Question About SAC & Suunto Dive Manager

Now that I got my Suunto PC Interface cable working, I was able to check out some of my dives in the Dive Manager (version 2.6). I had a couple of questions. I'm a new diver with 9 dives, typically dive in cold water of Monterey, and I am using a Suunto Cobra dive computer in case anyone wants to know.

1. I noticed from the plots that almost every time my depth increased, my SAC would also increase. Every time we dove a little deeper, my SAC would decrease. This seems to be a clear pattern, happening over multiple dives weeks apart. Just curious if any other divers out there get a better SAC when they dive deeper (or is the dive manager fooling me)? The only other explanation I can come up with is that for some odd reason I am more relaxed at deeper depths, couldn't explain why that is though. Seems like it should be the other way around since its supposed to be colder at 35' than it is at 15', I always thought cold water made you suck up your air faster. Thus when I dive deeper I should get a worse SAC and when dive shallower I should get a better SAC, but what I am seeing in the graphs is the exact opposite. (Note: my SAC rates don't vary much on a dive. I typically see rates between .35 and .5, but I can tell from the graphs that there are slight increases and decreases)

This brings me to my second question...

2. Does the Cobra record the actual PSI in your tank at each 20 second interval (or whatever interval you have your computer set to)? Or does it just record the starting and ending pressures, and then the dive manager estimates your tank pressure based on some sort of algorithm at each interval? If it uses the algorithm method to determine the PSI at each interval then that would mean that SAC is based on calculated values and not actual values, so the SAC graph the dive manager is showing me is not really my actual SAC during my dive, but really more of an estimate.

Now that I've totally confused the heck out of myself and probably anyone who reads this, this new diver does appreciate thoughts or input anyone has on this subject. Thanks!

I don't think Cobra records pressure at every interval, at least based on this information:

papers:suunto_vyper [Markus Fritze (MMM)]

Does DM shows separate chart for SAC?

Wow! Are you like a really small diver? .35 seems REALLY REALLY efficient!

Clammy:

Wow! Are you like a really small diver? .35 seems REALLY REALLY efficient!

Click to expand...

I don't think I'm that small. I'm a woman, 5'4", a bit overweight. That's one of the reasons I was asking this about other divers was that .35 seemed way to efficient for someone at my skill level. hmm... Maybe I'm just totally relaxed in the water? More likely I'm doing something wrong in either reading the data or configuring the dive manager properly.

I started at about .8 SAC in open water class using a rental oceanic dive computer. Bought a Cobra after that and have seen gradual improvements since the class. My last 2 dives (dives #8 and #9) I averaged a SAC rate of .4 cu ft/min, minimum looks to be about .35, unless the dive manager is tricking me.

I kid you not. That is what the suunto dive manager program is telling me. Here's a pic from my latest dive (dive #9 for me):

[/QUOTE]1. I noticed from the plots that almost every time my depth increased, my SAC would also increase. Every time we dove a little deeper, my SAC would decrease. [/QUOTE]

This statment seems to contradict itself, but I get the gist..the deeper you dive the lower the SAC......Your SAC will be higher at the begining of the dive, and will gradualy fall as you become more comfortable. Since this is usualy happening durning your decent I would guess that that is why your SAC rate seems to be falling with depth. Might be compleatly wrong though.Happens often. Narcosis can also cause a decrease in SAC rate, but I usualy only see that in myself when I dive below 45m (145 feet) on air.

I do not think your deeper dives are making you more relaxed:no It is your buoyancy that affects your SAC (by the way the proper name is Respiratory Minute Volume but let's stay with SAC as you posted).

At the surface your SAC is only going to be affected by the amount of air from your reg. Once you are under water, you have to adjust your buoyancy too. Where do you think your air is coming from????

When you are at 15 ft and wearing heavy exposure suit, plus more weight (compared with warm water) you have to adjust your buoyancy more often (release and add air to your BCD). At depth the bubble is more manageable because going 5 feet up or down is not causing, as significant changes in the air volume of your BCD, as compared to 15 ft.

The best SAC 0.5 I got on warm water dives with 3mm suit and 6 lbs of weight. 0.35 seems to be a little bit less realistic unless you are very fit person with perfect buoyancy diving warm water. Besides men tend to consume more air then women because of the body structure - more muscles.

As mentioned in the previous e-mail. Suunto does record initial PSI and ending PSI values then just averages air consumption based on your average depth. From the math point of view it is the same as integrating sampled data over time. The problem with this model is though you will not be able to study how depth, temperature etc affects your "dynamic" SAC (depth dependent).

-Roman

Romko:

At depth the bubble is more manageable because going 5 feet up or down is not causing, as significant changes in the air volume of your BCD, as compared to 15 ft.

Click to expand...

I hadn't thought about that. I was just thinking that you dive deeper, you need to add more air to your BC, your tank pressure drops, your SAC should go up. I forgot about the fact that its easier to control buoyancy at deeper depths. Makes sense though.

5'4" is small compared to a 6' guy for instance so your body will require less air. It seems a little hard to believe that your SAC rate went from .8 t0 .35 in 7 dives but possible I guess.

Next time just manually take note of your beginning and ending tank pressure as well and total dive time and average depth (which your computer will give you). Then you can manually figure your SAC rate and see if it compares to the Dive Manager.

I have a related question - Suunto DM's SAC/RMV calculation is always about 5-10% lower than when calculated by hand or by my homemade divelog database (FileMaker). I'm wondering if there is an error in my calculation somewhere?

Here's an example:


Am I doing something wrong??? I'd appreciate any advice/corrections!

Also, it sounds to me like the Cobra 2 is not, in fact, recording PSI in real time, but just using starting and ending pressures and calculating SAC/RMV based on a constant consumption rate. The curves that WaterFox provided show a straight line for tank PSI, which seems unlikely. I see exactly the same trends in Suunto DM when it simulates SAC/RMV for me - however, I have the Vyper 2 with no air integration.