Why SPG/BT over dive computer?

[Carribeandiver]

When I decided to adopt the DIR diving philosophy, I gave up my $900 dive computer and bought a SPG and BT/Depth gauge.
Can someone tell me why DIR divers abandoned dive computers in favor of the SPG, BT/depth gauge?

And a second question regarding gas planning. Because a certain individual here was kind enough to forward a very good article explaining many of the principles concerning SAC/RMV, turnaround pressure, etc., I learned that a AL80 with 3000 psi has roughly 26 cf of gas per psi and how to figure that number. Using that formula, I figured my LP77 with 2640 psi to be around 29 cf per psi.
BUT, what happens when the dive shop generously overfills? I have 2 LP77's with approximately 2900 psi. Surely they now have more than 77 cf, right? How do I calculate the total cf of gas so I can figure cf per psi?

 

[TSandM]

To answer your second question, if you divide the tank volume by the working pressure at which the tank is supposed to hold that volume (usually the +10% number for steel tanks), you get the cubic feet per psi for that tank. Multiply that by the pressure the tank's at, no matter what that is, and you have the current volume of the tank. For example, for my 95's, they should hold 95 cu feet at 2640 psi. Multiply that ratio by the current pressure of the tank, and I know my volume. (I cheat and do it with tank factors, which are approximations of cu ft per 100 psi. For my 85s, that's 6.4; means that if I have 2000 psi in them, I have approximately 128 cu ft, close enough for government work.)

For the first question, here is a "Baker's Dozen" list of objections to dive computers by Jarrod Jablonski. The basic argument is that computers don't give profiles for decompression diving that are what GUE believes are most efficient and safe, and that using computers for that purpose causes the diver to stop thinking about what he is doing.

Most of us used computers when we took Fundies, and used them for some time afterwards. It takes, I think, quite a bit of study and practice before you feel comfortable going without and doing all the monitoring and calculation in your head.

 

[PerroneFord]

(77*2900) / 2640 = cuft.

Computer diving (taken from the link in my signature)


Divers have three primary methods by which to calculate dive and/or decompression time, namely, tables, wrist-mounted computers, and personal computer decompression programs. There is a great deal of debate surrounding the use of wrist-mounted decompression computers; divers wear these to calculate dive time limitations and decompression obligations during the dive. All divers should learn the proper use of decompression tables in order to learn the actual process of decompression diving. Divers that choose to use computers should do so after becoming well-versed in diving limits and then using the computer primarily as an educational tool. To see a list of problems with computer diving, consult Doing It Right: The Fundamentals of Better Diving, Chapter six.



A Baker's Dozen: Problems With Computer Diving

Dive computers tend to induce significant levels of diver dependence, and undermine the awareness essential to all diving, but particularly essential to divers just beginning decompression diving.

Dive computers prohibit proper planning; they discourage divers from “studying” the impact of various mixtures and decompression choices.

Dive computers are of little educational benefit because they promote neither questioning nor proper planning discussions.

Dive computers often use algorithms that heavily pad decompression time; this sometimes results in odd and ridiculous levels of conservatism.

Dive computers are expensive, and prevent divers with limited resources from purchasing truly useful equipment.

Dive computers significantly limit the likelihood that divers will track their residual nitrogen groups, leaving them less informed in the event of computer failure.

Dive computers do not allow for diving helium in any format but the bulkiest and most questionable. It is very likely that new helium-based decompression computers will be inordinately conservative and suffer from all the limitations of air and Nitrox dive computers.

Dive computers often generate longer decompressions than an astute, well-educated, experienced diver generates.

Dive computers often confuse matters by providing the diver with too much useless information, sometimes even obscuring depth and time in favor of blinking CNS and/or decompression limitations.

Some dive computers become very difficult to use if a decompression stop has been violated. Some computers will lock up completely, while others will just beep or generate erroneous and distracting information.

Dive computers do not allow the educated diver to properly modify his/her decompression profile to account for advances in knowledge, e.g., the use of deeper stops in a decompression profile.

Dive computers do not offer divers much flexibility to generate profiles with varying conservatism. For example, the right mix would allow 100 minutes at 60 ft rather than 60 minutes at 60 ft, but a diver might prefer to do one or the other or a hybrid of the two. Computers confuse this issue by not providing divers with the proper information.

Dive computer users often ignore table proficiency and therefore do not learn to read tables properly. When faced with a situation where they can't dive a computer (e.g., failure or loss) these divers are seriously handicapped.

 

[Carribeandiver]

Thanks TS&M and PF.

