Computer VS. Tables

[Hawkwood]

That's too bad. I've found it to be a very valuable teaching tool, despite some if the glitches. It is generic, but that is the point. It is only to illustrate how to use computers in general, when diving

 

[boulderjohn]

If you did a 5 minute safety stop after a 15 minute dive to 100 feet, the tables will give you a nitrogen load of 20 minutes at 100 feet.

Your computer sees 100 feet as 100 and 15 feet as 15. If you want to be conservative, by all means follow the tables, but the Suunto algorithms have been used and tested for many years. And Suunto, by the way, has a reputation as being a conservative computer.

With most tables, your bottom time ends when you begin your ascent to the surface. It does not count either that ascent or the safety stop. If you are using the PADI tables, for example, and you start your ascent from 100 feet after 15 minutes, take 1.5-2 minutes to reach safety stop depth, do the safety stop, and ascend slowly to the surface, then you log the dive as 100 feet for 15 minutes. Those are the numbers you use to plan the next dive.

 

[DiveNav]

My PADI class had a book on computer use ....... but there was no online simulator.....

When a PADI Open Water student diver enrolles in new PADI Open Water Diver course that uses a dive computer as planning tool, s/he should receive the new PADI Open Water Diver Computer Crew-Pak (Product N. 60335) or the PADI Open Water eLearning Crew-Pak with Dive Computer Simulator (product no. 70821). This Crew-Paks includes a Dive Computer Simulator Access Card which contains a Unique Activation Code.

As part of our agreement with PADI, a student enrolled in the PADI Open Water diver course is eligible to receive a 12 month free license for divePAL Nitrox, 12 month of free use for a model-specific dive computer simulator and a substantial discount on any of our online classes for model-specific dive computers.

divePAL is a multiplatform application that allows student divers to plan, execute, log and analyze simulated (or real) dives. The intuitive graphical interface makes it easy for a student diver to input a dive profile, analyze it, and view the trends and relationships between depth, time, nitrogen loading and tank pressure.

divePAL is also a powerful dive computer simulator that can be customized with sever model-specific dive computer simulators that appear and function like the real dive computers student divers may use during their Open Water training.

Specifically to this thread, divePAL includes also a tool to ESTIMATE a pressure group of a dive (using the average depth approach). This tool should NOT be used for planning real dives as its main intent is for education.
For more info on this tool ... keep reading......

**************************************************************************

When you took your Open Water course, among other things, you learned that, when we dive, our body accumulates excessive Nitrogen and then when we surface this extra Nitrogen gets released.

You also learned that this intake/release of Nitrogen is somehow a function of depth and time and most likely you were taught to track it by using a dive table:
..... select the depth column .... go down until you find your bottom time (rounded-up) .... go right to find the pressure group at the end of the dive ..... look for your surface interval time (range) .... go down to find your pressure group at the end of the surface interval ..... and if you are doing a repetitive dive ..... turn the dive table around ..... find the column with the Pressure group at the beginning of the dive .... look for the row with the depth of the repetitive dive .... find the cross-section .... write down the Residual Nitrogen ..... turn the table around and repeat procedure ... remembering to add the RN to the bottom time ......

But, have you ever wondered how a dive table really works?
Do you know that a dive table has a shape that tracks the nitrogen loading during the dive and is not just a large collection of boxes with numbers in it?

Let's see an example together.
In the figure below we have a dive profile of a real multi-level dive with a maximum depth of 90ft and bottom time of 40 minutes (including a safety stop of 3 min).

The first thing we notice is that this dive would NOT be possible using a dive table (40 minutes at 90ft is way into deco) .... this is because dive tables assume squared profiles .....

The second thing we notice is that this dive is actually feasible for most AOW recreational divers - equipped with dive computers - as it is NOT a deco dive.

In fact, if we analyze this dive using divePAL (ZH-L16C - moderate conservatism), we find out that the maximum value of nitrogen loading during the dive was ~88% and the N2 load at the end of the dive was ~76%.
See figure below.

So, since this dive is feasible, what should its Pressure Group be?
Or even better, how does the Pressure Group change during this dive?
What is the shape of the dive table for this dive?

