Are dive computers making bad divers?

[packrat12]

DEPTH(ft):TIME (mins)

60:60
70:50
80:40
90:30
100:25
110:20
120:15
130:10


There is a simple method called Depth Averaging that magically gives you those "lost minutes" that your computer will eventually arrive to after "real time depth calculations. A mathematical average is defined as the sum of all the numbers on a list divided by the total number on the list. In case of the wreck which starts at 80 and ends at 100, we will add 80 + 100 and divide that by 2 to get to 90 feet. From memory, I will know that I can now stay for 30 minutes instead of 25.

So let me get this straight,

I can spend 11 minutes a the top of a wreck at 70', go to the sand at 130' for 11 minutes and surface?????. My average depth is 100' which has a limit of 25 minutes so my 22 minutes is well within that limit......

Tables are not built for average depths.

 

[phoenix31tt]

This thread shows that there is a significant diving population out there that does not understand how tables work. Yet they are convinced that they understand how decompression works. In other words they have understood how decompression manifests itself on the computer display while having no understanding how it manifests itself in the form of parallel columns where depths meet No Deco times!

I am not suggesting that they are bad divers because in the age of computers, you can buy a computer for less price than a bottom timer (seriously) but it is still important to know what our computer is doing to decide what value we wish to attach to that service.

While I dive with a computer, I have the following NDLs memorized in my mind. I remember this table listed below just like I remember my mothers name.

DEPTH(ft):TIME (mins)

60:60
70:50
80:40
90:30
100:25
110:20
120:15
130:10

This means that holding a computer or even looking at a table in dives shallower than 60 feet is totally useless as long as surface interval is more than an hour and dive is less than an hour.

On dives deeper than 60ft, the memorized table means that as long as I do not exceed the depths in the left column I can stay in the water for the duration on the right column. Once memorized, this is purely instinctual and habitual and requires no math, computer or tables. Just pure memory.

Since these are squared out linear profiles, it can be argued that holding a computer in my hand will allow me to extend those times a little more due to the multi-level nature of most dives. In other words,if I am diving a wreck that is 100 feet to the sand and the top of the wreck is at 80 feet and I am exploring the entire wreck going up and down then computer might let me prolong the overall dive of 25 minutes to a few more minutes.

True but hold on ...

There is a simple method called Depth Averaging that magically gives you those "lost minutes" that your computer will eventually arrive to after "real time depth calculations. A mathematical average is defined as the sum of all the numbers on a list divided by the total number on the list. In case of the wreck which starts at 80 and ends at 100, we will add 80 + 100 and divide that by 2 to get to 90 feet. From memory, I will know that I can now stay for 30 minutes instead of 25.

Instead of calculating NDLs using maximum depth of 100 feet now and then ending at 25 minutes, we can use averaged out NDL of 90 feet and finish the dive in 30 minutes. 5 minutes will be added by this highly idiotized and over simplified math of depth averaging and as long as you don't go above the top of the wreck (80 feet as there is nothing there anyways) and you don't go below sand (100ft because you cant) your computer will also add around 5 minutes to the dive with all that "real time and depth calculations."

So depth averaging is a simple way of adding those lost minutes for which we normally carry computers. For someone properly trained on tables, the computer really does not do anything for the first dive. It shows its value on the second dive by saving the need to cast that one glance at the new NDLs on the table which replaces the ones memorized for your first dive. After a few dives on tables that occasional glance may not even be necessary.

Once the above is understood, each person can then decide how much value they assign to this simple function being performed electronically.

So let me get this straight,

I can spend 11 minutes a the top of a wreck at 70', go to the sand at 130' for 11 minutes and surface?????. My average depth is 100' which has a limit of 25 minutes so my 22 minutes is well within that limit......

Tables are not built for average depths.

not to mention... why in the world would i want to do such an average when my computer can give me a more accurate figure?

Why in the world would i want to manually add a bunch of numbers and divide by the number of numbers to find the average when i can just use the avg function in excel? (as a simple example)...

As the OP your question was about computers making bad divers... if that's what you mean by bad divers then heaven help us all... alot of us are bad divers

 

[boulderjohn]

not to mention... why in the world would i want to do such an average when my computer can give me a more accurate figure?

Why in the world would i want to manually add a bunch of numbers and divide by the number of numbers to find the average when i can just use the avg function in excel? (as a simple example)...

As the OP your question was about computers making bad divers... if that's what you mean by bad divers then heaven help us all... alot of us are bad divers

There are agencies (at least one) that teach people to use their average maximum depth to determine their ascent profile, and they teach them to compute that average in their heads as they dive. These divers are told not to use computers, because computers can make mistakes.