 

[Carribeandiver]

Thanks TS&M and PF

 

[Bismark]

It might help if you quit thinking in terms of PSI and start thinking in terms of cubic ft. You don't breath PSI, you breath a volume of air, which is measured in cubic ft (or litres for you metric folks). Also, learn your tank factors and with a little practise it becomes pretty easy to calculate how much gas you have in your cylinders at any given time. Also, combined with knowing your SAC (which is measured in cubic ft by the way, and not PSI) you can easily do dive planning. Once you start doing this you will see how "be back on the boat with 500 psi" becomes problematic at the best. 500 PSI of my low pressure 95's mean I have approximately 17.5 cubic feet of gas remaining while 500 psi of an al 80 means I have approximately 13 cubic feet of gas remaining. What was the 500 psi comment trying to accomplish? That I have 13 cubic feet or 17.5 cubic feet of gas? What was the point of buying a larger cylinder if I can't take advantage of it? If your truck has a 50 gallon tank and someone tells you to bring it back with "1/4 showing on the gauge" is that even close to telling my buddy with a small mazda car with a 20 gallon tank to bring it back with "1/4 showing on the gauge". Calculating your volume of gas is not beyond the average person, so why don't we insist everyone do it in their open water class? Good question.

 

[Charlie99]

Because a certain individual here was kind enough to forward a very good article explaining many of the principles concerning SAC/RMV, turnaround pressure, etc., I learned that a AL80 with 3000 psi has roughly 26 cf of gas per psi and how to figure that number. Using that formula, I figured my LP77 with 2640 psi to be around 29 cf per psi.

Your numbers are off by a factor of 1000. 26cf per 1000psi, or 2.6cf per 100psi for the AL80. 2.9cf per 100psi for the LP77

BUT, what happens when the dive shop generously overfills? I have 2 LP77's with approximately 2900 psi. Surely they now have more than 77 cf, right? How do I calculate the total cf of gas so I can figure cf per psi?

The cf per psi doesn't change. It is a characteristic of the tank. It is nothing more than Rated Cubic Feet/Fill pressure for that rating.

77cu/2640psi = 0.029 cf/psi = 2.9 cf/100psi = 29 cf per 1000 psi.
In real life, since we read SPGs to the nearest 100psi, and we calculate gas needs to a few cubic feet, the 2.9cf/100psi is the most useful version.

2.9cf/100psi * 2900psi reduces to 2.9 * 29 hundreds of psi = 84 cu ft.

You could of course get the same answer by realizing that your LP77's are overfilled by the fraction 2900/2640. 2900/2640 * 77 also equals 84 cu ft, but isn't as easy to do in your head.

 

[rjack321]

All good stuff you've gotten so far...

One correction, an AL80 has about 2.6cf of gas per 100psi (not 26).

2.6 x your current pressure in hundreds = the amount you have

 

[Charlie99]

It might help if you quit thinking in terms of PSI and start thinking in terms of cubic ft. You don't breath PSI, you breath a volume of air, which is measured in cubic ft (or litres for you metric folks).

Both PSI and cubic feet have their places in diving.

When I'm diving, I measure my gas by looking at an SPG, which is in psi, NOT cubic feet. Even if I originally calculated my rock bottoms and turn gas in cubic feet, they get turned into PSI for the dive.

That's why it is nice to be able to easily convert back and forth, using factors such as 2.6 cf / 100psi or 39 psi / cu ft.

In many cases, since your SAC varies from dive to dive, being 4% off by using the simpler to work 2.5cf/100psi or 40psi/cu ft isn't going to make a material difference in your calculations.

 

[MXGratefulDiver]

It might help if you quit thinking in terms of PSI and start thinking in terms of cubic ft. You don't breath PSI, you breath a volume of air, which is measured in cubic ft (or litres for you metric folks).

Actually, you breathe the gas in your cylinder ... what I teach my students is that it's important to think in terms of CF for gas planning (prior to the dive) and in terms of PSI for gas monitoring (during the dive). That's because before the dive you want to know that you have adequate gas for the dive you're planning ... and because during the dive the only "feedback" you have on gas consumption is your SPG. Therefore understanding your consumption rate in both terms is useful.

Calculating your volume of gas is not beyond the average person, so why don't we insist everyone do it in their open water class? Good question.

As an instructor who has attempted to do just that, I found that I would "lose" OW students with too much information. I settled for teaching them the concepts of rock bottom, turn pressure, and a few rules of thumb ... and emphasizing that there was much more to learn.

With the exception of lengthy OW classes like you typically get in a university, I haven't found it practical to get into things like tank factors and SAC calculations in OW. There's just too much else to cover in a limited amount of time. Frankly, since consumption rate is really not a number, but rather a value range based on several variables, one needs several dives worth of data to make it meaningful. And that's not something that an OW student is going to have at their disposal anyway. I decided, therefore, that this information was more appropriate to be taught at the AOW level.

... Bob (Grateful Diver)