In the version 0.12 of divePAL Windows, we introduced the feature Estimated Pressure Group that visualize how the PG progresses (changes) during a dive.

In the image below we can see that 10 minutes into the dive the EPG was B.

At half point into the dive it was I, then it peaked at Q at about minute 33 and adjusted at that level till the end of the dive.

When I look at the dive tables in this way I almost like them

 

[industrious95]

With most tables, your bottom time ends when you begin your ascent to the surface. It does not count either that ascent or the safety stop. If you are using the PADI tables, for example, and you start your ascent from 100 feet after 15 minutes, take 1.5-2 minutes to reach safety stop depth, do the safety stop, and ascend slowly to the surface, then you log the dive as 100 feet for 15 minutes. Those are the numbers you use to plan the next dive.

That's not the way I was taught back in 1986. PADI AOW. We were taught to use the total dive time as our bottom time. But I guess things have changed in 28 years.

How would you calculate an ocean shore dive, where you might get down to 45 feet, but spend 15 minutes swimming back to shore, slowly getting shallower until you exit?

 

[TMHeimer]

Good point. When I shore dive (usually 30' or at most a bit deeper), I do use the total bottom time from descending at shore to surfacing at shore. Of course, other factors end the dive other than NDL (or no stop time, as it is now referred to). For deep boat dives I have the computer and constantly check remaining bottom time, but I really still think tables since my dives are usually square profile. I still count the whole time since I usually shoot down the anchor line fast. Plus it gives me more conservatism.

 

[boulderjohn]

That's not the way I was taught back in 1986. PADI AOW. We were taught to use the total dive time as our bottom time. But I guess things have changed in 28 years.

How would you calculate an ocean shore dive, where you might get down to 45 feet, but spend 15 minutes swimming back to shore, slowly getting shallower until you exit?

It is possible that you misremembered your instruction--the original copyright on the PADI RDP, the directions for which have not changed, was in 1985. Before that they used the U.S. Navy tables, which were done the same way. It is also possible that you were misinformed by your instructor. For nearly the past two decades, we have relied heavily upon home study as the primary method for students to learn the academic material. A conflict between what the instructor says and what the course content says should be very apparent. When you learned it was more likely to be presentation by lecture, so misinformation of that kind can be transmitted easily to students.

In the scenario you describe, you are doing a multi-level dive, and the tables are not designed for that. With tables, you treat the entire dive as if it were conducted at the deepest point. You would calculate it as if it were a square profile, with the entire dive having been conducted at 45 feet. When you decide you have done an ascent with a safety stop and when you have just been extending the dive is really arbitrary. That is an obvious weakness in table design.

Because of that, PADI created the wheel, a complex table that allowed divers to calculate the dive in up to three levels. In the scenario you describe, you would create three levels, depending upon the bottom contour. Let's say you choose 45', 35', and 25'. You begin your dive at the deepest point and work your way shallower. Count the number of minutes between 45-35 feet. That is your first level time. Then count the number of minutes between 35-25 feet for your second level time. Then finish with the time above 25 feet until you head directly to the surface for your third level time. The wheel would then give you your final pressure group.

Pretty arbitrary, huh? Especially if you do the last 20 minutes above 20 feet. Is that part of your bottom time, or is it part of your extended safety stop, which doesn't count? With computers, counting mutli-level dives that way does not make much sense for that reason. Consequently PADI advises students that when they go out into the world and log their dives, they can do whatever makes sense to them--traditional bottom time or total dive time. When I work with students in the computer version of the course, we do total dive time.

---------- Post added August 19th, 2014 at 04:05 PM ----------

Here is another example of the weakness of tables in calculating a dive such as was described in post #24. I once did a dive to a small wreck at a depth of about 100 feet. It was not much of a wreck, and we did not stay there very long. We then went briefly to a middling depth, but we ended up drifting gently along on the top of the reef for so long that our total dive time, with AL 80 tanks, was over 80 minutes. How do you handle that on a table, when the maximum bottom time for a 200 foot dive is 20 minutes? When did our ascent to the surface begin? When did our safety stop begin?