 

[CT-Rich]

And you can do all of this math in your head, accurately at 100+ fsw? I don't think I would trust my math skills at depth when my safety is on the line. I think a good analogy for the current should we trust PDC discussion would be to look back at advent of BCDs or SPGs. You could go back to 60s and 70s easily argue that a Real Diver can plan his weights so that he should never use a BCD and that knowing his gear and reading how his regulator breaths, a divers should be able to know his air supply and to kick in his reserve valve to head up. People did things this way and it was pretty safe. And there were hold outs as the power inflators were incorporated in the BCDs and gauges became widespread.

Could you say the same thing about PDCs? Sure you can rely on a computer to monitor your relationship with the NDL. To be honest I trust the computer to time my dives, the temperature and depth accurately. I respect it when I see where I am in relation to the NDL. Does it make my dives safer? Yep. Could I dive with out it? Yep. I could go back to a cheap watch and depth gauge and a laminated dive table in my BCD pocket. Do I trust my ability to read and use the table at 100'? Nope. Have I ever forgotten to set the bevel on my watch? Yep. Have I ever forgotten to note my max depth. Yep.

Diving is different today. than it was pre-BCD, pre-guages and pre-computer. I don't know anyone that would decide not to take advantage of the modern convenience of having a BCD or gauges or an octopus for diving. You totally can, but why bother? Noobs should learn to plan a dive with tables and understand why they are important, but really, they are safer in the water with a computer that HELPS them monitor their dive, the same way they can use a BCD to fine to their buoyancy and an octopus makes air sharing easier in the rare cases of OOA. Some people are going to be more technology dependent than others, but the Tech is there to make the overall experience more enjoyable and safer.

 

[dberry]

This means that holding a computer or even looking at a table in dives shallower than 60 feet is totally useless as long as surface interval is more than an hour and dive is less than an hour.

I use a computer, but also plan with tables (mainly because I'm new, and a geek, and like to see "how things work".) I'm good with how you figure your first dive (it's basically the 120 rule, with some extra minutes at the deep end), but I'm confused about how you handle repetitive dives. Probably just my newbie-ness showing, so I'm ready to be educated.

Let's say dive 1 is 60min at 60ft. By the NOAA tables that leaves me in Group K, and after 1 hour SI I've degassed all the way back to Group, er, J. Which means I've got a RNT of 97 minutes if Dive 2 is to 40 ft, and a max NDL of 66 min. Now 66 min at 40 ft isn't exactly a short dive, but this scenario could leave an experienced, air-sipping diver with a (minor?) deco obligation during dive 2. But it gets uglier if we're out for a 3-tank trip, no?

So am I mis-using the tables for repetitive dives?

On dives deeper than 60ft, the memorized table means that as long as I do not exceed the depths in the left column I can stay in the water for the duration on the right column. Once memorized, this is purely instinctual and habitual and requires no math, computer or tables. Just pure memory.

Again, that seems pretty reasonable to me for the first dive, but it seems to me staying NoDeco on dives 2 (or 3) could be challenging if you're relying only on memory.

 

[boulderjohn]

I use a computer, but also plan with tables (mainly because I'm new, and a geek, and like to see "how things work".) I'm good with how you figure your first dive (it's basically the 120 rule, with some extra minutes at the deep end), but I'm confused about how you handle repetitive dives. Probably just my newbie-ness showing, so I'm ready to be educated.

Let's say dive 1 is 60min at 60ft. By the NOAA tables that leaves me in Group K, and after 1 hour SI I've degassed all the way back to Group, er, J. Which means I've got a RNT of 97 minutes if Dive 2 is to 40 ft, and a max NDL of 66 min. Now 66 min at 40 ft isn't exactly a short dive, but this scenario could leave an experienced, air-sipping diver with a (minor?) deco obligation during dive 2. But it gets uglier if we're out for a 3-tank trip, no?

So am I mis-using the tables for repetitive dives?


Again, that seems pretty reasonable to me for the first dive, but it seems to me staying NoDeco on dives 2 (or 3) could be challenging if you're relying only on memory.

It is difficult to combine tables and computers unless you are doing truly square profiles. A computer will follow your multi-level dive and give you very different results as a consequence. When I was on my first dive trip, I simply could not use the tables because of it--I was immediately off the charts. If you are using the tables ONLY to know your maximum time at maximum depth on the approaching dive and then following a computer, then that will only be accurate for the first dive. If you want to know your true maximum time for the approaching repetitive dive, you should use your computer's dive planning function instead.

To answer your specific question, I will use the PADI tables with their different figures because I don't have NOAA tables with me. It should still work the same for the main idea of the answer. Going to 60 feet for 60 minutes exceeds the NDL, so I will say 55 minutes, putting you in group W. After 1 hour, you will be in group I. For a 40 foot dive, that will give you 100 minutes of bottom time and 40 minutes of Residual nitrogen time. If you exceed 100 minutes on that dive, then, yes, you will have a decompression obligation. If you go all the way to 100 minutes, you will surface in the Z group. If you do another 1 hour surface interval, you will be in the J group, but only in theory. According to the PADI tables, if you do 3 or more dives and get into the Z group, you will need a 3 hour surface interval before the next dive.

