Mares Nemo Air

[~Scuba_Steve~]

Zac....will this computer also give your SAC rate? Do you have the manual in PDF if so please send it to greenshovel@hotmail.com I am considering this for my primary computer and will move my RBGM to my back up

 

[Zak OHara]

Hi Steve,

I just emailed it over. Hope it helps. Maybe it's early pre coffee, but please clarify SAC. I have a hard enough time remembering all of our 3 letter acronyms. (DFC, SCS, MRS, etc....) LOL

 

[~Scuba_Steve~]

Thanks Zak, got it . SAC stands for Surface Air Consumption Rate.... This below is from the ScubaToys website, I am such a theif, lol.:no I will read the PDF it will show me in there. Agian thanks.

The compressibility of gasses is also an important consideration for divers due to its affect on how long a diver can stay underwater. Scuba regulators are designed to deliver air to a diver at the same pressure as the surrounding water pressure, at ambient pressure. That means that when a diver fills his lungs at a depth of 33 feet, he is taking in the equivalent amount of air as two breaths at the surface. Obviously then, a tank will only last half as long at 33 feet as it would at the surface. A tank that would last 1 hour at the surface would only last 1/3 as long, or 20 minutes, at a depth of 66 feet, etc.

It can be beneficial to be able to estimate how long a scuba tank might last at a given depth when dive planning. To determine this, it is first necessary to determine a divers Surface Air Consumption (SAC) rate. For example, if you are diving at 33 feet, and use 500 lbs of air in 10 minutes, it is easy to determine that you are using 50 lbs per minute. This is only true for this depth however. How much air would you use at 66 feet, or 99 feet?

The first thing we must do is calculate SAC rate. In the preceding example, a diver using 50 pounds per minute at a depth, would use 25 pounds per minute at the surface. His surface air consumption rate is 25 pounds per minute. With our SAC number of 25, it is easy to calculate our consumption rate for depths of 33, 66, 99 feet etc. We know we are under 3 times the pressure at 66 feet, so we would use 3 times as much air, or 75 pounds per minute at this depth.

The process becomes slightly more complex if depth consumption rate (DCR) is determined at a depth that is not in even atmospheres. (Not at 33, 66, 99 feet etc.) For this situation we use a formula that is simply an adaptation of Boyle's Law to determine our SAC rate:

SAC Rate = (DCR x 33) / (Depth + 33)

Let's look at an example. Suppose you did a 50 foot dive for 25 minutes and used 1700 pounds of air. This would mean our DCR is 1700/25 or 68 pounds per minute. Using this in our formula we get:

SAC Rate = (68 x 33) / (50+33)

or: SAC Rate = 2244/83 or about 27 pounds per minute.

We can then turn the equation around to determine our DCR for any depth.

DCR = SAC Rate x (Depth + 33)/33

Let's assume our SAC Rate is 25 and we want to know how fast will we use 2000 pounds of air at a depth of 75 feet.

Dropping our numbers into the equation we get: DCR = 25 x (75 + 33)/33 or DCR = 25 x 108/33 or DCR = 81.81

This means at a depth of 75 feet, we will use 81.81 pounds of air per minute. Dividing this into the 2000 pounds, we see this amount of air would last 24.4 minutes.

It is important to note that SAC Rate takes into account the assumption that you are exerting the same amount of energy at any given depth, and you are using the same size tank as you used when calculating your DCR.

For example, under strenuous diving conditions, you can consume air 4 times faster than when sitting still taking pictures. Also it is obvious that a 50 cubic foot tank would not last as long as an 80 cubic foot tank, even if they were both filled to the same pressure.

 

[Zak OHara]

Wow.. Ok here is another 3 letter acronym way TMI!!!! I just want to go underwater and have my computer tell me when it is time to come up. No worries no thinking.

I hope the Nemo Air hits all your needs. Thanks for looking!

Sincerely,

 

[~Scuba_Steve~]

Right, lol. Thanks agian Zak

 

[scubatoys]

Wow! I feel honored you grabbed some of my old ramblings! Important to note, most times when dealing with SAC rates - we covert them to Cubic feet per minute - and in my math examples - we used PSI. If always using the same size tanks, it's almost easier to know PSI as that will let you know that you have 12 minutes left... or 5 minutes left because you will go through so many pounds per minute... But cubic feet works better if one day you are using a 50, another day an 80, then next day twin 120's... then you also have to convert the cubic feet to the psi as well...

Or as Zak pointed out... just use an air integrated computer like the new mares!

Nemo Air Nitrox Dive Computer, Nitrox, Mares, Nemo Air Nitrox Dive Computer

 

[~Scuba_Steve~]

Wow! I feel honored you grabbed some of my old ramblings! Important to note, most times when dealing with SAC rates - we covert them to Cubic feet per minute - and in my math examples - we used PSI. If always using the same size tanks, it's almost easier to know PSI as that will let you know that you have 12 minutes left... or 5 minutes left because you will go through so many pounds per minute... But cubic feet works better if one day you are using a 50, another day an 80, then next day twin 120's... then you also have to convert the cubic feet to the psi as well...

Or as Zak pointed out... just use an air integrated computer like the new mares!

Nemo Air Nitrox Dive Computer, Nitrox, Mares, Nemo Air Nitrox Dive Computer

Thanks, I will be looking at yall when it is time to purchase this, more than likeyl at the end of the year. Agian thanks.