If you were to treat your tables like a computer, we did a 7 minute dive to 100 feet followed by a 7 minute dive with no surface interval to say 50 feet followed by a 66 minute safety stop. I could probably calculate my dive that way, but that would certainly not be how the tables were intended to be used. If I logged it that way, my log entry would not truly represent the dive, since I was actively diving that entire time, not hanging in the water waiting to surface for three minutes, which is what a safety stop implies. Although I did have the physiological effect of a 66 minute safety stop, in terms of what I was actually doing on the dive, I did not do a safety stop at all.

 

[MichigandiverJon]

When a PADI Open Water student diver enrolles in new PADI Open Water Diver course that uses a dive computer as planning tool, s/he should receive the new PADI Open Water Diver Computer Crew-Pak (Product N. 60335) or the PADI Open Water eLearning Crew-Pak with Dive Computer Simulator (product no. 70821). This Crew-Paks includes a Dive Computer Simulator Access Card which contains a Unique Activation Code.

As part of our agreement with PADI, a student enrolled in the PADI Open Water diver course is eligible to receive a 12 month free license for divePAL Nitrox, 12 month of free use for a model-specific dive computer simulator and a substantial discount on any of our online classes for model-specific dive computers.

divePAL is a multiplatform application that allows student divers to plan, execute, log and analyze simulated (or real) dives. The intuitive graphical interface makes it easy for a student diver to input a dive profile, analyze it, and view the trends and relationships between depth, time, nitrogen loading and tank pressure.

divePAL is also a powerful dive computer simulator that can be customized with sever model-specific dive computer simulators that appear and function like the real dive computers student divers may use during their Open Water training.

Specifically to this thread, divePAL includes also a tool to ESTIMATE a pressure group of a dive (using the average depth approach). This tool should NOT be used for planning real dives as its main intent is for education.
For more info on this tool ... keep reading......

**************************************************************************

When you took your Open Water course, among other things, you learned that, when we dive, our body accumulates excessive Nitrogen and then when we surface this extra Nitrogen gets released.

You also learned that this intake/release of Nitrogen is somehow a function of depth and time and most likely you were taught to track it by using a dive table:
..... select the depth column .... go down until you find your bottom time (rounded-up) .... go right to find the pressure group at the end of the dive ..... look for your surface interval time (range) .... go down to find your pressure group at the end of the surface interval ..... and if you are doing a repetitive dive ..... turn the dive table around ..... find the column with the Pressure group at the beginning of the dive .... look for the row with the depth of the repetitive dive .... find the cross-section .... write down the Residual Nitrogen ..... turn the table around and repeat procedure ... remembering to add the RN to the bottom time ......

But, have you ever wondered how a dive table really works?
Do you know that a dive table has a shape that tracks the nitrogen loading during the dive and is not just a large collection of boxes with numbers in it?

Let's see an example together.
In the figure below we have a dive profile of a real multi-level dive with a maximum depth of 90ft and bottom time of 40 minutes (including a safety stop of 3 min).

The first thing we notice is that this dive would NOT be possible using a dive table (40 minutes at 90ft is way into deco) .... this is because dive tables assume squared profiles .....

The second thing we notice is that this dive is actually feasible for most AOW recreational divers - equipped with dive computers - as it is NOT a deco dive.

In fact, if we analyze this dive using divePAL (ZH-L16C - moderate conservatism), we find out that the maximum value of nitrogen loading during the dive was ~88% and the N2 load at the end of the dive was ~76%.
See figure below.

So, since this dive is feasible, what should its Pressure Group be?
Or even better, how does the Pressure Group change during this dive?
What is the shape of the dive table for this dive?

In the version 0.12 of divePAL Windows, we introduced the feature Estimated Pressure Group that visualize how the PG progresses (changes) during a dive.

In the image below we can see that 10 minutes into the dive the EPG was B.

At half point into the dive it was I, then it peaked at Q at about minute 33 and adjusted at that level till the end of the dive.

When I look at the dive tables in this way I almost like them

Thank you for the great explanation!

---------- Post added August 19th, 2014 at 09:03 PM ----------

It is possible that you misremembered your instruction--the original copyright on the PADI RDP, the directions for which have not changed, was in 1985. Before that they used the U.S. Navy tables, which were done the same way. It is also possible that you were misinformed by your instructor. For nearly the past two decades, we have relied heavily upon home study as the primary method for students to learn the academic material. A conflict between what the instructor says and what the course content says should be very apparent. When you learned it was more likely to be presentation by lecture, so misinformation of that kind can be transmitted easily to students.

