Computers: Bottom time /vs/ Ascent time? Integrated?

[Rred]

OK, I'm just computerizing, I still like tables but that tends to make other folks get nervous.

And I've yet to see anything explicit in any computer manual about how or if ascent time is accounted for in the total "bottom time" and no decompression time limits shown.

For instance, if a computer is saying "six minutes" recommended ascent time, and "twenty five minutes NDL" does that really mean I've got 19 minutes and then I'm just magically expected to begin ascending? Or does it mean, I should expect to spend 25 minutes down AND THEN begin my six minute ascent? (25 and 6 are numbers just pulled out of a hat here, please don't get hung up on them.)

Is there a standard way the two are integrated and dealt with? Or will that vary by the maker?

 

[Centrals]

Do not really understood the question! Silly me.

This is what my non-AI computerS will do
They only give ndl but never "ascent time" together.
However, if I had overstayed the "ndl" then my computer will show mandatory deco time required as well as "total ascent time to the surface".

 

[CWK]

Your NDL is the time you have left at your present depth before you go into deco. The ascent time is the time you should take to get to the surface from your present depth. So, in your example, you can spend up to 25 minutes at that depth then ascend over 6 minutes.

 

[scubadada]

Oceanic air time remaining is ascent time plus all stops, safety or deco. When time equals zero, normal ascent plus stops will allow you to surface with selected reserve pressure, ie. 500 psi or whatever you choose. Works amazingly well assuming you comply with ascent criteria

 

[doctormike]

OK, I'm just computerizing, I still like tables but that tends to make other folks get nervous.

I don't understand this part.

And I've yet to see anything explicit in any computer manual about how or if ascent time is accounted for in the total "bottom time" and no decompression time limits shown.

For instance, if a computer is saying "six minutes" recommended ascent time, and "twenty five minutes NDL" does that really mean I've got 19 minutes and then I'm just magically expected to begin ascending? Or does it mean, I should expect to spend 25 minutes down AND THEN begin my six minute ascent? (25 and 6 are numbers just pulled out of a hat here, please don't get hung up on them.)

Is there a standard way the two are integrated and dealt with? Or will that vary by the maker?

I can't speak for every DC manufacturer out there, but I'm pretty sure that the following is standard. My DC (Petrel) uses TTS (Time to Surface) instead of "ascent time", but I think that it's the same thing.

NDL 25 minutes means that if you stay at the current depth (and continue to breathe the current gas), you can stay for another 25 minutes before you will incur a decompression obligation, and no longer be able to safely ascend to the surface without at least one staged decompression stop.

Ascent time of 6 minutes means that if you begin your ascent right now, it will take you six minutes to make it to the surface if you ascend no faster than the specified safe ascent rate (e.g. 30 FPM). I'm pretty sure that the standard for this is not to include a safety stop, but then again, AT / TTS is mainly relevant for decompression diving, where no SS is assumed. If you are diving within recreational limits, you wouldn't be in a position where you need to know your ascent time during the dive to see if you have enough gas to do it, so it's not really a very helpful number.

It probably says in the manual whether or not the AT includes a SS. To use your arbitrary example, if your computer is giving you AT of 6 minutes and you have 25 minutes of NDL, then I have to assume that it is calculating an AT including a safety stop, because otherwise 6 minutes at 30 FPM means a depth of 180 feet, and you certainly wouldn't have 25 minutes of NDL. Including a 3 minute SS for that example means a depth of 90 feet.

So assuming that you don't go past NDL, if your computer gives you an ascent time from your current depth, that should stay the same as long as you stay at that depth, right up to the point that your NDL counts down to zero. Once you start incurring a decompression obligation, your AT will go up to include the required stops.

For NDL diving, I would ignore AT and focus on knowing how much gas you need to safely surface, including a margin for error (i.e. rock bottom planning).

 

[Bob DBF]

And I've yet to see anything explicit in any computer manual about how or if ascent time is accounted for in the total "bottom time" and no decompression time limits shown.

There is no total bottom time on a computer since it calculates your ongoing NDL. When you ascend to the surface at the specified ascent rate it will calculate it properly but it is not differentiated to you because you may stop to look at the reef and it has to calculate that it was not your direct ascent to the surface, but another level of the dive. Since the computer works in milliseconds, a hesitation in the ascent rate and it will calculate that extra time at depth into the mix.

The computer gives you the run time of the dive, and tracks NDL for the dive, there is no equivalency to the tables unless perhaps you do an exactly square profile and your computer is running an algorithm like your tables.

If you have heard of the PADI Wheel, it was made to calculate a multilevel dive from the tables. The dive computer does that faster and more accurately. If you haven't, you are not missing anything.



Bob

 

[KenGordon]

Ask yourself this, how useful would a computer be that gave a current NDL of 25 minutes but after that 25 minutes the ascent took you into deco? It wouldn’t have really been the NDL would it. So the user would need to think about and calculate the actual NDL based off of this fake one and the depth.

So my expectation is that the NDL allows for the surfacing time and so the ascent rate etc. Mostly though an ascent is short, getting shallower and doesn't significantly add to the gas loading if done at the expected (by the computer) rate.

Note also that the time to surface number can cause confusion to those unused to deco dives. Once the NDL reaches zero and the computer starts showing TTS it may jump to some quite large number. From 30m ther will be 3 minutes of ascent, 3 minutes of safety stop and maybe a minute of actual deco, plus a minute to the surface. So from an NDL of 1 there is suddenly a TTS of 7 or 8 minutes giving the impression of a lot of deco. Some computers leave out the safety stop time so aren’t quite so dramatic but I have found the entry level ones most likely to include it.

 

[ScubaBadger32]

Silly question, but is this time something i can rely on? TAT i mean.

 

[scubadada]

Nearly 4 years later... what is this TAT? Do you mean time to surface TTS? For my Shearwater Teric TTS is the ascent time and deco/safety stops. It is quite accurate is you ascend at the prescribed rate of 33 ft/min and complete the deco/safety stops as dictated. Or, do you mean air or gas time remaining, ATR, GTR? For my Teric GTR includes the ascent but no safety stop (a relatively small amount of gas for 3-5 min at 15 ft). GTR on the Teric is disabled when you go into deco. For my Oceanic, ATR includes the ascent and safety or deco stops. I have used this for 20 years and it has proven quite accurate for no stop and light deco.

 

[GreggS]

You don't say what computer you are using, or if it is AI or not, but none that I've ever used shows an ascent time. Mine, which is hoseless AI, shows NDL in large numbers, my current tank pressure, and how much air time I have left at the current depth. And the only bottom timer I have seen on a computer shows you how long your dive has been to that point. It is not a timer to show you how how much bottom time you have left. As others have said, the NDL number is how much more time you have before going into deco at your current depth.

Let's say I'm diving a reef at 100 fsw, have been at that depth for 15 minutes and my NDL shows I have 5 minutes left. If I ascend to 80 feet, my computer will recalculate my NDL and it may show 8 minutes at that depth. The shallower I get, the more NDL time I will have. As long as I have more air time left, which will also increase as I ascend, than I have NDL time, I'll have plenty of air to do my safety stop and get to the surface. (And like you, these numbers are just ones thrown out there for illustrative purposes and may or may not be accurate at those depths.)

So, just don't exceed your NDL time and don't run out of air. Those are your 2 most important numbers and they both are constantly being recalculated as you descend and ascend.

ETA: I just realized after posting that this thread is 4 years old.