to know your TTS with the current gases, I made sure the current product has that feature. The VR3 does this too.

On the GF, you have two gas sets, CC and OC. Each gas set needs to be set to the gases you are actually carrying for correct TTS and time at stop.

On the VR3, you turn gases on and off to tell it what gases to use. I'm not sure how the HS does it.

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bgpartri

I have updated the manual and website with the above information.

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Good judgment comes from experience. Experience comes from bad judgment.