Why the dislike of air integrated computers?

[DaleC]

The problem with not knowing the basics of dive planning probably won't become apparent on many dives one does recreationally in benign conditions... until you do a dive where there is some current (no big deal), boat traffic (no big deal) and someone has a gas problem (no big deal). All those "no big deals" can add up to a "pretty big deal", pretty fast.

 

[boulderjohn]

Please tell us all more about these magical devices that can sense what size tank you're diving....

Spoiler: sorry kids, it's really important to program an accurate cylinder volume if you want to rely on an Air Time Remaining calculation you don't understand beyond 'the expensive glowing box says I'm OK!'

It's actually not a difficult mathematical calculation. The computer does not actually need to know the size of the tank. All it needs to know is how fast you are going through the PSI in that tank and how many PSI are left. Afte that, it is simple arithmetic to calculate how long the remaiing PSI will last based on current consumption.

 

[stuartv]

Yeah, what John said. My Atom does not even have a way to tell it the tank size. I set the tank size in SubSurface, after I download the dive data from the Atom, and SubSurface calculates the RMV.

From the Atom's perspective, all it needs to know is that I burned 3000 psi down to 1500 psi in 30 minutes for it to be able to tell me that, at 1500 psi, I will be empty in another 30 minutes. Assuming I am at the same depth the entire time. GROSS example, just to reinforce John's point.

 

[Dr. Lecter]

It's actually not a difficult mathematical calculation. The computer does not actually need to know the size of the tank. All it needs to know is how fast you are going through the PSI in that tank and how many PSI are left. Afte that, it is simple arithmetic to calculate how long the remaiing PSI will last based on current consumption.

Which is fine, as long as you don't care about the accuracy of the number it's giving you. Without the volume of the container in which it is measuring pressure changes, ATR will not be accurate in the face of sudden changes in consumption rate.

 

[stuartv]

Without the volume of the container in which it is measuring pressure changes, ATR will not be accurate in the face of sudden changes in consumption rate.

How would knowing the volume of the container change the accuracy of an ATR calculation in the face of a sudden change in consumption rate?

The only thing the computer is measuring is tank pressure and time. Tank volume is a constant, for a given dive. So, the computer can directly calculate time left based on change in pressure over time. Or it can take the same measurement (of pressure and time) and do the arithmetic to convert it to gas volume (at surface pressure) change over time and then calculate how long to run out. Either way, the precision of the calculation is based on the same two measurements - tank pressure and time.

And, really, just sticking with tank pressure would be easier, to give accurate results, because the proper calculation would be how long until the tank pressure drops to the current ambient pressure - not to 0. Trying to calculate it based on tank volume and actual cubic feet of gas used would just make it more difficult, because the result would tell you how long until there was 0 cubic feet of gas left in the tank, which will never happen. To be accurate that way, you'd end up having to convert back to pressure, to compare against ambient, anyway.

 

[boulderjohn]

Which is fine, as long as you don't care about the accuracy of the number it's giving you. Without the volume of the container in which it is measuring pressure changes, ATR will not be accurate in the face of sudden changes in consumption rate.

Nothing will be accurate in the face of sudden changes in consumption rate.

 

[KenGordon]

That's why AI computers are a big safety advantage for rec divers, and the less experience they have or gas planning they do, the more the computer adds to their safety. All they do is plug into the computer that they will want to be on the surface with "x" psi (call it 500 for discussion sake). Since 500 is an almost universal standard for rec diving, they probably only have to enter it once and then forget it.

That 500 psi is the contingency gas. For example should you need to share air with an OOG buddy it is the gas available to get them to the surface. Surely that depends on depth, buddy sac etc. is this calculated on the fly by the computer or does it need to be planned?

 

[stuartv]

The computer is doing all the ATR (Air Time Remaining) calculations on the fly, based on how quickly the tank pressure is dropping. It knows your depth, so it knows that when you ascend to a spot that is half the ambient pressure, your gas consumption will be half (as far as the number of psi you are using per minute). It can factor all that into its calculation of when you need to start ascending in order to arrive on the surface with 500 psi left.

If a buddy goes OOA and you start sharing, it will take the computer some amount of time to detect that your psi in the tank is now dropping twice (or more) as fast, but then it will recalculate your ATR based on the new consumption rate. The exact details of how it does this calculation is going to be up to each computer manufacturer.

For the computer to figure out your consumption rate - which it does by monitoring your tank pressure over time - there is a fine line it has to walk. If it uses too long a slice of your history, it will be too slow to react to sudden changes in consumption. If it uses too short a slice of history, the ATR will vary by a lot just as you are floating there staring at it.

