My new Uemis SDA

[Michael Gerhartz]

I'm still waiting for some last details from Switzerland on which formula they use. I just want to point out, that I tried several scenarios with both imperial and metric settings yesterday and that the difference in the SAC rate was so small, that I am not sure if it was because of the calculation or the physical conditions during the dive.
If someone does not trust the imperical calculation, there is a simple fix:
Switch from Cuft to Litre, then enter the tank size ( not the total air volume it holds at a rated pressure, just the internal volume of the tank, i.e. 11,1 liter for an aluminium 80 ).
After that change the settings back to imperial and you are good to go. The whole process takes less than a minute....Since the metric calculation only need the internal tank volume and the air pressure, this gives you the most accurate reading.
Like I said, I tried several different variations the other day with no significant difference in the SAC rate ( okay, I have to admit that I froze my .... off after the 3rd dive, but that is a different story...)
Knowing for how many other AI computers the manufacturer of Uemis have written the software succesfully, I'm convinced that it works properly. But if someone has some fency numbers coming up on the display, I would certainly like to know about it.
Just for the recordthis issue would not come up in a metric calculation
Michael

 

[DiverLaura]

here is the screenshot where I enter it.. imperial.. .sorry it took so long, had to get some she-p's packed, UEMIS brochures sent out and the dog for a jog as she was driving me batty

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.

 

[CompuDude]

SG- I figured you knew about an 80 (and my post said so!), I just wanted to be clear for other coming along.

Ok, gotcha re the 102. New one for me... I had never noticed the old PST HP100s were actually 102s. (even when I owned one)

Edit: SG, no pic shows in your post above. (#52)

 

[Marc Bryan]

here is the screenshot where I enter it.. imperial.. .sorry it took so long, had to get some she-p's packed, UEMIS brochures sent out and the dog for a jog as she was driving me batty

Thanks for the pic, but that is a setting for a pressure alarm during the dive. You set it when you want the uemis to alert you that you have reached a certain pressure, like if you were diving thirds and needed a reminder. It has nothing to do with the baseline or rated pressure of the tanks.

I agree that you will not see a large sac difference in an aluminum 80 under standard dive shop fills. And I do not see a large difference between entering 104 or 133, however, that does not mean that I do not want it to be as accurate as possible.

Ps - you are now a uemis distributor correct? Edit: oops just noticed the brochure comment. And I know how the dogs can be. I have a German Shepherd puppy. Between teaching and the puppy and the wife I barely have time to breathe.

 

[Marc Bryan]

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? NO it does not. That is my point. The only entry is tank size. That is it. 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? Yet that is exactly what I am doing. Now maybe the uemis has some type of internal calculation that are not list in the manual, and that I do not understand, but I still don't see any way it can properly calculate it without all of the relevant information. It's like one of those math questions where the answer is D: not enough information.

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.

You said yourself that you read the manual. Did you see anywhere in the manual a section for entering tank information, besides tank size? I just want to make sure I have not overlooked something.

 

[Marc Bryan]

I also want to make clear that I believe the people at uemis know what they are doing or I would not have bought the SDA, they obviously have the track record. I just want to know how the uemis is doing it.

 

[disbelief]

It seems to assume a working pressure (3000psi, from a back of an envelope calculation) from which the cf is derived. This gives internal volume, which is what you actually need.

The manual ought to read: "enter cylinder size in cf volume at xxxx psi".

 

[Michael Gerhartz]

When you enter the tank size as the imperial unit, I was told the computer uses a baseline of 3000PSI, which can, theoretically cause some slight differences in the SAC rate. Now, before the screaming starts, let me explain: As Laura has pointed out and as I have said before, we both did several dives with all kinds of different tanks and setups and starting pressure and everything...the difference in the SAC rate was always so minor that at least I do not know if it was because of the calculation or due physical conditions of the dive itself. For 99% of all dives, those differences are not a factor. Do not forget that the computer can only calculate based on the information the user puts in. So if you are diving an 80 Cuft alu tank, the true capacity at the rated pressure is 77Cuft, if you dive a 80 Worthington, it is actually 81,3 Cuft. For Worthington 119 it is actually 123,4Cuft, so you have to specific all the way if you want to get the most accurate SAC.
IF the Sac rate is really that important for your dive planning, it is recommended to enter the nominal tank capacity as a metric measurement and switch to imperial settings afterwards. For calculating the SAC rate, the unit always converts to metric, since this is way more accurate. Only the end result is then converted back (if desired) in Cuft.

Michael

 

[Marc Bryan]

Thanks Michael. I just want it to work properly. So if I understand correctly, when entering tank size, it should be the volume of whatever tank you are diving at 3000psi. Then, if it is filled to 4000psi, the uemis makes the adjustments on the fly. So for all cylinders rated at 3000 (obviously the vast majority) there is no need for adjustment, but for all of us diving steels, we need to calculate the volume at 3000psi and enter it as tank size.

I know this seems trivial to some of you, but when you are over a mile from any open air source and 170ft deep, the little things matter in planning. It also may not seem like a big deal to do the imperial/metric/imperial swap if you are only diving a few times a month, but for people who do 30-40 dives per month, it just seems to be a nuisance.

Thanks again Michael for the info.

 

[Marc Bryan]

It seems to assume a working pressure (3000psi, from a back of an envelope calculation) from which the cf is derived. This gives internal volume, which is what you actually need.

The manual ought to read: "enter cylinder size in cf volume at xxxx psi".

That's what I think as well.