Reverse dive profiles

[Don Burke]

cornfed once bubbled... This is basic dive planning.

"Regular" profiles have no inherent safety factor over reverse profiles. If you plan your dives to push the NDLs you're decreasing safety, it doesn't mater if they're forward or backward.

I'd like to see your figures for a 30 minute dive to 70 feet followed by a one hour surface interval and then a 30 minute dive to 50 feet as compared to a 30 minute dive to 50 feet followed by a one hour surface interval and then a 30 minute dive to 70 feet.

I can't make the reverse profile work on the PADI tables without exceeding the no-stop limits.

 

[cornfed]

Don Burke once bubbled...

I can't make the reverse profile work on the PADI tables without exceeding the no-stop limits.

I don't have tables with me right now so take any number I give with a grain of salt. But even if I did it shouldn't matter. With that...

I think you missed my point, Don. Your example is like trying to plan a 60 minute dive to 70 feet and complaining that you can't stay within the NDL.

When you plan a dive you have to keep it within the NDLs. The sequence leading up to that dive doesn't effect this. It effects the calculation but not the axiom, if you will, that you need to keep it within the NDL. You could do your deepest dive first, you could take a one hour SI, you could take a four day SI, it doesn't change anything. When you plan your dive you need to stay within the no-stop times.

Now, you clearly do have control offer the events leading up to a particular dive. For example, I recently did two back to back 95 ft dives. Given the short bottom times at that depth I found I needed to put some thought in to my dive plan. I tried to choose a bottom time for the first dive and a surface interval which would maximize my total bottom time for the two dives. You can have a long first dive and a short second dive. You could lengthen your SI. You could have to shorter dives. There are a lot of different ways to do this and you're free to do it as you see fit. You're even allowed to choose a dive series which makes the second dive virtually impossible (in terms of NDL).

Now bear with me, the following is an abstract argument...

Let's say you do a dive to some depth, d1, for t1 minutes followed by a dive to d2 for t2. Now this is a regular dive so d1 > d2 and you calculate a NDL for the second dive. Let dt be the difference in bottom time and NDL for the second dive, dt = NDL - t2 > 0.

Repeat this for a reversed profile series, ie d1' for t1', d2' for t2', where d1' < d2' and dt' = NDL' - t2' > 0.

It shouldn't be too hard to pick values such that dt = dt'.

Where's the increase in safety? I don't see it.

My point all along has been to show ScubaJeep the benefit in "regular" profiles is that they help maximize bottom time, which is what most divers care about.

 

[Don Burke]

cornfed once bubbled... I don't have tables with me right now so take any number I give with a grain of salt. But even if I did it shouldn't matter. With that...

I think you missed my point, Don. Your example is like trying to plan a 60 minute dive to 70 feet and complaining that you can't stay within the NDL.

When you plan a dive you have to keep it within the NDLs. The sequence leading up to that dive doesn't effect this. It effects the calculation but not the axiom, if you will, that you need to keep it within the NDL. You could do your deepest dive first, you could take a one hour SI, you could take a four day SI, it doesn't change anything. When you plan your dive you need to stay within the no-stop times.

Now, you clearly do have control offer the events leading up to a particular dive. For example, I recently did two back to back 95 ft dives. Given the short bottom times at that depth I found I needed to put some thought in to my dive plan. I tried to choose a bottom time for the first dive and a surface interval which would maximize my total bottom time for the two dives. You can have a long first dive and a short second dive. You could lengthen your SI. You could have to shorter dives. There are a lot of different ways to do this and you're free to do it as you see fit. You're even allowed to choose a dive series which makes the second dive virtually impossible (in terms of NDL).

Now bear with me, the following is an abstract argument...

Let's say you do a dive to some depth, d1, for t1 minutes followed by a dive to d2 for t2. Now this is a regular dive so d1 > d2 and you calculate a NDL for the second dive. Let dt be the difference in bottom time and NDL for the second dive, dt = NDL - t2 > 0.

Repeat this for a reversed profile series, ie d1' for t1', d2' for t2', where d1' < d2' and dt' = NDL' - t2' > 0.

It shouldn't be too hard to pick values such that dt = dt'.

Where's the increase in safety? I don't see it.

My point all along has been to show ScubaJeep the benefit in "regular" profiles is that they help maximize bottom time, which is what most divers care about.

I don't see your point. Reply again when you get a set of tables in your hand and can try it with real numbers. The simple linear algebra you are trying to use will fall apart.

