how much air do you use?

[Warren_L]

I don't find PSI/min all that useful as you are correct in implying that this value indicates different gas volumes with different tanks. As well, this value is dependent on the depth as well. Deeper dives will have higher PSI/min rates.

SAC rates are usually measured in SCFM (surface cu. ft. per min.). This rate is independent of depth and provides a relative measure of gas consumption.

I did a dive on the Munson earlier this year. Down to 110'. SAC was 0.43 scfm and 54.6 psi/min. Compare this to the Marine Museum dive me, you, Chris, and Bruce did a couple of weekends ago at 35'. SAC was 0.49 scfm and 33.8 psi/min. So my SAC was actually higher on the shallower dive than the deeper dive, but the psi/min was higher on the deeper dive than the shallower dive.

 

[Seadiver5]

IDEA makes a great air consumption wheel that is very easy to use.

Sounds to me like you are doing just fine.

Kevin

 

[FreeFloat]

The formula we were given to use is corrected for surface air consumption. I find it's much more accurate using one's average depth for the dive, as opposed to the max recorded depth.

The dive we did at the Marine Museum, I recorded a SAC of 14.6 psi/min. (personal best) Just prior, on the Cora W Post ("red brick barge") I recorded 19.0 psi/min.

I asked my instructor how accurate those figures actually are, or whether they're just a personal benchmark. (He dives tech) He said they become indispensable later on when it comes to predicting gas usage. I suppose my best job mow is to keep pretty close track of my figures until they settle down into a pretty close range. So far my SAC still seems to vary depending on quite an interesting assortment of reasons.

So how many cu.ft are in a steel tank (on average)?

 

[Warren_L]

Just to clarify - the rates I quoted were off my computer, so it's not based on the deepest depth of the dive. Really I was just trying to indicate that the first dive on the whole was much deeper than the second, and the SAC rate was lower on the deeper dive.

My computer is set for a 20 sec sampling rate, so the pressure/depth data is collected throughout the dive.

 

[MikeS]

My Cobra air integrated computer rots my brain by calculating not only the amount of air used in psi and volume but also the SAC for the dive. Wait! Perhaps that’s a good thing. Then I compare air consumption to the depth profile and other details such as equipment configuration, temperature, etc., to learn the effect of those factors on air consumption.

For example, diving in a drysuit as compared to a wetsuit tends to increase my SAC as does colder water. This process must be turning by brain to mush!

Mike

 

[scubajoe]

erubic once bubbled...


drbill, i am a little confused here(new to diving) you dove to 90ft for 60 min and than 80ft for 60 min. I probably just don't understand correctly but these seem really long dives. Besides being decompression dives you are exceeding the limits by a lot. ex. 90ft is 25 min. max for a no decompression dive. Other than that I can´t believe that you could make a tank last that long at a 90ft dive without using nitrox or some other combination of gases. Also you would need a really long surface interval correct? please help me understand this!

eric

Eric, there is a simple explanantion for these profiles. They were probably multilevel dives. I'm surprised Drbill hasn't responded. Since I wasn't there I won't attempt to explain his dives. If he doesn't reply in a day or two shoot him a PM and he'll fill you in.

Joe

 

[Boogie711]

I don't think PSI per minute is a useful statistic at all. Even if it were adjusted to account for surface atmospheric pressure, it still gives you no idea what your breathing rate is.

When you say "So how many cu.ft are in a steel tank (on average)?" that's the equivalent of saying "How many feet are in a ball of string?"

For example - there are two steel tanks in my office right now. One gives me 100 cf at 3442 psi. One gives me 95 cf at 2640 psi. For mathematics sake, lets say I use one cubic foot of air per minute at the surface - using your math, Im using either 34.42 psi/min OR 27.78.

If you use the example of an E8-149 and a HP80, the number swings from 23.1 psi/min to 43.75 psi/min -- all for the same breathing rate.

 

[nemo06489]

MikeS once bubbled...
My Cobra air integrated computer rots my brain by calculating not only the amount of air used in psi and volume but also the SAC for the dive. Wait! Perhaps that’s a good thing. Then I compare air consumption to the depth profile and other details such as equipment configuration, temperature, etc., to learn the effect of those factors on air consumption.

For example, diving in a drysuit as compared to a wetsuit tends to increase my SAC as does colder water. This process must be turning by brain to mush!

Mike

Mike i have a cobra also i didnt even know it calculated Sac for my dives. I was asking how to figure out how to calculate Air comsumption and it was allready being done for me.

 

[DeepScuba]

Nemo, don't let the DIR police hear that. They'd have another bakers dozen of reasons NOT to use a computer. (And that might be a good thing in this case).

Since it's easy to learn, I'd suggest learning it.....and then letting your brain rot afterwards!

 

[Warren_L]

As long as you are well informed, and know the risks, do whatever you are most comfortable with. If it's with tables and a bottom timer, great. If it's with a computer, great.