Nitrox course. What's the point?

[Dody]

LOL. And dividing by 33 is just dividing by 100 (really easy!) and multiplying by 3.

In fact, it is easier than that. Just divide or multiply by 3. 3ft are 1 meter (I purposedly round). So if you have feet, either multiply by 3 and divide by 10 to have meters or just divide by 3. It is not perfect but it gives a pretty good order of magnitude. The beauty with 3 is that we know that there are only 2 possibilities. Let’s take any number. Divided by 3, it is either a round number, x.33 or x.66. Practical example. Let’s take 22. Dividing by 3, 21 will give the round number, 7. So 22 will give 7.33, 23 will give 7.66 and 24 will 8. A round number. You can do that with all number and you will always have .33 or .66.

 

[BRT]

Modern computers calculate in real time. TTS, (time to surface,) is an estimated figure, based on current (real time) data about the dive, (and diving history) an assumed ascent rate, (potentially an assumption on what decompression gas will be breathed during the ascent).
If you deviate from the assumed elements used to calculate the ascent, , the computer will recalculate the TTS.
NOTE - this is why, when you have a decompression stop to do, and you choose to decompress deeper than the ceiling stated by the computer, e.g. decompress at 6m rather than the 3m, your decompression time will be greater than stated. Modifying your TTS. In the UK, we often choose to decompress at 6m, rather than the computers preferred decompression depth of 3m. Surface conditions often preclude shallow stops. Most UK divers allow for this in their dive planning. If I am on a no-stop dive, I will do safety stops at 6, 4.5 and 3m, the computer will not acknowledge the safety stop at 6m, but will once at 4.5m or shallower.

It should also be remembered that there are differing theories on how to calculate decompression requirements resulting in different no stop time for the same dive, dependent on table (theory used). This variation is increased in the dive computer market, where additional elements may be added, conservatism, padding, adjustment for number of repetitive dives, poor dive practice, etc.
A manufacture may have multiple models across its range e.g. Suunto, which uses RGBM, RGBM 2, Fused RGBM and Tech RGBM. Some of the Tech computers allow you to change models dependent on your preference, e.g. Bulhmann or VPM.

I quoted NST figures for a dive using differing mixes earlier in the thread, I qualified the statement by stating WHICH model was being used. Differing models, differing times!

Yes, so true about computers calculating in real time. When you are out of NDL the computer makes no assumptions about your time to the surface. It simply continues to calculate. If you stay there it calculates deco times , if you go up it calculates what it needs to to decide on your remaining NDL. I suppose there are computers that once they decide you are in deco they require you to do the predetermined deco stop at the predetermined depth irregardless of the depths you continue your dive at, but this is illogical. When my computer puts me in deco it has always cleared before I reach the surface. In fact it has cleared long before reaching the deco stop. Does this seem to indicate requiring a longer stop at deeper depth? No, the computer simply continues to calculate.

 

[BRT]

If nitrox is used the computer calculates the assent is also on nitrox, if air is used it calculates the assent is on air. It allows extra bottom time on the assumption that nitrox will be used for the entire dive including the assent. If you were to do the bottom time on nitrox and switch to air for the assent you’d run the risk of getting bent. O2 on deco only accelerates the deco and shortens the time at the stops if you complete the stops on air the same nitrogen is removed.
P.S. I don’t use a computer in the water.

I can tell you don't use a computer in the water.

 

[boulderjohn]

When you are out of NDL the computer makes no assumptions about your time to the surface. It simply continues to calculate.

That will be true of a single gas recreational computer. If I have my computer in tech mode and have preprogrammed in my deco gases, it will assume I will make logical steps as the dive progresses, telling me my TTS (Time to Surface) based on an assumption that I will switch to the appropriate deco gas at the appropriate depth. If I continue to dive at depth, it will constantly recalculate that.

I suppose there are computers that once they decide you are in deco they require you to do the predetermined deco stop at the predetermined depth irregardless of the depths you continue your dive at, but this is illogical.

I don't know of any computer that does not do that. My first computer, a Suunto I purchased nearly a quarter century ago, did that. It told me what depths I had to do my deco stops at. Back when I used it, I was exclusively a recreational diver, but there were occasions when I did get into deco with it, and I did stop at those depths as instructed. The PADI computer-based OW course tells students that computers do that.

When my computer puts me in deco it has always cleared before I reach the surface.

