HUD Mask

[fellowtraveler44]

I do not endorse damaging reefs. I consider my bouyancy to be pretty good, but I admit to having kicked coral while attempting to take that perfect shot with my camera. I have seen others do it as well countless times, and I would like to hear one underwater photographer deny ever doing that. I no longer have that problem - that was my point.

As for extending my bottom time, yes, I generally remain below the surface until it's time to surface, and my computer helps me determine when that time arrives. The Compumask helps remove some of the guesswork out of that determination. It can be used conservatively or otherwise, and I have used it both ways. I suppose one could argue that calculating in one's head how much air is needed to surface with 500 psi is unsafe, but having a computer calculate for me how much air I need to safely reach the surface with a safety stop seems at least as safe if not more so. If that concerns the user, simply use it conservately, and ascend sooner. I have done that during most of my dives.

I do not understand your use of the term "rock bottom reserves." The Compumask calculates the amount of air needed to slowly ascend, make a 3 minute safety stop, and surface with 300 psi. On the occasions when I utilized that feature, it worked flawlessly. It also alerted me (by flashing, I believe) when the bottom timer reached 5 minutes and again (by audible alarm, I believe) when the bottom timer reached 2 minutes. I was diving with my buddy, who on those dives had more bottom times than I did. If my buddy needed to ascend earlier, he let me know, and we ascended. Since he was using a traditional computer, he had to estimate how much air he would need for his ascent; I did not have that problem.

 

[GrumpyOldGuy]

Rock bottom is generally the amount of air needed for you and your buddy to safely ascend from the max planned depth under stress. To be reasonably accurate you need to estimate the air consumption for both you and your buddy, then compute a reasonable stress factor, ascent rates and stops plus a tolerance for SPG error. Then calculate from volume to PSI based on your tank. (This of course is not really needed if you are just hanging shallow burning your last bit of gas.)

The additional information you provide on the alarm at 5 and 2 minutes is comforting, definitely a step in the right directions.

As a programmer, I know the computer could be programmed to calculate rock bottom for you given an estimated stress SAC for you and your buddy. They could do it on the fly and compensate for deviations in depth. However, I have not seen any that actually do this yet. They merely monitor you current SAC and predict how much air you need based on current use compensated for depth and ascent rate with a small fudge factor. This is accurate, until you toss in a buddy leaching off your air, add some stress, etc... and your safety margin may be smaller than you think.

 

[kanonfodr]

fellowtraveler44:

Congratulations on your new purchase. I'm a bit jealous of you as that little invention is super nice, but I misplace masks too easily to allow myself to shell out the cash for one.

What I believe GrumpyOldGuy was getting at is that you make it seem as if you are leaving all your underwater thinking to the computer in your mask, as opposed to using the one between your ears. Especially when surfacing with only 300 PSI in your cylinder, unless you are using a positively freakish cylinder that is not nearly enough to rescue yourself, much less anyone else, if anything on the dive were to go south near the end.

I personally leave gas for my buddy and like it when he leaves gas for me, on the chance that if something happens to either of our gas supplies the other can rescue us. The amount of gas we leave is entirely dependent on the dive. This is our rock bottom, at that point we are headed up. You made it sound like you were not leaving enough gas for your buddy in your first post, kinda scary to me.

BTW, the argument that it could be seen as unsafe for me to calculate my gas reserves to surface at 500 psi is very easily blown away. I could do the math on a bar napkin while sipping a beer. However I don't get a warm fuzzy by trusting my life to some matchbox sized calculator that I have no idea how the damn thing works, so I think it's prudent that all divers should know how to calculate their Rock Bottom requirement.

Once again, congrats on the purchase and I hope it provides you many years of trouble-free operation.

Peace,
Greg

 

[sff]

As a programmer, I know the computer could be programmed to calculate rock bottom for you given an estimated stress SAC for you and your buddy. They could do it on the fly and compensate for deviations in depth. However, I have not seen any that actually do this yet. They merely monitor you current SAC and predict how much air you need based on current use compensated for depth and ascent rate with a small fudge factor. This is accurate, until you toss in a buddy leaching off your air, add some stress, etc... and your safety margin may be smaller than you think.

Actually there are computers that calculate monitor breathing rate during a dive and adjust remaining bottom time based on that calculation. Ie, the bottom time is adjusted if you are breathing heavily (exertion). The one I use, UWATEC SmartCom does it.

Also, I would like to suggest that arguments that surfacing with 300 psi or something about how you MUST account for your buddy are groundless. There are no hard fast rules. Each situation has to be evaluated based on the circumstances of your dive. So if I am diving with my wife, who is superb diver and we are diving in Cozumel on my friend's boat, I am going to assess the situation much differently than if I am diving with an unknown buddy off a boat I don't know in an area I don't know.

In fact, this is heresy but I would much rather dive solo than dive with an unknown buddy.

But back to the question. In a situation where I am diving with less confidence in the location and conditions or the buddy, I am going to add a safety consideration into my mental calculations. I don't need the computer to do so. I want the computer to just tell me the facts and let me make the judgement. With my wife, I am much more relaxed and do push more the limits.

Finally, I suggest that everyone try at some point to test your equipment. How well it breathes as 300 psi or below is a function of your regulator and your confidence. It is good to know how your regulator will breath as the pressure drops. Some get harder to breath. How well your gauge measures pressure is also very important. It is good to know how much air you really have when your gauge says 300 psi. I've dived down to 50 psi under pretty controlled situations. I also know that I can sit at 10 feet for 15 minutes with 300 psi in my tank (assuming 72 cuft). I think this is important so that the day when things aren't going right, you know how your equipment will perform and your confidence level will be that much higher.