 

[cornfed]

Don Burke once bubbled...
I don't see your point. Reply again when you get a set of tables in your hand and can try it with real numbers. The simple linear algebra you are trying to use will fall apart.

Take any two dive series that end in the same general proximity of the no-stop limits. How could one be safer then the other?

 

[Don Burke]

cornfed once bubbled... Take any two dive series that end in the same general proximity of the no-stop limits. How could one be safer then the other?

That isn't the issue.

The issue is two dives to different depths. In one order they will result in a certain amount of residual nitrogen which reflects in the tables as a pressure group and a relationship to the no-stop limits. In the other order, the residual nitrogen, pressure group, and relationship to the no-stop limits will be different.

When you get a set of tables in hand, you'll see this.

Here is a pretty good site for tables:

http://www.flash.net/~table/table/p0000065.htm

Just tell me what set you wish to use and I'll calculate along with you.

 

[cornfed]

Don Burke once bubbled...
That isn't the issue.

The issue is two dives to different depths. In one order they will result in a certain amount of residual nitrogen which reflects in the tables as a pressure group and a relationship to the no-stop limits. In the other order, the residual nitrogen, pressure group, and relationship to the no-stop limits will be different.

Alright, quick question...

Using the "New Doppler Line Modified Navy Tables"...

A 60 minute dive to 50 ft and a 40 minute dive to both put you within 10 minutes of the respective NDL. My argument centers around not being able to say one of these dives is "safer" than the other. What am I missing?

 

[Don Burke]

cornfed once bubbled... Alright, quick question...

Using the "New Doppler Line Modified Navy Tables"...

A 60 minute dive to 50 ft and a 40 minute dive to both put you within 10 minutes of the respective NDL. My argument centers around not being able to say one of these dives is "safer" than the other. What am I missing?

I'll have to crunch those for a while. I've already screwed it up a couple of times.

 

[Don Burke]

cornfed once bubbled... Alright, quick question...

Using the "New Doppler Line Modified Navy Tables"...

A 60 minute dive to 50 ft and a 40 minute dive to both put you within 10 minutes of the respective NDL. My argument centers around not being able to say one of these dives is "safer" than the other. What am I missing?

This is not about a single dive. It is about repetitive diving.

The example you gave won't stay on the tables. I'm assuming you meant the 40 minute dive to go to 50 feet.

If I shorten the times to 30 minutes at 50 feet and 25 minutes at 60 feet, I can keep them on the tables.

50 feet for 30 minutes makes you an E diver. A one hour surface interval makes you a D diver. The RNT for a D diver going to 60 feet is 24 minutes. That plus 25 minutes results in a total bottom time of 49 minutes, one minute short of the no-stop time for 60 feet.

60 feet for 25 minutes makes you an E diver. A one hour surface interval makes you a D diver. The RNT for a D diver going to 50 feet is 29 minutes. That plus 30 minutes results in a total bottom time of 59 minutes, eleven minutes short of the no-stop time for 50 feet.

We can debate the accuracy of the tables, but I like being 11 minutes short of the limit as opposed to one minute.

 

[Charlie99]

cornfed once bubbled...
Take any two dive series that end in the same general proximity of the no-stop limits. How could one be safer then the other?

Quite easily if the model strays significantly from reality.

Models based solely on dissolved gas, such as the DSAT/PADI RDP don't call for deep stops. They also don't have any penalty for reverse profiles. RGBM does.

 

[cornfed]

Don Burke once bubbled...
This is not about a single dive. It is about repetitive diving.

The example you gave won't stay on the tables. I'm assuming you meant the 40 minute dive to go to 50 feet.

That quick question didn't have anything to do with repetative dives.

I was asking a question about two seperate, unrelated dives.

Can you answer my question about safety as it applies to this scenario? If you do it will help make my point clearly later on.

If I shorten the times to 30 minutes at 50 feet and 25 minutes at 60 feet, I can keep them on the tables.

50 feet for 30 minutes makes you an E diver. A one hour surface interval makes you a D diver. The RNT for a D diver going to 60 feet is 24 minutes. That plus 25 minutes results in a total bottom time of 49 minutes, one minute short of the no-stop time for 60 feet.

60 feet for 25 minutes makes you an E diver. A one hour surface interval makes you a D diver. The RNT for a D diver going to 50 feet is 29 minutes. That plus 30 minutes results in a total bottom time of 59 minutes, eleven minutes short of the no-stop time for 50 feet.

We can debate the accuracy of the tables, but I like being 11 minutes short of the limit as opposed to one minute. [/B]

You and I agree. You just don't see it yet.