Why no accurate computers?

[OTF]

Unfortunately, your thinking that Nitrox and diving deep go together, is a common mistake. When diving on Nitrox your Maximum Operating Depth is shallower than air. To dive deep you should consider trimix not Nitrox. But remember this is a new diver forum before jumping into this hole.

I was not thinking that. But you're right that it is a common misconception.

Anyway, my point was that OP should take some further training to stay down longer if the NDLs given by computers on air feel too limiting. It's a better, safer solution than rejecting computers or diving GF99/99 all the time.

 

[boulderjohn]

You cannot plan a reversed profile dive with table.

Why not?

Sure, the PADI tables say not to do it, but that advice has a specious history, and it is the only thing stopping you. The first time that advice appeared was in a 1972 PADI OW manual, and it was a suggestion. There was no explanation for the suggestion, and when PADI was queried about it two decades ago in the workshop that determined reverse profiles were OK, PADI did not know why that suggestion was made or who made it. As the years went by, other agencies adopted it, and it eventually became a rule. No one knew why.

If you plan repetitive dives in different orders, you will see the most likely reason for that original suggestion, you will see the most likely reason--you get shorter surface intervals if you plan the deeper dives first, and the difference can be significant. In 1972, everyone, including PADI, was using the US Navy tables, with their longer surface intervals. Plan the shallower dive first, any you would be out of the water for a long, long time between dives.

So, go ahead and plan reverse profile dives with the tables, knowing that the only impact will be on your surface interval. If you think using a computer does away with that, think again. You just can't see it happening as easily.

 

[boulderjohn]

My instructor taught me to use tables and they were safe. I was taught to plan my dive and dive my plan.

I hesitated to respond to this because I was not sure what he meant. I frequently cringe when I read the phrase "plan your dive and dive your plan" because too often it reveals a very limited mindset. Too often, the phrase is used by people who were taught one and only one way to plan a dive, and they therefore believe that any other way of planning a dive really means not planning at all.

People using that phrase usually plan by predetermining the depths and times of the dive. "I will dive to 78 feet and stay there for 26 minutes before ascending." To them, any of the following plans constitute not planning at all, and anyone diving like this is to be scorned. There is, however, no reason the word "plan" does not apply to them all.

• I will dive to a maximum depth of 78 feet and stay there until either my gas supply reaches 900 PSI or I am within 4 minutes of decompression limits before ascending.
• I will explore the bottom levels of the wreck and work my way to the top, watching my gas supply and NDL times as I go. The top of the wreck is at about 50 feet, and I will make sure I have at least 800 PSI or 3 minutes of NDL when I ascend from there.
• I will go to the bottom of that coral spire (bommie), which is about 80 feet deep, and I will circle it, gradually spiralling my way to the top as I monitor gas supplies and NDLs.
• We will enter the cave and swim the upstream channel, going into the flow. We will stay on the mainline throughout, taking no jumps. We will turn the dive and return to the entrance when one of us reaches the designated turn pressure following the rule of thirds.

A ScubaBoard thread on this topic about a decade ago included a diver very well known for wreck exploration. The "plan your dive and dive your plan" group was fairly screaming at him that he had to plan his dive by knowing exactly how long he was going to be at every depth at every time. In replying that it was impossible for him to do that on an exploration dive, he commented along the lines of "you people must only do really, really boring dives." His point that you can only do that if you know exactly what you are going to encounter on the dive before you encounter it applies to all levels of diving, including beginning OW dives.

 

[Edward3c]

You cannot plan a reversed profile dive with table.

Of cause it’s possible. Just accept the second dive will either be much shorter or will require a long deco obligation. U.K. tables have always included mandatory deco calculations.

 

[chillyinCanada]

I hesitated to respond to this because I was not sure what he meant. I frequently cringe when I read the phrase "plan your dive and dive your plan" because too often it reveals a very limited mindset. Too often, the phrase is used by people who were taught one and only one way to plan a dive, and they therefore believe that any other way of planning a dive really means not planning at all.

People using that phrase usually plan by predetermining the depths and times of the dive. "I will dive to 78 feet and stay there for 26 minutes before ascending." To them, any of the following plans constitute not planning at all, and anyone diving like this is to be scorned. There is, however, no reason the word "plan" does not apply to them all.

• I will dive to a maximum depth of 78 feet and stay there until either my gas supply reaches 900 PSI or I am within 4 minutes of decompression limits before ascending.
• I will explore the bottom levels of the wreck and work my way to the top, watching my gas supply and NDL times as I go. The top of the wreck is at about 50 feet, and I will make sure I have at least 800 PSI or 3 minutes of NDL when I ascend from there.
• I will go to the bottom of that coral spire (bommie), which is about 80 feet deep, and I will circle it, gradually spiralling my way to the top as I monitor gas supplies and NDLs.
• We will enter the cave and swim the upstream channel, going into the flow. We will stay on the mainline throughout, taking no jumps. We will turn the dive and return to the entrance when one of us reaches the designated turn pressure following the rule of thirds.

A ScubaBoard thread on this topic about a decade ago included a diver very well known for wreck exploration. The "plan your dive and dive your plan" group was fairly screaming at him that he had to plan his dive by knowing exactly how long he was going to be at every depth at every time. In replying that it was impossible for him to do that on an exploration dive, he commented along the lines of "you people must only do really, really boring dives." His point that you can only do that if you know exactly what you are going to encounter on the dive before you encounter it applies to all levels of diving, including beginning OW dives.

My dive plan is often, I'm going to go down, swim around looking at wonders of delight and then I'm going to return to the surface.

 

[tursiops]

• I will dive to a maximum depth of 78 feet and stay there until either my gas supply reaches 900 PSI or I am within 4 minutes of decompression limits before ascending.

• I will explore the bottom levels of the wreck and work my way to the top, watching my gas supply and NDL times as I go. The top of the wreck is at about 50 feet, and I will make sure I have at least 800 PSI or 3 minutes of NDL when I ascend from there.

• I will go to the bottom of that coral spire (bommie), which is about 80 feet deep, and I will circle it, gradually spiralling my way to the top as I monitor gas supplies and NDLs.

• We will enter the cave and swim the upstream channel, going into the flow. We will stay on the mainline throughout, taking no jumps. We will turn the dive and return to the entrance when one of us reaches the designated turn pressure following the rule of thirds.

These are interesting examples because none of them can be done using tables, except maybe the last bullet in limited circimstances...as described it might well be a deco dive.

 

[scubadada]

Hi @boulderjohn

Nice post 100. Most of my planning looks something like the 2nd and 3rd bullet. I almost always take a look at my planner to get an idea of what kind of NDL time I will have

 

[boulderjohn]

These are interesting examples because none of them can be done using tables, except maybe the last bullet in limited circimstances...as described it might well be a deco dive.

Correct. I especially included the cave dive because cave diving is often considered the ultimate in dive planning, yet that "advanced planning" is really no different from what is done by typical beginning OW divers who are scorned by some as not planning at all.

One of my most memorable NDL dives took place on the big island of Hawai'i. We were led by a DM who described a plan that I frankly cannot remember because we did not do it at all. On our way to our destination, we spotted a manta ray below us. It was at a cleaning station at 100 feet, where it was being serviced by cleaner wrasse. We discarded our plan and descended to have a look. After a while, the manta cleared out, and we were about to do the same when an eagle ray arrived for a cleaning. We watched that for a while until we checked each other's computer situations and decided we needed to ascend up the reef. As we ascended, we came upon an interesting sight--a helmet conch stalking a decorator urchin. We stopped to watch, and our wait was rewarded when, in a sudden move I have never seen before or after, the conch pounced (I didn't even know they pounced), covering the decorator urchin with its shell as it began its meal. Our group then ascended to the top of the reef, close to the boat, where we poked around until our gas supplies were low, at which point we took our thoroughly satisfied selves to the surface.

That dive description must have "plan your dive and dive your plan" purists writhing in anger.

 

[chillyinCanada]

Correct. I especially included the cave dive because cave diving is often considered the ultimate in dive planning, yet that "advanced planning" is really no different from what is done by typical beginning OW divers who are scorned by some as not planning at all.

One of my most memorable NDL dives took place on the big island of Hawai'i. We were led by a DM who described a plan that I frankly cannot remember because we did not do it at all. On our way to our destination, we spotted a manta ray below us. It was at a cleaning station at 100 feet, where it was being serviced by cleaner wrasse. We discarded our plan and descended to have a look. After a while, the manta cleared out, and we were about to do the same when an eagle ray arrived for a cleaning. We watched that for a while until we checked each other's computer situations and decided we needed to ascend up the reef. As we ascended, we came upon an interesting sight--a helmet conch stalking a decorator urchin. We stopped to watch, and our wait was reward when, in a sudden move I have never seen before or after, the conch pounced (I didn't even know they pounced), covering the decorator urchin with its shell as it began its meal. Our group then ascended to the top of the reef, close to the boat, where we poked around until our gas supplies were low, at which point we took our thoroughly satisfied selves to the surface.

That dive description must have "plan your dive and dive your plan" purists writhing in anger.

"I'm gonna go down, look around, and then I'm gonna go back up again."

 

[boulderjohn]

"I'm gonna go down, look around, and then I'm gonna go back up again."

Sounds like a plan.