This means you have barely gotten into deco, and a slow ascent cleared it for you. Go a few minutes further into deco and it will be different.

 

[Gareth J]

Yes, so true about computers calculating in real time. When you are out of NDL the computer makes no assumptions about your time to the surface. It simply continues to calculate. If you stay there it calculates deco times , if you go up it calculates what it needs to to decide on your remaining NDL. I suppose there are computers that once they decide you are in deco they require you to do the predetermined deco stop at the predetermined depth irregardless of the depths you continue your dive at, but this is illogical. When my computer puts me in deco it has always cleared before I reach the surface. In fact it has cleared long before reaching the deco stop. Does this seem to indicate requiring a longer stop at deeper depth? No, the computer simply continues to calculate.

I can't speak for all computers. We are really talking about calculated TTS when you have incurred decompression stops. If I change gases on my computer it recalculates my TTS and my stops. If I deviate from the computers expected ascent rate, my TTS updates. If I follow the computers assumed ascent rate, my calculated TTS and the actual TTS are basically the same.

I used to do a lot of runtime dives which where preplanned prior to mixed gas computers becoming readily available. I still like runtimes, and always carry bailout runtime tables when diving.
All my dives where preplanned using dive planning software. The software assumed an ascent rate, you could set the gases being used. It would give you plan A, if you modified the plan, e.g. adding a deep stop, you got a different ascent profile and time. If you changed your ascent rate, another set of numbers. The software was constantly making calculations based on data you set in the computer, modify the gas, ascent /descent rate or add an additional stop the forecast profile changed.
The whole point about runtime plans is that you must follow the details used in the decompression profile. The gases, used, the depths planned, the bottom time, the ascent rate and the planned stops. Deviating from the plan compromised your safe decompression. Whilst being shallower, and / or shorter was not an issue, the biggest issue with new divers and runtime was failure to follow the correct ascent rates and gas switch, at the correct time. Ascending slowly was a particular issue, in real time it would have added significant time to the shallow stops, which on runtime meant you where skipping required decompression time.

Dive computers are similar to the planning software (in fact some of the planning software is supplied by the computer manufacturer and uses the same algorithm your computer is using), the differences is that they are using the actual dive data to date to calculate a new TTS, or remaining NST. Some of the air integrated computers are also taking your breathing rate and available gas into account, not only warning you about he decompression profile, but how the TTS impacts the available breathing gas. and vice versa.

A few of the people I dive with use the newer Suunto technical computers and preplan their dives on DM5. This has proved to be quite accurate. However, when diving with me they need to be careful, I don't like to ascend as quickly on CCR as they would on OC, staying with me adds time on that they are not expecting. I don't mind too much if we are in a group on the shot or trapeze, I can get out when my decompression has completed and leave them with one of the other OC pairs , mind you they are irritated by the extended stop time. Big benefit for me, I get the biscuits (cookies) and cake before they surface.

 

[Celt]

I can tell you don't use a computer in the water.

I’ve never used a computer in the water, I plan for a few different scenarios and must stick with one. My dives are solo and 90% are deco so have little wiggle room. I would like to try a computer in the water though.

 

[dmaziuk]

Personally, I think dividing by any number is equally simple as dividing by any other number.

How about 22 / 7 ?

 

[lermontov]

I’ve never used a computer in the water, I plan for a few different scenarios and must stick with one. My dives are solo and 90% are deco so have little wiggle room. I would like to try a computer in the water though.

well after diving for 4 decades i think you should treat yourself before you pop off

 

[dmaziuk]

Yes, so true about computers calculating in real time. When you are out of NDL the computer makes no assumptions about your time to the surface.

It may or may not, it's up to the programmers. At the safe ascent rate of 9 m/min coming up from the recreational depth of 40 m will take ~4.5 minutes. That's half-time of the fastest tissue compartment in many models; some have the fastest TC at 2+ minutes so it's twice the half-time of that. I.e. it can be significant. I haven't run the numbers but I strongly suspect part of the reason "safe ascent rate" is safe is because it's slow enough to off-gas the fastest TC safely.

For slower compartments it shouldn't really matter on no-stop dives. Once you're in deco, math changes somewhat.

 

[Celt]

well after diving for 4 decades i think you should treat yourself before you pop off

Diving scuba 50 years this year, photo I took 10 years ago of my friend coming to the deco stop he’s 87 this year, but you’re never to old to learn so I’m going to take your advice. Thanks