My new Uemis SDA

[BB1]

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.

Ok, not being funny here - I'm trying to learn a bit more.

If 2 tanks the same size are filled to the same pressure how can they have different volumes contained within them???

Maybe I'm a bit to new at this still but the engineer in me seems to say "same size and pressure should equal same volume?!!?!

 

[formula1mb@aol.com]

Here's the thing, the computer doesn't give a damn about how much actual volume is in the tank, it only cares about the tank factor for the given tank, in other words x psi = 1 cubic foot of standard air. If you input the RATED VOLUME at the RATED PRESSURE of a given tank, than you can divide the RATED PRESSURE by the RATED VOLUME and figure about the tank factor. So for the AL80 at 3000 psi lets say, 3000 psi divided by 80 = 37.5 psi for every cubic foot. Now you mentioned if you overfill, underfill, whatever........that does not change the tank factor. If you fill it to 3800 psi, you have 101.3333 cu ft at 3800 psi, if you divide 3800 psi by 101.333 cu ft you still get 37.5 psi = 1 cubic foot of standard air. The point is for a given tank, all you need is RATED tank size and RATED fill pressure.

I don't have the Uemis but it sounded like from scootgirl that it asks you to enter both the tank size and tank pressure. So using that you can figure out the tank factor and calculate the SAC. You're confusing total volume with rate of consumption, as long as the computer knows the rate at which the pressure is dropping and the tank factor, you can get the SAC.

Maybe your mistake is you're entering the actual volume of air in the tank when you overfill them which yes, would throw off the tank factor and SAC. If you just enter the rated pressure and rated volume, you should be good to go.

 

[Michael Gerhartz]

I think what Mark is talking about is the baseline for the computer to know when the tank has 80 Cuft of air in it, which would be at 3000PSI for an aluminium 80.
I forwarded this topic to Switzerland and I'm still waiting for some details back.
In the metric settings it is easy, you set your tank volume ( 10 liter, 11.1liter, 12 liter or 15 liter or whatever ) and that's it. The transmitter gets the tank pressure, lets say 212 bar and that gives you the total air volume of 2120 liters if you are diving a 10l tank.
I must say, that I did several dives with both imperial settings as well as metric settings on double 120's and that the difference in the SAC rate was so minimal that I do not know if is due to the calculation or physical factores during the dive.
The SDA calculates the the SAC in metric and converts the result in imperial. But I will post more details once I have them, since I do not want to post anything that I can't back up.

Michael

 

[DiverLaura]

okay kids... calm down a bit...

so there is a key difference that people are missing.

imperial and metric computers have slightly different screens.

when i go into my adjust gas window (I'm in seattle, washington, USA) and dive Cuft and psi) I get a screen that says "cu ft" and "max pressure". then the 'reserve' for the low gas alert.

so if i'm diving double 102, i put in 204, and i put in 3500psi.

if i'm diving single 80, etc...

the imperial window for this only asks for the 'liter' size of the tank, because it doesn't matter what the standard EU pressure would be, because they are entering in actual internal volume 9l, 10, 12, etc.. internal volume times the pressure will give you the volume of gas, change in pressure times internal volume will give you the consumption.

So from what i gather, it calculates on the pressure change (based on what the transmitter is telling it), and converts that to a number we see as "consumption rate". as long as you give it the correct tank size in metric, you are good to go. as long as you give it both the correlative tank size and rated psi in imperial, you are good to go.

what a lot of people may be surprised at is in fact their 'actual' consumption rate. when we average our rate and fudge it on the surface, we can make the number look pretty. when it's staring at you on your wrist when you are swimming against current, it can be a bit disconcerting.

air integrated computers made by these same people have been around for a long time. they did not reinvent the wheel


lol... michael beat me to it

 

[Marc Bryan]

Scooter Girl - what screen in your uemis are you finding max pressure? I see tanks size, oxygen, maxPO2, tcd, gas limit, reserve. This is in the dive settings>gas menu.