---------- Post added December 18th, 2015 at 10:13 AM ----------

And you can do all of this math in your head, accurately at 100+ fsw? I don't think I would trust my math skills at depth when my safety is on the line.

Nope. That is one of the reasons I don't dive with the group that taught me to average my depths any more.

 

[Storker]

There are agencies (at least one) that teach people to use their average maximum depth to determine their ascent profile, and they teach them to compute that average in their heads as they dive. These divers are told not to use computers, because computers can make mistakes.

Are those the same agencies that teach that the maximum END should not exceed 30m/100'?

I don't trust myself to do a running depth average at 30m on air. Much less than I trust my PDC to keep track of my N2 loading. And I'm a science type of guy who usually don't have a problem with mental math...

 

[Patoux01]

Once the above is understood, each person can then decide how much value they assign to this simple function being performed electronically.

Except that's not how it works...

 

[boulderjohn]

Are those the same agencies that teach that the maximum END should not exceed 30m/100'?

They do advocate that.

 

[KenGordon]

The scheme below make assumptions about deco being a linear function of depth and that the time order (ie profile) of a dive does not matter. It also assumes that a user can record, or at least notice, how long they spend at whichever depths and then do the maths required to do the averaging.

Deco is not a linear function of depth. The models used vary from a little bit complicated to really properly complicated. The simplest involve calculating multiple integrals over exponential decay functions and then a bunch of limit calculations to figure out an NDL. Nobody will be doing those in their head.

Over some depth ranges, for some given starting conditions the NDL + time = a constant is sort of true, given the very rounded nature of tables, plus or minus the odd minute or so. Perhaps the poster and others get away with it as much diving is not square profiles and ends shallower.

As for the averaging ploy... Anyone who deals with numbers on a daily basis beyond checking their change in the pub ought to see why this is wrong. You cannot average over various values of X for a non linear function f(X).

Through a couple of extreme examples such as doing 30 minutes at 10 feet and then 30 at 130 for and average hour at 70 and you can clearly see this does not work.

If anyone teaches this I expect there is a bunch of special conditions and limits which prevent it being completely deadly.

Software is cheap and easy. You can calculate repetitive, multilevel trimix to your heart's content on your phone for very little money. Or you can look at your tea leaves and think "Do I feel lucky today?"

This thread shows that there is a significant diving population out there that does not understand how tables work. Yet they are convinced that they understand how decompression works. In other words they have understood how decompression manifests itself on the computer display while having no understanding how it manifests itself in the form of parallel columns where depths meet No Deco times!

I am not suggesting that they are bad divers because in the age of computers, you can buy a computer for less price than a bottom timer (seriously) but it is still important to know what our computer is doing to decide what value we wish to attach to that service.

While I dive with a computer, I have the following NDLs memorized in my mind. I remember this table listed below just like I remember my mothers name.

DEPTH(ft):TIME (mins)

60:60
70:50
80:40
90:30
100:25
110:20
120:15
130:10

This means that holding a computer or even looking at a table in dives shallower than 60 feet is totally useless as long as surface interval is more than an hour and dive is less than an hour.

On dives deeper than 60ft, the memorized table means that as long as I do not exceed the depths in the left column I can stay in the water for the duration on the right column. Once memorized, this is purely instinctual and habitual and requires no math, computer or tables. Just pure memory.

Since these are squared out linear profiles, it can be argued that holding a computer in my hand will allow me to extend those times a little more due to the multi-level nature of most dives. In other words,if I am diving a wreck that is 100 feet to the sand and the top of the wreck is at 80 feet and I am exploring the entire wreck going up and down then computer might let me prolong the overall dive of 25 minutes to a few more minutes.

True but hold on ...

There is a simple method called Depth Averaging that magically gives you those "lost minutes" that your computer will eventually arrive to after "real time depth calculations. A mathematical average is defined as the sum of all the numbers on a list divided by the total number on the list. In case of the wreck which starts at 80 and ends at 100, we will add 80 + 100 and divide that by 2 to get to 90 feet. From memory, I will know that I can now stay for 30 minutes instead of 25.

Instead of calculating NDLs using maximum depth of 100 feet now and then ending at 25 minutes, we can use averaged out NDL of 90 feet and finish the dive in 30 minutes. 5 minutes will be added by this highly idiotized and over simplified math of depth averaging and as long as you don't go above the top of the wreck (80 feet as there is nothing there anyways) and you don't go below sand (100ft because you cant) your computer will also add around 5 minutes to the dive with all that "real time and depth calculations."

So depth averaging is a simple way of adding those lost minutes for which we normally carry computers. For someone properly trained on tables, the computer really does not do anything for the first dive. It shows its value on the second dive by saving the need to cast that one glance at the new NDLs on the table which replaces the ones memorized for your first dive. After a few dives on tables that occasional glance may not even be necessary.

Once the above is understood, each person can then decide how much value they assign to this simple function being performed electronically.