Average or Maximum?

[nannymouse]

I have been using a dive computer that tells both average depth and maximum depth.
Which number should I use for surface interval and following dive calculations? -N

 

[radiocraig]

i am assuming that if you are using a comp that you dont use tables...your post is confusing to me unless i am missing something

if you are just checking your comp to tables, they wont come anywhere close...

if you are using the comp as a bottom timer then use your max depth, just like you would if you didnt have your computer

once i got a computer ive never used tables...you get more time with a comp and thats the point of using one.

 

[ClayJar]

I have been using a dive computer that tells both average depth and maximum depth.
Which number should I use for surface interval and following dive calculations?

The table rules are the table rules, so if you wanted to do tables, you would have to follow the table rules and use maximum depth. Since the tables assume square profiles while your dive computer calculates multi-level dives as you go, you can easily "bend the tables" (i.e. go into deco or even off the tables completely) even while your computer shows you to be well within limits.

(Trying to shoehorn computer data into table calculations is like mixing oil and water, except that it's a lot harder to make a delicious vinaigrette with dive data. :biggrin

The one place where you would *absolutely* use average depth would be in calculating surface air consumption. Using maximum depth in that calculation gives you incorrect results and will necessarily result in SAC numbers that are less than your true SAC.


If you want a simple, rudimentary approximation from which to estimate planned surface intervals (just as a guideline, of course), you can go to the computer's PLAN mode to see how much time it would allow at a given depth. Find the letter group that would give you an adjusted maximum dive time of approximately that long at that depth, and use that letter group as a rule-of-thumb value for your current letter group. Then go into the surface interval table from there as if you just came up from a dive at that letter group. It's just a loose approximation, and you'll check your actual planning using the computer, but it can give you a very rough idea of where you probably are.

 

[pasley]

The table rules are the table rules, so if you wanted to do tables, you would have to follow the table rules and use maximum depth. ....

Exactly right.

 

[don Francisco]

Clayjar nailed it. I have only one thing to add. Use the plan mode to determine the SI

As a starting rule of thumb assume a minimum SI of about 1 hour. As you get close to 1 hour use the Plan mode to verify that you can do your next dive. Say you want to dive to 50 feet for 50 minutes, wait until the computer says that's possible, then dive. With experience, you'll find that you can pinch this slightly, or might have to allow a little more time, because actual dives are never exactly like theoretical square 50' dive profiles.

If you're using a computer, you might it use all it's features.

 

[nannymouse]

Good advice all around.

I will have to sit down with the manual and go through it step-by-step.

My dives have all been relatively easy, with long SIs, so I'm not haired, but I plan on getting a bit deeper soon.

Thanks! -N

 

[24940]

Depth averaging works quite well with tables, but I wouldn't recommend learning the technique on the internet.

 

[Blackwood]

Depth averaging works quite well with tables

With some caveats...

For example, the nitrogen loading at the end of these two dives with identical average depths will be substantially different. As such, proper parsing of the profile based on an understanding of on/offgasing is very important.

So, like you said, learning the technique over the internet may not be a great idea.

 

[AbyssalPlains]

I was confused by your post, too. Please forgive me if I just misunderstood, but my first thought was, This person is obviously not certified. If this is the case, then the answer is simple: You will learn all the answers to this question and everything else you need to know during your certification class.

If you are certified, you may want to take your class room material off the shelf, dust it off, grab a cup of coffee, and refresh your diving memory.

To calculate your tables/nitrogen exposures/intervals, you always go by your max depth.

Hope this helps.

 

[DA Aquamaster]

The OP's question makes sense when you consider he only has 25-50 dives. Most classes are taught with tables and the usefulness of some computer functions are not immediately obvious. It makes sense for a new diver to assume that tables use max depth because that is the only number you have (which is true) and it is not out of the realm of possibility for a new diver to wonder if the average depth offerred by a computer might work better. I think SDI teaches based on computers, but computer A is different than computer B so some learnig still occurs later.

Either way, I'd cut the OP some slack on asking the question and not go with the the mostly unhelpful "you'll learn that in class" or "you need to review it" responses.

Average depth is very useful for figuring the SAC rate/RMV for the dive.

1. Determine the cu ft of air used. For example if you started with 3000 psi in an AL80 and ended with 1200 psi.
77 cu ft / 3000 psi = .02567 cu ft/psi
.02567 X 1800 = 46.2 cu ft.

2. divide the gas used by the dive time. For example assume the dive time was 33 minutes.

46.2/33= 1.4 cu ft/minute

3. Convert the average depth to atmoshperes. For example your average depth was 57 feet.

(57/33) + 1 = 2.73 ATM

4. divide the cu ft used per minute at the average depth by the atmospheres of the average depth to determine your surface consumption rate.

1.4 / 2.73 = .513 cu ft per minute.

On your next dive under similar conditions you can use this to estimate the gas you will use at a different depth.

For example, If I am diving to 105':

1. figure out the ATM's at 105'

(105/33) +1 = 4.18

2. Take your SAC of .51 and multiply it times the ATM
.51 X 4.18 = 2.13 cu ft / minute

3. Figure your available gas minus the reserve

1/3 in reserve (1000 psi reserve) in an AL 80, (77/3) X 2 = 51 cu ft

4. divide the available bottom gas by the depth use rate of 2.13 cu ft/ minute

51/2.13 = 23.94 minutes maximum bottom time.

NOTE: The minutes available from a gas planning standpoint may exceed your NDL at a given depth with a given RNT and a given mix.

NOTE #2: Your gas consumption may vary and be higher or lower than planned due to workload, current, water temp, moon phase, etc.

It has its limitations and cautionary notes, but over time you will develop a pretty good feel for your SAC rate under various conditions and you will find it will help give you some firmer numbers for gas planning purposes.

In the good old days we had SAC rate computers that were special purpose circular slide rules. (they show up on e-bay now and then). SAC rate is by definition based on psi and is tank dependent while RMV is based on volume in cu ft or liters and is not tank dependent, but the terms are now more or less interchanged regardless of the psi or volume format used.

With a SAC rate computer, you just find the average depth and align it with the psi used on the outer wheel and turn the inner wheel to 33 minutes and then read the SAC rate in PSI in the window - 20 psi per minute in this case.

You could then either convert it to cu ft:

77 cu ft/3000 psi = .0257, .0257 X 20 = .514 cu ft/ min

Or if you use the same size tank, you could just figure every thing in psi by setting the depth, time and SAC on the SAC computer and read the psi used.

Set what ever you think may be appropriate for a bottom time for the bottom time, 20 psi/minute in the SAC window and then read the psi used across from any given depth. Adjust the time up or down to get more or less reserve left at the end of the dive.