Average or Maximum?

[Teamcasa]

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

Like others have said, if you are using the computer then don’t worry too much about the tables. That said, you must learn how to read your computer and fully understand what it is telling you. Many newer divers use a fancy computer for nothing more than a electronic depth, temperature and timer. Most computers are so much more.

One thing I found helpful when using a new computer is to do a shallow dive, sit on the sand and just play with all of the buttons until you fully understand what your computers features and limitations are. Then have fun.

 

[SPG3K]

Insert EXTREMELY INFORMATIVE AND LONG POST BY DA here.

Aquamaster I have a question for you.

This may become stupidly obvious to me after I use my new VT3 (Wooo Fathers Day!) but I have not had a chance to use it underwater yet, or, obviously, download any profiles yet.

Being as how the computer is Air Integrated, will it show me the SAC rates for the various portions of my dives or just an overall average?

It seems it would be really useful to know my SAC based on depth instead of average. The manual doesn't really say which it does, just that it calculates SAC when you download a profile.

 

[peterbj7]

This question intrigues me, as it shows a fundamental lack of understanding of even the basics of decompression theory. Even PADI teach more than this in their Open Water course. Where were you when decompression theory was covered in your course?

 

[Blackwood]

Being as how the computer is Air Integrated, will it show me the SAC rates for the various portions of my dives or just an overall average?

Read the manual.

I don't have one, so I'm going to be guessing.

Most likely the unit will display a single number representing the entire dive, but the downloaded data will have throughout-the-dive figures.

This question intrigues me, as it shows a fundamental lack of understanding of even the basics of decompression theory. Even PADI teach more than this in their Open Water course. Where were you when decompression theory was covered in your course?

Hmm, I don't recall anything beyond table instructions being covered in my PADI OW course.

 

[DA Aquamaster]

I am not familiar with the VT3. I do know that virtually all air integrated computers sample and record a lot more data than the download software that most of them have can actually use.

The average depth function is nice as it gives you an average for the entire dive. In the past the best we could do was go to a fixed depth and swim at a constant rate for a known amount of time and record how many psi was used.

This is still worth doing. I will for example go to 60 ft, swim at a fast swimming speed for about 5 minutes to stabilize my o2 and metabolic rate, then record the tank pressure, and then swim at the same continous rate for 10 minutes at a constant depth for 10 minutes and then record the ending pressure. I normally get a SAC around .6 under those conditions in cold water.

On the other hand with computer averaging, I get a SAC around .5 on an active dive due to the time spent in less active portions and on safety stops.

At the other extreme, on a deco stop where I am just hanging on the line, a SAC around .20 to .25 is pretty normal for me.

For planning purposes, then I use the fast swimming SAC of .6 for the bottom portion of the dive and I use a middle of the road SAC of .4 for deco.

 

[Bob Vincent]

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.

Now you can do the above math. Which that is one heck of a job I have to say. Or you can follow your computer. Heck I'm still doing the old tables 60/for 60 70/for 50 80/for 40 in my head with no safety stop. So the computer is way better than me doing the math. The 1st computer I bought was a versa pro you don't think I didn't trust them? After diving it and checking it on tables (the new ones) time after time I now trust them. You can't beat them for bounce or deeper dives. Now talk about 2 & 3 gas computers are just ... way cool!

 

[nannymouse]

This question intrigues me, as it shows a fundamental lack of understanding of even the basics of decompression theory. Even PADI teach more than this in their Open Water course. Where were you when decompression theory was covered in your course?

Thank you ever so much for your terribly kind and immensly helpful response.
Once I have the computer sussed, I will write a dissertation on the subject of decompression theory.[/sarcasm]

I have a PADI eRDP. I have been using it for my dives, taking the max depth shown on the rental gauges that I have been using for the calculations.

I got my own computer and have not yet immersed myself in the manual. I still take the max depth and use it in my calculations.


"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." -DA Aquamaster

Thank you for your understanding and mathematical expertise!
This is the kind of information helps people advance to better comprehension, instead of just doing it the same way over and over.

"Like others have said, if you are using the computer then don’t worry too much about the tables. That said, you must learn how to read your computer and fully understand what it is telling you. Many newer divers use a fancy computer for nothing more than a electronic depth, temperature and timer. Most computers are so much more.

One thing I found helpful when using a new computer is to do a shallow dive, sit on the sand and just play with all of the buttons until you fully understand what your computers features and limitations are. Then have fun." -Teamcasa

I think that between doing this, and getting the PC cable to download the information, I will be well informed. Thanks Team!

 

[ucfdiver]

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.

The problem I have with computers are that they ignore your deep safety stops, and keep adding nitrogen during ascent (and not figuring that you off gas at this time).

DIR-diver.com - Using the min deco table

Here's GI3's attempt to explain it.
The First Stop / The Deep Stops | Global Underwater Explorers

 

[ClayJar]

The problem I have with computers are that they ignore your deep safety stops, and keep adding nitrogen during ascent (and not figuring that you off gas at this time).

You seem to show a fundamental misunderstanding of computers (although it could simply be something lost in translation). They *do* figure you offgas during deep stops and ascents. They *also* figure you ongas during deep stops and ascents.

At points during your ascent, the slow compartments in the model *are* still taking on nitrogen even while the faster compartments are offgassing. Unless you are in the gelatinous blob stage of a slime mold's life cycle, modeling your body as multiple compartments with varying half times is quite logical, and such models seem to do a decent job.

 

[Meng_Tze]

The problem I have with computers are that they ignore your deep safety stops, and keep adding nitrogen during ascent (and not figuring that you off gas at this time).

What do you base this assertion on?