Todays Dive: A few thoughts

[windapp]

I dove on a beautiful reef today, and I learned some things, and I wanted to share something that I already believed.

I dove with a novice dive buddy (6 dives), and we had discussed diving together the week before. I felt that I would much rather dive with someone who has 6 dives, and will talk about, and plan the coming dive than with someone who has 100 dives, and doesn't think that he/she needs a dive buddy.

My new buddy admitted that she doesn't have very good breathing yet (at 6 dives, who does). I was renting the bigger tank, and so we swapped. I was glad that we did because we both stayed under until the divemaster called time for the dive (It was a bit early because of the poor vis and surge).

Two things reaffirmed my like of the buddy system. First, my buddy got caught up in the buoy line as we tried to leave the surface (low vis and crowding created a problem). The only reason this wasn't an emergency was because she had a buddy to assist in untangling. I mean, the two situations (buddy or not) are like night and day when it comes to that sort of issue. The second was that I was able to observe her trim, and let her know that it was absolutely perfect. It can be pretty difficult to tell if you have your trim right on your own. Mine sucked because I was a touch overweighted so I was glad she didn't comment on it.

I also learned the value of remembering your SPG pressure at key points in the dive. I was able to calculate my sac rate (1.25 for the first 10 minutes, and 0.55 for the next 25 minutes), and I know have a much better understanding of where I tend to burn through my air and what part of the dive I really need to work on. I am actually thinking of investing in an interface cable for my computer so I can be more precise in my sac calculations (average depth is a guess on my part based on the depth I was at the majority of the time).

 

[LowDrag]

I know for me, I tend to breathe a bit harder at the beginning of a dive then settle in. The PC interface is a good idea. I have the infrared USB interface and use it with SmartTRAK from ScubaPro. My computer is a SubGear XP -10, it works pretty good and I can see my dive profile in detail minus air consumption. It only shows me my SAC rate because it gives me a place to log my start and end pressure and gauges it from there. Eventually when I get an air integrated PC it will be more accurate.

 

[TSandM]

The data on gas consumption are interesting. I know that, for the first six months or so that I dove, I was always a little apprehensive on getting in the water. I never calculated my SAC rate by dive segments, but I'll bet it was highest at the beginning, before I relaxed and settled in.

 

[LowDrag]

I have been wearing my GoPro camera when diving and you can hear my breathing settle into a rhythm once I have been down for a few minutes. Usually doesn't take long. Even my wife notices the difference in my breathing on the GoPro. You should try it even if it is for nothing other than to hear your own breathing. I just use the head strap under my hood and never have to worry about it so there is no task loading there for me to deal with.

 

[rx7diver]

Windapp,

One thing that I routinely do during any recreational dive is, if at all possible, pause during initial descent at 10 or 15 ffw for a minute or so, to do gear and resource checks and settle my breathing and heart rate. If the swim to the downline was especially strenuous, I might pause even longer (to calm down). This helps me tremendously. (This is something that carried over from my long-ago tech training.)

Safe diving,

rx7diver

 

[altaskier]

Another factor can appear to affect your SAC rate at the start of the dive. If you're going into cold water, the pressure in your tank drops as the tank and then the air inside cool off. Go from 80 F to 70 F water and it's a small change (10/(460+80))=1.9% or 60 PSI - and of course for 50 F water and 80 F air you triple the PSI drop.

The 460 is because temperature is on an absolute scale - and of course most of the world knows that absolute zero is -273 C instead of -460 F...

 

[tdtaylor]

Thanks altaskier. I was recently shore diving in Massachusetts, and the water was about 55 degrees. Before that, I have been diving in plenty of cold water from 41 to 58 degrees. The air temp has been in the 70s or warmer, sometimes up to 95 degrees (fun in a drysuit). I haven't noticed this effect as much with warm water diving.

Especially with the shore dives, I could not understand why my SAC rate was about 1.0, then after 3 to 5 minutes settled to about 0.50- 0.56. Very consistent SAC for 70-80 minute dives after the initial 5 minutes. Most were relatively easy entries, and nothing difficult or alarming about them.

Thanks again- I believe you solved the mystery. FYI- I have a D4i that tracks all this info every 20 seconds, and can be set for different tracking times.