Sac Rates

[Morg_NZ]

Thought that I'd post this here as many of you use dive computers !!!
What I was after is an indication of SAC rates that everyone is getting? I know it's a personal thing but a recent dive showed me with a rate of 2.14 BAR/Minute ?? surely something is a miss here!!!
What sort of rates are you all getting and secondly, does the type of computer that you use affect the calculation at the end?

 

[The Kraken]

Don't know what you in England use to express the SAC, but over here we express it in the amount of cubic feet of air that one consumes at the surface. I think about .4 to .5 cu. ft. of air is about normal, whatever "normal" is.

One thing you really, really need to determine your SAC rate is the average depth of your dive.

You determine the amount of gas available by knowing what volume your tank holds at nominal pressure. You determine your percentage of usage, divide that by the time of the dive in minutes and then multiplying that by the average depth of your dive in ATA's.

Let's say you use an aluminum 80, which at a nominal pressure of 3000 psi holds 77.4 cubic feet of gas.
Your beginning pressure is 2712 psi - this is right at 70 cubic feet of air
Your ending pressure is 776 psi - this about 20 cubic feet of air
Your dive time was 45 minutes
Your average depth was 66 feet - 3 ATA
So (50 cu ft / 45 min) / 3 = a SAC rate of about .37

 

[Diver0001]

Assuming you're using a 10 litre tank then you're getting about 22 litres a min RVM, mate. That's a little higher than average.

How did you calculate it? Let's start with that.

....hmmm.... I'm getting a clue from your profile picture but I can't quite put my finger on it.......

R..

 

[Charlie99]

What I was after is an indication of SAC rates that everyone is getting? I know it's a personal thing but a recent dive showed me with a rate of 2.14 BAR/Minute ?? surely something is a miss here!!!

Your computer doesn't know what tank you have, so it can only give you pressure used/minute. As Diver0001 posted, multiply your pressure/min times the tank size to get liters per minute.

The cubic foot of the USA/imperial system is about 28.3 liters.

22 liters/min or 0.8cu ft/min is relatively high SAC, but not unusual for new divers. Only a few divers will have SAC less than half that amount, 11lpm or 0.4cfm.

Charlie

p.s. Good profile photo. It looks like you found my old scuba rig.

 

[Morg_NZ]

Assuming you're using a 10 litre tank then you're getting about 22 litres a min RVM, mate. That's a little higher than average.

How did you calculate it? Let's start with that.

....hmmm.... I'm getting a clue from your profile picture but I can't quite put my finger on it.......

R..

Cheers for the remark!!

The details are as follows.
I use a 15 Ltr Faber
Depth was 30 Meters
Start BAR 200 and end was 50 BAR

I personally don't use a computer and my question/ SAC rate was worked out with a sensus pro and software. My bubby uses a suunto and ended up with a much lower rate BUT started with 200 BAR and ended with 45 BAR{he also complained later of a "head ache" i.e CO2 was built up} ???

Hows that figure? I'm basically trying to figure if there is something wrong with my rate and possible solutions to this.

 

[Charlie99]

1. Is 30 meters the AVERAGE depth? That's what you need for calculation.
2. How long was the dive?

 

[Morg_NZ]

1. Is 30 meters the AVERAGE depth? That's what you need for calculation.
2. How long was the dive?

Average depth was 18 meters and total BT was 36 minutes

 

[Charlie99]

I use a 15 Ltr Faber
Depth was 30 Meters
Start BAR 200 and end was 50 BAR
Average depth was 18 meters
total BT was 36 minutes

You used a total of 200-50=150 bar.
Your average consumption at depth was 150bar/36 min= 4.166 bar/min/
Your average depth was 18 meters, or 2.8ata.
So your surface equivalent bar/min was 4.166/2.8= 1.49 bar/min/ata
with a 15 liter tank this is 15x1.49= 22 liters/min.

That's on the high side of normal, but not unusual for a new diver.

The standard recommendations are

1) dive, dive, dive. Experience helps a lot.
2) get rid of excess lead, and stay neutral. You should be able to stop finning at any time and stay at the same depth. A lot of people with high air consumption are negatively buoyant and constantly finning at a 45 degree angles to maintain depth.
3). Take it slow and easy.
4). Know your SAC, make sure your buddy knows it, and plan your dives accordingly.

 

[Snowbear]

....hmmm.... I'm getting a clue from your profile picture...

When I started diving, my dad told me stories about his bucket and hand pump diving trials.... He said he could always tell when the shore (or boat) guy was getting tired from pumping as the water level in the bucket would rise :11:

 

[Morg_NZ]

You used a total of 200-50=150 bar.
Your average consumption at depth was 150bar/36 min= 4.166 bar/min/
Your average depth was 18 meters, or 2.8ata.
So your surface equivalent bar/min was 4.166/2.8= 1.49 bar/min/ata
with a 15 liter tank this is 15x1.49= 22 liters/min.

That's on the high side of normal, but not unusual for a new diver.

The standard recommendations are

1) dive, dive, dive. Experience helps a lot.
2) get rid of excess lead, and stay neutral. You should be able to stop finning at any time and stay at the same depth. A lot of people with high air consumption are negatively buoyant and constantly finning at a 45 degree angles to maintain depth.
3). Take it slow and easy.
4). Know your SAC, make sure your buddy knows it, and plan your dives accordingly.

Thanks for the advice,but I've been diving for about 7 years, I'm normally not using any lead because of twin tanks and as for slow!!!!.....well I'm a lazy ******* underwater!!!!!!!!
The only thing that I can think of is "it was a bad day", I'm completely bambozzeled by the rat I got???

Thanks for the pointers :bravo: