Optimum Depths / Dive Time Ratios

[Scuba Brad]

I have less than 100 dives under my belt and have often felt that I should be able to figure out what my optimum dive time WILL BE for my breathing patterns. For that matter is there a good formula you use to determine how long you will be down BEFORE you dive so that you can Maximize your dive once in the water? (Now having said all of that I use a computer and can read it as well as anyone. If only it could forcast..)

Any help? Or rules of thumb that are helpful???

Happy Diving

 

[Hyper-limits]

Of course there is, it call RMV/SAC rate.

SAC=surface air consumption
RMV=respiratory minute volume

How to find you RMV/SAC rate.

If you diving an aluminium 80 with a rated fill pressure of 3000psi

80cuft divided by 3000psi =

80/3000= 0.266ft3/psi

Let’s round that off to .027 cubic feet of gas per psi of cylinder pressure.

Next, pick a course to swim, one that will allow you to stay at a constant depth. Mark you time and pressure, and begin a moderate finning as if you were doing a normal sight seeing dive, continuing for 5 minutes.

Your start pressure was 2550 and after 5 minutes the end pressure is 2090 psi.

So you use 460 psi at 60 feet

Now multiply by the cylinder value of .027

460 x .027 = 12.42 cuft

Now divide by the number of minutes for each segment

12.42 / 5 = 2.484 cuft

Now, convert the depth to pressure for each

(60ft / 33) + 1 = 2.8 ATA

Finally, divide the cubic feet per minute by the segment pressure.

2.484 / 2.8 = 0.88 cuft min.

RMV/SAC is 0.88cubic feet per minutes at the surface. Now you can use this number to plan any dive.

Remember, these numbers are not fixed in stone,
They will vary depending on a huge number of factors like current, fatigue and stress. They will also vary day to day, dive to dive, but they give a diver a point of reference for planning purposes.

 

[Doc Intrepid]

I have less than 100 dives under my belt and have often felt that I should be able to figure out what my optimum dive time WILL BE for my breathing patterns. For that matter is there a good formula you use to determine how long you will be down BEFORE you dive so that you can Maximize your dive once in the water? (Now having said all of that I use a computer and can read it as well as anyone. If only it could forcast..)

Any help? Or rules of thumb that are helpful???

Happy Diving

Brad, ...let me get this straight: you want to predict your 'optimum dive time' in advance, based on your breathing patterns?

And/or you want a good formula to use to determine how long you will be down before you actually conduct the dive, so you can maximize your dive once in the water?

I think we went to different open water courses...

Mostly guys start by setting a depth limit, or having one set for them by underwater terrain or dive site. Then they calculate (or predict) the amount of time they can spend at that dive site (in advance) using either tables if its a square profile (like a wreck) or a wheel or a computer if its more of a random swim-around-the-reef type of dive. In terms of gas planning, many divers identify a psi-level at which they will turn or terminate the dive, allowing themselves adequate reserve gas for emergency response.

But in all cases its considered safest to maintain the same basic in/out breathing pattern - aside from this facilitating proper trim and bouyancy it also prevents carbon dioxide build-up which can result in unconsciousness (shallow water blackout) as a result of "skip-breathing".

Your breathing pattern has nothing to do with your optimum dive time.

There ARE formulas to calculate a diver's Surface Air Consumption (SAC) or Respiratory Minute Volume (RMV), which identify the rates at which a diver consumes their gas (in cubic feet per minute).

Using a second formula, which calculates the beginning volume of gas in the diver's tank, and dividing by the rate of consumption, provides the total time potentially available: Total cubic feet of gas -:- cubic feet per minute consumed = number of minutes that total cubic feet of gas will last.

Is that what you meant? While helpful to identify overall dive parameters, that is not considered 'gas planning' in terms of setting limits in advance on the time/depth of your dive. (?) For gas planning you want to identify how much gas you plan to use, how much you plan to hold in reserve, and how you expect your dive profile to look in terms of using that gas (max depth for X minutes, when do we turn the dive and head back to the anchor line, amount of gas you'll use on safety stops, etc.)