For example, if the computer uses your consumption over the previous 10 minutes, then, when you turn your dive and start swimming against a current, your consumption immediately goes way up, so your ATR should immediately go way down. But, with a 10 minute history, the computer could take several minutes to even start to reflect your sudden increase in consumption.

On the other hand, if your computer uses a 10 second history, one really long, really slow exhale could fool the computer into thinking you now have a REALLY low SAC and calculate your ATR as being MUCH longer than it really is.

Bottom line in my mind is that it's nice to have the ATR shown. But, I would always just take it as an educated guess more than an exact number. And be aware of what my circumstances are and how they would be affecting the number shown. E.g. if I know I started my dive by swimming against the current, and now I'm done and just floating with the current back to where I started, I will know that the ATR shown is incorrectly low (while I'm swimming) and take that into account. I generally treat mine sort of like my NDL. I conduct my dive so as not to be down for longer than what the ATR says (factoring in some intelligence about my circumstances). But, I might very easily and often conduct it to get out well before the computer says I'll run out of air. If the ATR turns out to be way off in the conservative direction, and I get out much earlier than I needed to, well... that just hasn't happened, so I'm not too worried about it.

 

[Kharon]

... On the other hand, solar would require us to leave the thing laying out where it could get sun. On a live-aboard or other high dive frequency trip, that could make it vulnerable to theft.

Also, I've yet to see a computer that doesn't warn against leaving it exposed to full sun for a length of time.

 

[guyharrisonphoto]

There are several misconceptions about how ATR works.

Dr. Lecter, my post (and some from others) explained why cylinder volume is irrelevant to the ATR and does not need to be programmed in. The calculation is based on change in pressure over time. A small cylinder will drop in pressure faster than a large one for the same volume of gas consumption. The computer registers the rate of drop and calculates your ATR accordingly. If you have a large cylinder, the rate of drop will be slower and (no surprise) the computer will show a longer ATR. Vice versa for a small cylinder. The computer does not need to "know" the cylinder volume to do this; it only needs to know the starting pressure.

Dale C, Boulderjohn and Dr. Lecter, the computer in fact does take into account sudden changes in breathing rate. I think the sampling rate on mine is every 5 (maybe 10) seconds. The adjustment for changes breathing rate is very fast, maybe it will use 30 to 60 seconds of sampling to re-calculate just so one or two deviant samples do not control, but it is fast, and accurate in my experience. It is far more reliable and a much bigger safety margin than trying to do mental calculation while suddenly under stress or a heavy workload.

Kharon, I use SPF 50 on my computer

---------- Post added July 31st, 2015 at 12:15 PM ----------

That 500 psi is the contingency gas. For example should you need to share air with an OOG buddy it is the gas available to get them to the surface. Surely that depends on depth, buddy sac etc. is this calculated on the fly by the computer or does it need to be planned?

No so much. Remember, I am talking about rec diving, with no hard ceiling or mandatory stop. The 500psi reserve is "your buddy's" gas because if you follow the ATR you will not be breathing it. The ATR will send you up with more than 500 psi from depth. As an example, it might send you up with 800 psi if you are at 90 feet. That means you will use 300 doing a standard ascent and safety stop, which leaves 300 for your buddy, and still 200 at the surface. It is only if the ATR would send you up at over 1000 that the concern would arise (ie if it sent you up at 1100, you would need 600 and only 500 would be there for your buddy). This has never happened to me, or even come close.

Plus, my planning for an ascent with an OOG buddy gives a lot more cushion than what the computer is assuming. I have only had to do this once (from 140' long ago) but that experience taught me that getting up while still alive is the top top priority. So, on a rec dive, where we are not in deco, I am going to push the ascent rate to 60 fpm until I hit 30' or so, and then 30 fpm to the surface, with no safety stop. This does two things. First, it rapidly increases the available gas as you get shallow quickly, and it decreases gas consumption by getting to the surface directly and without a stop. The minor increase in DCS risk is an easy trade-off for getting out of the far more lift-threatening situation.

I did an exercise in "rock bottom" calculation a while back, and found it very imprecise and based on a host of assumptions (square profile, assumed elevated SAC rates unchanging during the ascent, assumed standard ascent and assumed safety stop). The bottom line was that on AL80s at depth, such calculations might significantly cut dive time short of NDL on the actual dive itself. I decided not to plan every single dive of my life as involving an inevitable ascent for two divers at high consumption, at regular rates and through safety stops. I would have lost many precious hours underwater had I done this, and I am not going to start now. the AI computer has actually improved my knowledge regarding my gas use in real dives and I am comfortable with the planning and even the 500 psi surface reserve setting. If, for some reason, I had to dive to 130 on an AL80, I might get more conservative by setting the surface reserve at 750.