Big Mistake

[Hoppy]

I've just read the whole thread thinking I was gonna get something out of this one.

The Cr*p in the middle is just that, but at the beginning I was learning. Thanks for posting this one SJ I at least appreciate it.

I will re read my Vyper manual a little more carefully.

Can I ask one question on sac rates, the posts earlier confused me with regard to Suunto DM software.

The working pressure of the cylinder, actual pressure etc. has me lost now.

If my Cylinder is a 12 litre Faber steel which I beleive is rated at 232 bar (correct me if I'm wrong) is that the "working pressure" ?

My DM software makes no change in SAC depending on what is in working pressure, yet you guys appear to say that it should ? Or am i just just lost ? (Maybe)

What in litres would be a reasonable sac rate for a relatively inexperienced diver ?

How (apart from practice) does one try to help get a SAC down?

Techniques would be appreciated, I am currently the original underwater horse ! And it's getting embarrasing, at this rate I'm gonna have to learn how to sling stages for a rec dive to 20 meters.

BUT not till I've been trained how to Nanook so don't worry !

Seriously folks I could do with some help.

Cheers
Hoppy

 

[SA-Diver]

I have to agree with you Hoppy, there was a lot of good info in this thread but also a lot of crap.
The 'working pressure' in the Suunto DM software is as you describe it, and is not used to calculate SAC.
The software uses starting and ending pressures to calculate SAC.

To get your air consumption down - dive,dive,dive.

 

[Nanook]

RavenC once bubbled...
Posting insults and attitudes that are driving this now aren't constructive. They aren't helpful. Let's get back to the intended purpose and off the insults. Or just walk away and go to another post.

Not everyone is going to agree on everything but I would bet that many would agree that the attitude and tone here is out of hand and it all started with insults.

Let's not be mean.

I'm glad to see somebody agreeing that seajay started insulting those trying to help him. (along with the pm's from 5 different people thanking me for my posts)



By the way: it is great to use the internet to reinforce your knowledge, just don't use it as your only source of knowledge.

Dive within your training.

 

[jonnythan]

SA-Diver once bubbled...
I have to agree with you Hoppy, there was a lot of good info in this thread but also a lot of crap.
The 'working pressure' in the Suunto DM software is as you describe it, and is not used to calculate SAC.
The software uses starting and ending pressures to calculate SAC.

To get your air consumption down - dive,dive,dive.

The way I see it, the Suunto software uses working pressure and rated capacity to determine cf/psi

If I put in a working pressure of 3300 and a capacity of 77.4 cf, that's .02345.

However, if I put in the actual values of 3000 and 77.4, that's .0258 cf/psi, a 10% difference.

Using the former, draining a tank from 3000 psi to 300 psi gives 63.3 cf consumed.

Using the latter, the software would calculate 69.6 cf consumed.

Ignoring the working capacity/pressure, any software can calculate SAC in psi/min, but you need the capacity and pressure to turn that to cf/min. If those values are inputted incorrectly, then your SAC is wrong.

 

[Hoppy]

troll looks like.

I asked for some help here and lo and behold off we go again !

If Mike F or UP or Spectre or some of the other people I respect are here, maybe they could PM the advice ?

Getting bored now and used to like it here :upset:


Hoppy

 

[SA-Diver]

What was that all about Hoppy ?

If you don't like the replies you get just ignore them.
The more you dive the more relaxed you will become and the better your consumption will become to a point.
There is no magic formula.

Jonnythan :

I tried the following with Suunto DM.
I dive metric.

15L steel cylinder with a working pressure of 232 bar.

Starting pressure 190 bar.
Ending pressure 50 bar.

SAC Rate 12.3 SLM

Pressure used 140 bar.

Rate used 2.1 bar/min.

Total gas in cylinder 2850.0 liters

Volume of gas used 2100.0 liters

I then changed the working pressure for the same cylinder on the same dive to 200 bar with the same starting and ending pressures and the resulting SAC rate, pressure used, volume used and rate used were exactly the same.

If you get different results using imperial or doing something different to what I was trying here, please publish - I am interrested to see.

 

[jonnythan]

SA-Diver once bubbled...
What was that all about Hoppy ?

If you don't like the replies you get just ignore them.
The more you dive the more relaxed you will become and the better your consumption will become to a point.
There is no magic formula.

Jonnythan :

I tried the following with Suunto DM.
I dive metric.

15L steel cylinder with a working pressure of 232 bar.

Starting pressure 190 bar.
Ending pressure 50 bar.

SAC Rate 12.3 SLM

Pressure used 140 bar.

Rate used 2.1 bar/min.

Total gas in cylinder 2850.0 liters

Volume of gas used 2100.0 liters

I then changed the working pressure for the same cylinder on the same dive to 200 bar with the same starting and ending pressures and the resulting SAC rate, pressure used, volume used and rate used were exactly the same.

If you get different results using imperial or doing something different to what I was trying here, please publish - I am interrested to see.

It doesn't sound like you're entering capacity *at* working pressure, rather you're entering capacity at atmospheric pressure.

Maybe the imperial software is different.

I've never used either.

However, it's impossible to calculate a SAC in cf/min correctly if the software thinks that the tank holds 77.4 cf at 3300 psi.

 

[Spectre]

When I wake up a little more I'm going to have to think about your world a bit. I know our tank ratings are calculated in unpressurized gas volume. e.g. an 80 cu ft tank [77.4, but using 80 for the math] has 80 cu ft in it at 1 ATA when released. 40 cu ft when released at 2 ATA, etc...

With the imperial system, SAC rate is in cubic foot per minute, and it's gained from the change in PSI during your dive. If you used 500 psi, from a 100 cuft tank, how many cubic feet is that? It depends on at what psi the tank has 100 cuft. e.g. if it's a lp tank rated to be 100 cuft at 2640, then 500 psi is 19 cuft. If it's a high pressure tank, 100 @ 3500 psi, then you're taking 14 1/4 cuft for 500 psi. If your talking one of those funky filberglass 5000 psi tanks, then you're now talking 10 cuft. So if you accidentally have your working pressure 5000 psi, when it's actually 2500 psi, then you're SAC rates are actually half what they really are.
[these numbers are tweaked for ease in sleepy-math, and not reflective of actual tank ratings]

Metric tank ratings are rated in water volume, not gas capacity. So there may be a difference there. I'll have to think about it more, but I'd think it the same thing...

 

[Hoppy]

Sorry , we posted at about the same time, the troll comment was aimed at Nanook not you.

I had a feeling the advice was gonna be DIVE and do it some more and some more

I get the same result as you as I'm using metric.

The help file does suggest it calcs differently in Imperial, which seems strange ?

So whats a decent SAC then ?

I'm not publishing mine ! Too embarrasing

Hoppy

 

[SA-Diver]

Jonnythan

You are correct.

I did the same exersise with Suunto DM software using imperial and the results differ from what happens with metric.

The Suunto DM calculations are done differently depending on whether you use metric or imperial.

I would have loved to dive imperial, but in South-Africa it would be a big issue as I would be the only diver on the dives using imperial.