If you want software that will forecast gas consumption for dive planning, here is what I use. There are others just as good, I simply like this one:
http://www.hhssoftware.com/v-planner/

This software allows you to plan and analyze (compare and contrast) different dive plans and profiles by allowing you to alter one variable or another and see what impact it has on the dive profile. Its inexpensive and easy to download, and extremely helpful in understanding the relationships between various parameters in dive planning. This may also be what you had in mind.

Not entirely certain what you were looking for, but hope this was helpful.

Regards,

Doc

 

[Charlie99]

Another calculation you should be doing is figuring out at what pressure you need to turn around or ascend. http://www.scubaboard.com/showpost.php?p=401869&postcount=65 is a good description of one way to calculate the "rock bottom" amount needed for you AND your buddy to ascend while sharing air.

 

[DiverBuoy]

Just remember that "rock bottom" keeps in mind any buddy your swimming with too ... it's the amount of gas required to get both of you (breathing off the same source) safely back to your dive exit. This is scaled based on the type of diving your doing. In one type of diving you have to exit back the same way you came and must get to a specific spot like say a mooring line to return to the boat in heavy current, but in another scenario it's just a matter of getting both of you to the surface safely and you can then swim back on the surface ... clearly in the first scenario more gas will be required for two divers to breath off the same source for a decent swim back to a chain and to ascend (including safe ascent/decompression if necessary/safety stops) vs the second scenario which you can just ascend safely directly to the surface.

 

[Scuba Brad]

Doc,

No we went to the same course.........What i was trying to get to was to say, "I want to stay down the longest I can........at say 50'" And yes your calcs are just want I was looking for.

I am a big guy and consume more air than most yet the longer I dive I'm finding myself getting better and diving longer.

I am just trying to learn more.........most of the good stuff....rules of thumb....I'v elearned from DM's on dive trips. May I just say thanks to all those who are so willing to help those of us who are willing to still learn.

Every once in a while I will get a "why in the world are you asking that" sort of rely. But most times I get really excellent well thought out replys.....so THANKS!!

Happy Diving

 

[The Kraken]

Brad,
Here's a simple formula that you can use to determine how much gas you will use at a given depth for a given time:

SAC x((D/33)+1) x T

Where:
SAC = Your personal surface air consumption rate
D = Depth of dive in feet
T = Planned dive time

the K

 

[Scuba Brad]

K, Thanks. I've taken good notes and will try both ratios. I'm heading to Cayman Brac shortly and will report back on how close we were.

Its always nice to test the theory!

Happy Diving

 

[The Kraken]

I developed some simple tables that deal with different tank sizes, depths and SAC rates.

What you have is your SAC rate, size of tank, dive depth and optimum dive time with a residual 500 PSI.

Here ya are:

SAC Tank Depth/time
.40 = Al 80 - 50/64 60/57 70/51 80/47
119 Cu Ft - 50/101 60/90 70/82 80/74

.50 = Al 80 - 50/51 60/45 70/41 80/37
119 Cu ft - 50/81 60/72 70/65 80/60

.60 = Al 80 - 50/42 60/38 70/34 80/31
119 Cu Ft - 50/68 60/60 70/54 80/50

.80 = Al 80 - 50/32 60/28 70/26 80/23
119 Cu Ft - 50/51 60/45 70/41 80/37

the K

 

[Charlie99]

Since I almost always use an AL80 the easiest way I find to estimate available time at depth is to use "surface equivalent minutes" or "ata-minutes".

1 minute at the surface is 1 ata-minute. 1 @ 35' is 2 ata minute. 1 @ 50' is 2.5 ata minutes; 1 @ 70' is 3 ata-min, 1 @ 100' is 4 ata-min (these aren't quite exact, but close enough since SAC varies widely).

Depending upon what I'm doing, I just assume that the first 2500psi/65 cu ft of an AL80 has 120-160 ata minutes. (that's for 0.4 to 0.55 cfm SAC)