Calculating bottom time?

[SWAMPY459]

Hi,

I have a question about dives that have more than one level, where one of the levels is very shallow. I think I have this correct, but want some input to make sure I'm right.

Let's use Troy Springs as an example dive...

There is a spring that has a 75 ft "chimney".... let's say I go down to that 75 ft depth for about 15 minutes...

Then I come up out of the chimney into the flat part of the springs and dive around in shallow water between 10 and 20 feet deep for 20 minutes.

Then I wait 30 minutes before my next dive....



When I'm calculating this I count...

15 minutes @ 75 ft

don't count any minutes at 10-20 ft (call this an extended safety stop) but don't count it as surface interval either

and 30 minutes as surface interval ...


right?


basically, I'm asking... "The time I spend diving at less than 8 meters, I don't count as bottom time or surface interval... it's sort of a blacked out time" right?




now, carrying that further,
if on a dive I go to 75 feet come up to 20 feet for a little while... then back down to say 50 feet... then up for a proper safety stop at the end of the dive.. I don't count the 20 feet portion at all...

example...

75 feet for 10 minutes... 20 feet for [-]12 minutes[/-]... 50 feet for 10 minutes... safety stop at 15 feet [-]3 minutes[/-]...

I could use my table to figure this as a 75 foot dive for 20 minutes...

right?

 

[oly5050user]

All bottom time is calculated from the surface to the deepest part of the dive .bottom time ends when making direct ascent to surface.DO not count ascent time or safety stop in calculation. Can you get credit for being shallow..yes you can, but not the way you are going about it. Safest way is to use a computer or plan out dive using padi erdpml.

 

[boulderjohn]

Actually, as I understand what you wrote (and I may have misunderstood), you are correct on the first part. Let me state it as I understood it to make sure.

1. You did a dive to 75 feet for 15 minutes.

2. You ascended to approximately 15 feet for a safety stop. Your safety stop lasted more than 3 minutes.

3. You had a 30 minute surface interval.

In the scenario I described, your 75 foot, 15 minute dive with safety stop put you in pressure group G on the PADI tables. After a 30 minute surface interval, you would be in pressure group D.

You are not correct on the second dive, though.

On the second dive, you did a multi-level dive. You were at 75 feet for 10 minutes, and then you were at 50 feet for the remaining time before you ascended for your safety stop. The time spent at 20 feet would be considered part of your time at 50 feet. Although there is a way to use the basic RDP as a multi-level dive planner, if you only have the RDP, then you would consider it a 75 foot dive for the total time prior to your final ascent. As oly5050user indicates, if you have the eRDPml, you can calculate it as a multi level dive in two phases, 75 feet and 50 feet. A computer will do a more complex analysis.

 

[Adobo]

Actually, as I understand what you wrote (and I may have misunderstood), you are correct on the first part. Let me state it as I understood it to make sure.

1. You did a dive to 75 feet for 15 minutes.

2. You ascended to approximately 15 feet for a safety stop. Your safety stop lasted more than 3 minutes.

3. You had a 30 minute surface interval.

In the scenario I described, your 75 foot, 15 minute dive with safety stop put you in pressure group G on the PADI tables. After a 30 minute surface interval, you would be in pressure group D.

You are not correct on the second dive, though.

On the second dive, you did a multi-level dive. You were at 75 feet for 10 minutes, and then you were at 50 feet for the remaining time before you ascended for your safety stop. The time spent at 20 feet would be considered part of your time at 50 feet. Although there is a way to use the basic RDP as a multi-level dive planner, if you only have the RDP, then you would consider it a 75 foot dive for the total time prior to your final ascent. As oly5050user indicates, if you have the eRDPml, you can calculate it as a multi level dive in two phases, 75 feet and 50 feet. A computer will do a more complex analysis.

Interesting.

Why would the 20ft portion of the dive be added to the time at 50ft? Is there ongassing taking place at 20ft when you dive air?

Also, what if dive 2 was like this:

• 20ft for 12 minutes
• 50ft for 10 minutes
• gas used = air

In this case, would you still count the 20ft segment as part of the 50ft segment?