In the scenario you describe, you are doing a multi-level dive, and the tables are not designed for that. With tables, you treat the entire dive as if it were conducted at the deepest point. You would calculate it as if it were a square profile, with the entire dive having been conducted at 45 feet. When you decide you have done an ascent with a safety stop and when you have just been extending the dive is really arbitrary. That is an obvious weakness in table design.

Because of that, PADI created the wheel, a complex table that allowed divers to calculate the dive in up to three levels. In the scenario you describe, you would create three levels, depending upon the bottom contour. Let's say you choose 45', 35', and 25'. You begin your dive at the deepest point and work your way shallower. Count the number of minutes between 45-35 feet. That is your first level time. Then count the number of minutes between 35-25 feet for your second level time. Then finish with the time above 25 feet until you head directly to the surface for your third level time. The wheel would then give you your final pressure group.

Pretty arbitrary, huh? Especially if you do the last 20 minutes above 20 feet. Is that part of your bottom time, or is it part of your extended safety stop, which doesn't count? With computers, counting mutli-level dives that way does not make much sense for that reason. Consequently PADI advises students that when they go out into the world and log their dives, they can do whatever makes sense to them--traditional bottom time or total dive time. When I work with students in the computer version of the course, we do total dive time.

---------- Post added August 19th, 2014 at 04:05 PM ----------

Here is another example of the weakness of tables in calculating a dive such as was described in post #24. I once did a dive to a small wreck at a depth of about 100 feet. It was not much of a wreck, and we did not stay there very long. We then went briefly to a middling depth, but we ended up drifting gently along on the top of the reef for so long that our total dive time, with AL 80 tanks, was over 80 minutes. How do you handle that on a table, when the maximum bottom time for a 200 foot dive is 20 minutes? When did our ascent to the surface begin? When did our safety stop begin?

If you were to treat your tables like a computer, we did a 7 minute dive to 100 feet followed by a 7 minute dive with no surface interval to say 50 feet followed by a 66 minute safety stop. I could probably calculate my dive that way, but that would certainly not be how the tables were intended to be used. If I logged it that way, my log entry would not truly represent the dive, since I was actively diving that entire time, not hanging in the water waiting to surface for three minutes, which is what a safety stop implies. Although I did have the physiological effect of a 66 minute safety stop, in terms of what I was actually doing on the dive, I did not do a safety stop at all.

Thank you BoulderJon, While it really should be obvious that tables were developed using an algorithm, I honestly never gave them much thought as to just HOW they worked and some of the concepts behind them. These have been some very eye opening discussions!

 

[nadenal]

Could you use your average depth on your tables to figure out your pressure group? Would that more inline with the computer on multilevel dives?

 

[mrdre]

What about this, guys ("Dive Computer" vs Tables vs "Smartphone or Desktop Computer")?

1. Throw away the whole idea of NDL (consider all dives deco).
2. Plan max depth and max time (assume square profile).
3. Use software to calculate how much deco time you will need.
4. On ascend do the 1-minute mini-deco stops and deco stop at 15-20 ft according to the plan.
5. On second dive - repeat, but double up the mini-deco and deco stop at 15-20ft.

All this "NDL" model is purely theoretical anyway and it will differ vastly from person to person. Why not just treat every dive deco and stay at 15-20 ft long enough?

Any criticism?

 

[Storker]

Well for once it's the same "purely teoretical" model underlying both no-stop limits and required deco stops. So how you can claim that no-stop limits are purely theoretical while planning a dive on deco software isn't, is beyond my understanding.

Let's assume, as an example, that we're both diving a simple multilevel profile: 25m 25min, 15m 25min. If you use VPlanner software "to calculate how much deco time you will need", you're basically employing the same algorithm as I am if I plan the dive with VPlanner¹. Except that you're adding about a ton of conservatism if you plan your dive as if all your 50min are spent at 25m.


¹ Theoretical example. I prefer Suunto's Dive Planner, as it uses the same algorithm as my computer does.