Formula - I agree. You make my point for me. You cannot enter 80cuft in your example of a cylinder with 3800psi and get an accurate reading. You must enter 101cuft. My issue is that the manual should say tank volume not size, because it is not the same thing. There is not an entry for tank factor like you suggested. The point you are making is my exact argument.

Sorry for short replies, I am on my phone at Ginnie Springs.

 

[Marc Bryan]

Ok, not being funny here - I'm trying to learn a bit more.

If 2 tanks the same size are filled to the same pressure how can they have different volumes contained within them???

Maybe I'm a bit to new at this still but the engineer in me seems to say "same size and pressure should equal same volume?!!?!

I just did this as a test. They did have the same volume. My point is the tank size is 104. However when filled to 3500 it actually contains 133cuft. So when the uemis asks for tank size, that would be 104, but using this it would return a consumption rate based on 104cuft not 133.

 

[DiverLaura]

i have not noticed a difference in my Consumption rate when using a tank that is an AL 80 at 3000 psi, that has been filled to >3300psi.

why? because it's basing the calculations on a tank factor (psi change = X cubic feet). Not the cubic footage of the tank. I believe it uses the data i give 80@3000psi, 102@3500psi, etc... to figure that, just like we are taught in class to figure 'tank factors'.

your consumption in an 80, a 100, a 204, etc.. isn't gonna change if the formula for calculations are correct. i have not noticed a variance in my consumption going from tanks as small as 40's to double LP104's overfilled to >3000psi.

I'm only basing this on my personal experience with the unit.

are we talking hypotheticals here, or has someone actually noticed a discrepancy in their gas consumption that would be beyond the conditions of the dive whilst overfilling a tank?

 

[DiverLaura]

ps... jealous that you are at ginnie

 

[CompuDude]

I think what Mark is talking about is the baseline for the computer to know when the tank has 80 Cuft of air in it, which would be at 3000PSI for an aluminium 80.

ScooterGirl and Michael, just for the record, I'm sure you both know an Al80 doesn't actually hold 80cf, right? (at rated pressure, at least) Al80 = 77.4 cf at 3000psi. So if people are entering 80cf for the tank size, they're not getting accurate calculations out of it. (And SG, what is a 102? I've never seen a tank rated to 102. HP100 and LP104, yes, but no 102's.) I assume both of you are speaking shorthand for ease of discussion, but I wouldn't want people who are unaware of this to think that's the way to do it.

Full disclosure: I don't own a Uemis, although it's high on the list of serious contenders for my next computer. I've read the manual (once) and read many reviews (end-user and otherwise) with great interest, however.

Does the Uemis ask for the working pressure of the cylinder and the rated capacity (in cf) at that working pressure? Because that's all it needs (other than the pressure readings it gets itself) to calculate the proper SAC. Doesn't matter if a tank is overfilled or not. If the fill pressure is higher than the working pressure, the math still works the same to determine actual gas in the tank. (Ignoring a discussion of ideal gas laws)

If a tank is rated to 95cf @ 2640 psi, it's easy math to see how much gas it holds at 3000 psi. (3000/2640*95 = ~108) Rated pressure is simply 2640/2640*95=95.

If the Uemis is NOT doing a calculation to determine how much gas is actually in the tank, and always assumes a 95cf tank is holding 95 cf at the start, that's (a) retarded, and (b) completely wrong. Marc Bryan would be 100% correct to be suspicious. I find that unlikely, however, otherwise in the all-too-common scenario of starting off with an underfilled (or partially-used) tank, the Uemis would again be wrong in all of it's calcs unless you calc out (by hand) the actual volume of gas in the tank and enter that as the "tank size" instead of the rated size. (Let's say 2200psi in an LP95, giving us 2200/2640*95=~79cf) It would be absurd to have to enter 79cf into the computer to get an accurate SAC rate out of it, so why would Mark need to enter 133cf to get an accurate calculation?

