My new Uemis SDA

[DiverLaura]

i have bounced back and forth between doubles, singles, HP, low pressure, etc..

if it's an 80 cuft tank at 3000 psi I enter 80 in size and 3000psi at max pressure. for a set of double 102's I enter 204 @ 3500psi.

so far my breathing rate has been accurate on all the dives so i have to assume that when i enter the max pressure it's saying "80cuft @ 3000psi". i assume that it uses the wording "max pressure" because there are tanks, such as the 104's, that are 104's at plus rating of 2690psi, but not at their rated pressure (2400psi)

cheers,

Laura

 

[formula1mb@aol.com]

Here's my assumption, as long as you know the tank size and fill pressure, the computer can calculate the psi / ft^3. So for example, if you have an 80 rated for 3000 psi, a drop of 37.5 psi equals one cubic foot of air used. Using that, it doesn't matter if you fill it to 2000 psi or 5000 psi, since the only thing you need is the rate at which the pressure is dropping.

So now all the computer has to do is keep track of how quick the pressure is dropping. If you average it out every minute of diving, you see what the psi is at the beginning of a minute, subtract the psi after one minute of diving, divide it by 37.5 psi/ft3 or whatever it is for your tank, correct for the depth and you have your SAC.

 

[Marc Bryan]

Your missing a key ingredient. The sda knows the tank size, not gas volume. It bases the gas volume off of the size of the cylinder. If tell it that you are diving an 80cuft tank, it assumes 80cuft no matter the pressure, whether you have 500psi or 3000psi. Can you see where it would show a much higher sac rate for the 500 psi?

 

[Marc Bryan]

i have bounced back and forth between doubles, singles, HP, low pressure, etc..

if it's an 80 cuft tank at 3000 psi I enter 80 in size and 3000psi at max pressure. for a set of double 102's I enter 204 @ 3500psi.

so far my breathing rate has been accurate on all the dives so i have to assume that when i enter the max pressure it's saying "80cuft @ 3000psi". i assume that it uses the wording "max pressure" because there are tanks, such as the 104's, that are 104's at plus rating of 2690psi, but not at their rated pressure (2400psi)

cheers,

Laura

Where are you seeing or entering this "max pressure"? i see max po2 which has nothing to do with tanks size or pressure at all.

Nice dive resume BTW!

 

[BB1]

The transmitter can see the pressure - that's what its there for!

 

[Marc Bryan]

I understand what the transmitter is for. What does that have to do with where she is entering tank size and max pressure?

You are totally missing the whole point. The psi in the tank is only used to determine the actual volume of gas. That is it! The components of SAC rate calculation are:

1. Volume of gas consumed (not psi, it's not the same)
2. Time elapsed during the gas consumption
3. Depth during time of consumption

Also psi drop only works if you know that the starting pressure was the rated pressure for the volume of gas you are calculating the sac on.

If you have 80cuft of gas at 80 psi then every drop of one psi is one cuft. This does not universally work. What if I have 80cuft of gas at a pressure of 800psi? Then every one psi drop is not one cuft, it is one tenth. So the example of 37.5psi drop given earlier only works IF the computer knows the rated pressure of the cylinder, which the uemis does not!

The uemis will give a correct sac if you enter 77 as tank size for an aluminum 80 (which has 77cuft not 80cuft at 3000psi), and you always start the dive with 3000psi. However, this will hardly ever be the case. Sometimes you will have 3300 and sometimes 2600.

My oceanic vt3 has the ability to set the rated pressure of the cylinder I am diving, then it calculates the actual volume of gas based on the psi at the start of the dive to give an accurate sac reading.

 

[BB1]

Your missing a key ingredient. The sda knows the tank size, not gas volume. It bases the gas volume off of the size of the cylinder. If tell it that you are diving an 80cuft tank, it assumes 80cuft no matter the pressure, whether you have 500psi or 3000psi. Can you see where it would show a much higher sac rate for the 500 psi?

Do you know this for a fact??