When I first read the question, my inclination was to treat the second dive as if it were 2 dives. The first part would be would be 75ft dive for 10 minutes with an "extended safety stop". Then there would be a zero time surface interval. Then second part of dive 2 would be a 50ft dive for 10 minutes.

 

[TSandM]

What you are running up against is that tables are based on square profile dives, where the assumption is that you spend the entire dive at or near the same depth. These dives exist -- many dives on small wrecks have this shape, and a lot of working dives (like the ones the Navy divers did when they used tables) are also like this. Tables count bottom time from the beginning of the dive to the beginning of a direct ascent to the surface, and do not count safety stops as bottom time. If you are going to use tables, you must use them as they were designed, which means you can't pick and choose what parts of the dive you add to your bottom time, as you are doing in the second example. And although one could argue that, even in the first case, you were not on a "direct ascent", I would do as you describe.

But if you are going to do sawtooth profiles, or heavily multi-leveled profiles, strict adherence to tables either verges on ridiculously conservative (your first dive, had you counted the 20 foot time as bottom time, for example) or simply doesn't fit (your second dive, with the sawtooth profile). The more the dive diverges from the square profile, the less good the fit with the tables is. This is the biggest reason for the overwhelming acceptance of dive computers, because they can do the iterative calculations that work better for typical recreational dives.

 

[boulderjohn]

Interesting.

Why would the 20ft portion of the dive be added to the time at 50ft? Is there ongassing taking place at 20ft when you dive air?

Also, what if dive 2 was like this:

• 20ft for 12 minutes
• 50ft for 10 minutes
• gas used = air

In this case, would you still count the 20ft segment as part of the 50ft segment?

When I first read the question, my inclination was to treat the second dive as if it were 2 dives. The first part would be would be 75ft dive for 10 minutes with an "extended safety stop". Then there would be a zero time surface interval. Then second part of dive 2 would be a 50ft dive for 10 minutes.

Here is my short agreement with how Lynne described it above:

1. What I described is how the tables are officially supposed to be used with their square format.

2. What you describe (treating it as two dives) is essentially the same as doing a multi-level dive. It is true that if you really know what you are doing, you can use a square table to calculate a multi-level dive, and I contemplated mentioning that. As I said in my response, the eRDPml is designed to do that, and it will do what you suggest.

3. Computers will also do what you want to do. They are constantly recalculating each moment of the dive as if it were a new dive.

Using a square profile tool (basic tables) for a multi-level dive can be done, but there are better ways to do it.

 

[DiveNav]

I have a question about dives that have more than one level, ..........example...
75 feet for 10 minutes... 20 feet for [-]12 minutes[/-]... 50 feet for 10 minutes... safety stop at 15 feet [-]3 minutes[/-]...

use a multi-level dive planner such as divePAL?

​

Alberto (aka eDiver)

 

[SWAMPY459]

DO not count ascent time or safety stop in calculation. .

This is what I'm asking about... let me rephrase the question...

Can my safety stop be an hour long, and moving around instead of sitting still... and still not count that hour in my bottom time OR my surface interval?

 

[Tigerman]

This is what I'm asking about... let me rephrase the question...

Can my safety stop be an hour long, and moving around instead of sitting still... and still not count that hour in my bottom time OR my surface interval?

No, not unless its very shallow..
Even at 10 meters you will load up on nitrogen, however very slowly, but it could get you into trouble ignoring this if youre already close to your NDL from the deeper part of the dive..

By "safety stop" if you mean 3 meters, you probably can, but I would not recommend doing so without some proper decompression theory training..

 

[SWAMPY459]

No, not unless its very shallow..
Even at 10 meters you will load up on nitrogen, however very slowly, but it could get you into trouble ignoring this if youre already close to your NDL from the deeper part of the dive..

By "safety stop" if you mean 3 meters, you probably can, but I would not recommend doing so without some proper decompression theory training..

10-20 FEET (3-6 meters)