Assuming the Uemis is properly calculating tank size/capacity/volume based on entering the appropriate numbers, Mark would be completely messing with the computer by entering 133cf for the tank size, if he's diving an LP104 overfilled to hold 133cf (but still enters the correct working pressure for LP tanks, 2640), and getting incorrect SAC calculations out of it as a result. Otherwise he's basically inventing custom tank sizes on the fly by entering a mythical tank that holds 133cf with a 3500 psi rating. The math will work out in the latter case, but only because he's forcing it to by doing a lot of (what I assume to be) unnecessary hand calculations before entering it. That must get old pretty quick if there's any variation in fills on a day-to-day basis.

 

[DiverLaura]

yes compudude, i am aware of the 'actual' volume of an AL80. *sigh* but entering 80 is easier from a keystroke standpoint, and changes my SAC rate marginally.

the HP102 from Pressed Steel. it is a 3500 psi tank from the early through late 90's, yes, it had a rated pressure of 3500psi. you may be more familiar with them as "those weird old tanks that have really narrow necks" if you know them as HP100's, that is cool, we just always called them 102's since that was their actual volume *poke poke*

Scuba Cylinder Specifications from Tech Diving Limited - 928-855-9400

I guess we will have to wait for the final word from Switzerland to figure out this conundrum.

BUT, again, for the record, starting with half full tanks or 3/4 full, or overfilled tanks, i have not noticed any odd variation of my Consumption Rate with the Uemis.

ScooterGirl and Michael, just for the record, I'm sure you both know an Al80 doesn't actually hold 80cf, right? (at rated pressure, at least) Al80 = 77.4 cf at 3000psi. So if people are entering 80cf for the tank size, they're not getting accurate calculations out of it. (And SG, what is a 102? I've never seen a tank rated to 102. HP100 and LP104, yes, but no 102's.) I assume both of you are speaking shorthand for ease of discussion, but I wouldn't want people who are unaware of this to think that's the way to do it.

Full disclosure: I don't own a Uemis, although it's high on the list of serious contenders for my next computer. I've read the manual (once) and read many reviews (end-user and otherwise) with great interest, however.

Does the Uemis ask for the working pressure of the cylinder and the rated capacity (in cf) at that working pressure? Because that's all it needs (other than the pressure readings it gets itself) to calculate the proper SAC. Doesn't matter if a tank is overfilled or not. If the fill pressure is higher than the working pressure, the math still works the same to determine actual gas in the tank. (Ignoring a discussion of ideal gas laws)

If a tank is rated to 95cf @ 2640 psi, it's easy math to see how much gas it holds at 3000 psi. (3000/2640*95 = ~108) Rated pressure is simply 2640/2640*95=95.

If the Uemis is NOT doing a calculation to determine how much gas is actually in the tank, and always assumes a 95cf tank is holding 95 cf at the start, that's (a) retarded, and (b) completely wrong. Marc Bryan would be 100% correct to be suspicious. I find that unlikely, however, otherwise in the all-too-common scenario of starting off with an underfilled (or partially-used) tank, the Uemis would again be wrong in all of it's calcs unless you calc out (by hand) the actual volume of gas in the tank and enter that as the "tank size" instead of the rated size. (Let's say 2200psi in an LP95, giving us 2200/2640*95=~79cf) It would be absurd to have to enter 79cf into the computer to get an accurate SAC rate out of it, so why would Mark need to enter 133cf to get an accurate calculation?

Assuming the Uemis is properly calculating tank size/capacity/volume based on entering the appropriate numbers, Mark would be completely messing with the computer by entering 133cf for the tank size, if he's diving an LP104 overfilled to hold 133cf (but still enters the correct working pressure for LP tanks, 2640), and getting incorrect SAC calculations out of it as a result. Otherwise he's basically inventing custom tank sizes on the fly by entering a mythical tank that holds 133cf with a 3500 psi rating. The math will work out in the latter case, but only because he's forcing it to by doing a lot of (what I assume to be) unnecessary hand calculations before entering it. That must get old pretty quick if there's any variation in fills on a day-to-day basis.