Do you have a UEMIS yourself so you are basing this on experience??

The Ideal Gas Law would seem to cover this or am I missing something??

The pressure, volume, temperature and amount of gas in a container are all linked. This is expressed using the Ideal Gas Law, which says that PV = nRT. In this equation, P is the pressure, V is the volume, n is the amount of gas (in moles), R is a constant, and T is the temperature (in Kelvin).

What the Ideal Gas Law says is that if the pressure goes up, something else much change, but it could be either the volume, the temperature, or the amount of gas.

Let me give a few examples.

Imagine some gas inside a piston. If you compress the piston, the volume will decrease, and the pressure will increase correspondingly.

Now imagine a solid container (like an oxygen tank for scuba diving for instance). If it is empty and you start filling it with gas, the pressure will increase. In this case the volume is constant, but the amount of gas is increasing ("n" in the PV = nRT equation). Same idea when you pump up your car or bike tires -- the volume changes more than with a metal container, but not much really.

 

[CompuDude]

I understand what the transmitter is for. What does that have to do with where she is entering tank size and max pressure?

Have you ever used an air-integrated computer? You're really over-thinking this.

All AI computers work the same way. The pressure sensor detects PSI in the tank, and the computer logs the pressure readings at various intervals. (Obviously I'm looking at this from an Imperial standpoint, but I'm sure you can translate the theory to Metric.)

Most computers can't determine SAC until you get back to the computer, and download the data into the desktop software program. Once you add the tank info (rated size and working pressure), the software can determine the SAC. That's all you need... rated size and working pressure, along with the logs of pressure measurements over time, and it's trivial math for a your average PC to work out. But this is how EVERY AI computer works, in essence, be it a Uwatec, Oceanic, Suunto, or Uemis.

The nice thing about the Uemis is it has an interface that will let you enter the tank data right on your wrist, and the processing power to do the calcs without the need for a desktop download first. That's unusual.

There are things I wish the Uemis had, but this is truly basic, and not something you need to worry about.

 

[Marc Bryan]

Yes I do have a uemis, actually two. I also know that you must enter the actual volume in the tank, not the tank size. I have tested this while side mounting with the same size cylinders, entering one at it's rated size of 104 cuft, and entering the other at it's actual volume of 133cuft. It produced two different sac rates! They were both filled to 3500psi.

 

[Marc Bryan]

Have you ever used an air-integrated computer? You're really over-thinking this.

I have three different manufactures. And I have two uemis.
All AI computers work the same way. The pressure sensor detects PSI in the tank, and the computer logs the pressure readings at various intervals. (Obviously I'm looking at this from an Imperial standpoint, but I'm sure you can translate the theory to Metric.)

Most computers can't determine SAC until you get back to the computer, and download the data into the desktop software program. Once you add the tank info (rated size and working pressure), the software can determine the SAC. That's all you need... rated size and working pressure, along with the logs of pressure measurements over time, and it's trivial math for a your average PC to work out. But this is how EVERY AI computer works, in essence, be it a Uwatec, Oceanic, Suunto, or Uemis.

This is EXACTLY my point. You are making it for me. It must have an entry for rated size at a specific pressure. Then the software on the PC can calculate the SAC based on actual volume of gas, not just the pressure. My point is that nowhere does the uemis offer this. I just wanted everyone to be clear that tank size as input into the uemis should actually be tank volume at the pressure it is filled to.


The nice thing about the Uemis is it has an interface that will let you enter the tank data right on your wrist, and the processing power to do the calcs without the need for a desktop download first. That's unusual.

There are things I wish the Uemis had, but this is truly basic, and not something you need to worry about.

I do not agree with the last sentence. It is not basic, it works fine for recreational divers who just use an aluminum 80 always filled to 3000psi, but not for divers who use cylinders that are not always filled to a lower working pressure, but sometimes overfilled, such as in cave country where 2400 rated cylinders are filled to 3600+ psi routinely.

Thanks for the good discussion by the way. This is what SB should be about, nice adult discussions.