Average or Maximum?

[perdidochas]

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

It's the same as using the dive tables. You use the maximum depth (even if only there for a second or two) rounded up to the next 10. 61 ft rounds to 70 ft. THe problem, as other posters have mentioned, is that computers and tables work based on different models. You're not going to get the same results between the computer and the tables.

 

[perdidochas]

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.

SAC is Surface Air Consumption. It's adjusted for what your air consumption rate is on the surface. When using it, you need to adjust it for depth. For example, at 33 ft, your air consumption would be twice your SAC (33 ft is 2 atmospheres). At 66 ft, your air consumption would be three times your SAC (66 ft is 3 atmospheres).

 

[peterbj7]

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]

Sorry, I wasn't trying to mock you, and if others weren't already doing a good job I'd be happy to answer your question.

But what I said is true. Your basic course should have included enough on decompression theory for you to know the answer to this most basic of questions. It isn't about computers, it's about the algorithm used for the tables. I know that there is sufficient in the PADI OW course to answer the question, assuming it was ever taught to you in the first place.

But that's probably the point. In another thread there has been a discussion touching on the quality of basic training, and all too often it is sadly deficient. Breach of standards yes, but true nonetheless. Even when taught properly it's usually taught so you learn it rather than understand it, and that's not just true of the recreational agencies. I've always believed that what you learn you forget, or far worse mis-remember, but what you understand you retain.

So please don't be offended by what I said, and enjoy your learning process. You're going about it the right way, by asking questions. It's just that some should have been answered even before you asked them.

 

[peterbj7]

SAC is Surface Air Consumption. It's adjusted for what your air consumption rate is on the surface. When using it, you need to adjust it for depth. For example, at 33 ft, your air consumption would be twice your SAC (33 ft is 2 atmospheres). At 66 ft, your air consumption would be three times your SAC (66 ft is 3 atmospheres).

I believe some models are self-calibrating, in that they work out your SAC based on your actual consumption, then extrapolate to a direct ascent as of now. Clearly what is shown as the gas needed to get to the surface changes as the dive goes on - as with all computers, the time shown (and hence gas required) assumes a direct and immediate ascent at the presumed ascent rate.

If you're going to start relying on what the computer tells you re gas requirements you need to understand the algorithm it uses very well. They always used to be pretty primitive though they may now be much more sophisticated. But asap start doing your own SAC computation and consumption predictions.

 

[peterbj7]

It's the same as using the dive tables. You use the maximum depth (even if only there for a second or two) rounded up to the next 10. 61 ft rounds to 70 ft. THe problem, as other posters have mentioned, is that computers and tables work based on different models. You're not going to get the same results between the computer and the tables.

But different only in detail. The general approach, and the input information needed, don't change. A computer is in effect a very fast table with lots of columns and rows. It also generally uses far more tissue groups, partly because it can and partly because decompression theory has moved on.

 

[Blackwood]

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).

Of course they do (figure off-gassing on ascent).

They may consider slow tissues as being equally important to fast tissues (which is a different paradigm than min-deco) and thus appear to ignore deep stops. But rest assured, unless broken, they're continuously updating gas loading.


It's easy to verify. Take one down, put it into deco mode, and then make a staged ascent. It will come out of deco and add more "NDL" time.

 

[ucfdiver]

What do you base this assertion on?

This past weekend my nitek went up a bar while doing a 40ft safety stop. From what I've seen, when I do a 40-50ft stop, my nitek is counting against the NDL limit. Now, when I do a 20ft stop, it certainly counts that one, because it's a predetermined stop, but from the max depth of 99ft up until 20ft it only showed NDL limits going down.

Do you think this is the result of tables being made off of a different algorithm than the computer, or just my specific model of computer?

Here's the dive profile if it matters (taken from a buddies computer)
http://jamesg.net/misc_stuff/message_boards/all_boards/08_june/dive2.png

 

[Blackwood]

This past weekend my nitek went up a bar while doing a 40ft safety stop. From what I've seen, when I do a 40-50ft stop, my nitek is counting against the NDL limit. Now, when I do a 20ft stop, it certainly counts that one, because it's a predetermined stop, but from the max depth of 99ft up until 20ft it only showed NDL limits going down.

Do you think this is the result of tables being made off of a different algorithm than the computer, or just my specific model of computer?

That's very strange. I've never seen a computer act that way (but then again I almost always dive in gauge mode).

The way you describe it, yours seems to be acting like a flat table (establishing NDL based on maximum depth rather than actual profile).

Before you edited your post, it said something like when I did a 40-50ft stop, my nitrogen loading bar clicked up. That I can believe, as the bar is likely displaying the highest loading in any of the compartments it's tracking. At that point in time, some of the very slow tissues may still be loading, so the bar could reasonably click up. But if the "NDL remaining" displayed is going down while you're going up, I'd bet your computer is malfunctioning.

 

[ucfdiver]

That's very strange. I've never seen a computer act that way (but then again I almost always dive in gauge mode).

The way you describe it, yours seems to be acting like a flat table (establishing NDL based on maximum depth rather than actual profile).

Before you edited your post, it said something like when I did a 40-50ft stop, my nitrogen loading bar clicked up. That I can believe, as the bar is likely displaying the highest loading in any of the compartments it's tracking. At that point in time, some of the very slow tissues may still be loading, so the bar could reasonably click up. But if the "NDL remaining" displayed is going down while you're going up, I'd bet your computer is malfunctioning.

Well, it did click one bar up. By showing NDL remaining, what I meant, was the Nitek displays the time you have at your current depth to avoid decompression, and that number was decreasing. Honestly, the only time I've seen that number go away is when I'm at a 2min stop.

I guess the most confusing point is that at the end of the day after dive 4, I was 1 bar into the red on the residual nitrogen graph. Now, I don't see how this is possible, just estimating by my bodies' feedback.

Let's say I do a rainbow river drift dive for 2 hours, I NEVER feel tired. Let's say i go to V. Blue and do 2 15 minute dives to 110ft, I ALWAYS feel tired. Everything I've read implies that the tired feeling has a lot to do with nitrogen in the body, since obviously my physical workload would be greater overall on the 2hr in water time than 30 minute dive times. Now, if I do the safety stops, my computer's nitrogen graph is at the same spot or one higher, yet lots of the fatigue goes away. So that would mean that your body gives you a good "warning sign" if you have residual nitrogen, would it not? (If I'm wrong here, please correct me).

 

[Meng_Tze]

Well, it did click one bar up. By showing NDL remaining, what I meant, was the Nitek displays the time you have at your current depth to avoid decompression, and that number was decreasing. Honestly, the only time I've seen that number go away is when I'm at a 2min stop.

I guess the most confusing point is that at the end of the day after dive 4, I was 1 bar into the red on the residual nitrogen graph. Now, I don't see how this is possible, just estimating by my bodies' feedback.

Let's say I do a rainbow river drift dive for 2 hours, I NEVER feel tired. Let's say i go to V. Blue and do 2 15 minute dives to 110ft, I ALWAYS feel tired. Everything I've read implies that the tired feeling has a lot to do with nitrogen in the body, since obviously my physical workload would be greater overall on the 2hr in water time than 30 minute dive times. Now, if I do the safety stops, my computer's nitrogen graph is at the same spot or one higher, yet lots of the fatigue goes away. So that would mean that your body gives you a good "warning sign" if you have residual nitrogen, would it not? (If I'm wrong here, please correct me).

I do not know what algo/model your computer is running. But when you go up in a dive, you are still on gassing if you have not reached critical super saturation. (i.e you are still in NDL). Many computers for the 'safety of the user' account for this and count down NDL.

After 4 dives in the day (no knowing what they look like) it is not surprising to see res. N2 to be high. What amuses me is that you can match how you feel with a bar graph on a computer screen and disagree with that....:blinking: