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DangerousDave
17-08-2015, 11:51 AM
Just wondering if someone could answer the following question -

DangerousDave has a 204 bar cylinder that is filled to 85bar of EAN36.
He'd like it topped off to EAN28.

What additional 02 pressure is needed to get this mix?

I have gone through the TDI manual several times but I'm having difficulty with this.

I ended up with
0.01 * 119 (119= 204- 85bar)
-----
.79

= 1.506xxxx

Is this right?

nickb
17-08-2015, 12:01 PM
Air top will be about right

Gareth J
17-08-2015, 12:03 PM
Just wondering if someone could answer the following question -

DangerousDave has a 204 bar cylinder that is filled to 85bar of EAN36.
He'd like it topped off to EAN28.

What additional 02 pressure is needed to get this mix?

I have gone through the TDI manual several times but I'm having difficulty with this.

I ended up with
0.01 * 119 (119= 204- 85bar)
-----
.79

= 1.506xxxx

Is this right?


Dave

The two most reliable methods are to use either a look up table. (For Nitrox, this is the quickest and best way, especially if you are on a boat.)
The second is to use software that will do the calcs for you.

There should be a table in the back of the manual.

Gareth

JPTaylor
17-08-2015, 12:05 PM
Use http://wetnotes.t-provider.de..... don't now cylinder size, but assuming 12L (needed for real gas model)

Add 1 bar of oxygen & air top, or basically don't bother adding any oxygen, just air top the cylinders, you'll be close enough....

MikeF
17-08-2015, 12:05 PM
add 1 bar O2. air top to 204.

or

just air top to 185 bar gives you 28%

204 air top gives you 27.4

Gareth J
17-08-2015, 12:05 PM
Air top will be about right

Yep, I make it about +3bar O2. Then air top.

gobfish1
17-08-2015, 12:06 PM
Yep 2 bar o2

BTS
17-08-2015, 12:06 PM
What Nick said... just hit it with air.. It will be about bang on the money.

When I had twins I would fill with 32, use half, fill with air and have 27%give or take a percent

JPTaylor
17-08-2015, 12:08 PM
Yep 2 bar o2

2 bar for ideal gas laws, 1 bar if real gas laws... but when adding this amount, more trouble than it's worth, you'll most probably add to much & end up with a richer mix! As others have said, just add air.

gpj
17-08-2015, 12:15 PM
Air top will be about right


What Nick said... just hit it with air.. It will be about bang on the money.

When I had twins I would fill with 32, use half, fill with air and have 27%give or take a percent

Yep, that's what I do - twin 12s, 32% use half then air top result 26 -27% (if I don't use the fill over 2 dives)

Gareth J
17-08-2015, 12:21 PM
Yep, that's what I do - twin 12s, 32% use half then air top result 26 -27% (if I don't use the fill over 2 dives)

Kind of how I used to dive Scapa before I switched to CCR.

Fill the twins with a 36% for the second dive, exit when gas is near to the threshold point for the following morning [1]
i.e. 120 bar for a 27% mix in the morning

[1] Warning, contents gauges are terribly inaccurate. You need to check with a more accurate (digital) gauge when you get back on the boat.

DangerousDave
17-08-2015, 12:53 PM
To everyone, thanks for the help so far.

I'm interested in understanding the maths (though I do appreciate that in the real world using tables / software is what you'd do.) Also grateful for the real world advice too.

I want to understand the calculations and I'm still not sure what the exact calculated value should be. It's the maths I'm interested in without wishing to sound ungracious/ungrateful to those who've responded.

JPTaylor thanks for the link it's really nice.

For those of you familiar with the TDI stuff - here's what I did

Want Have Need

.28 O2% .36 O2% 0.22

204 bar 85 bar 119 bar

57.12 30.6 26.52


.22 is derived from 26.52 / 119 (this is FW - fill pressure of cylinder)

So I use these values with the Partial Pressure With EAN formula;

FW - FTMx * Fill Pressure (in this example 119)
----------------
FMx - FTMx


so I end up with:

.22 - .21 * 119
-----------
.79 (1.00 - .21 using air as topoff)

= 1.506xxx

Have I made a mistake in these steps?

matt
17-08-2015, 01:13 PM
Just wondering if someone could answer the following question -

DangerousDave has a 204 bar cylinder that is filled to 85bar of EAN36.
He'd like it topped off to EAN28.

What additional 02 pressure is needed to get this mix?

I have gone through the TDI manual several times but I'm having difficulty with this.

I ended up with
0.01 * 119 (119= 204- 85bar)
-----
.79

= 1.506xxxx

Is this right?

85bar EAN36 comprises O2 and Air as follows:
Bar EAN36=85bar
Bar O2=[[(36/100)-0.21]/0.79]*85=16.1 Got Bar O2
Bar Air=85-16.1=68.9 Got Bar Air

You want 204 bar of EAN28 which is composed of O2 and Air as follows:
Bar EAN28=204
Bar O2=[[(28/100)-0.21]/0.79]*204=18.1 Want Bar O2
Bar Air=204-18.1=185.9 Want Bar Air

If you want 18.1 bar O2 and got 16.1 then you need (18.1-16.1)=2bar of O2, balance air.

More exotic example here: http://www.thediveforum.com/showthread.php?789-Diving-Sums&p=5608&viewfull=1#post5608

Matt.

matt
17-08-2015, 01:18 PM
2 bar for ideal gas laws, 1 bar if real gas laws... but when adding this amount, more trouble than it's worth, you'll most probably add to much & end up with a richer mix! As others have said, just add air.

I've never bothered with "ideal" - when it comes to the practial stuff it's a bit voodoo anyway.

For this example I'd either stick 5 bar O2 in and top to 220 or just top to 200. Depends on the dive.

Matt.

Ian_6301
17-08-2015, 01:36 PM
Dangerous Dave needs a 232 bar cylinder and a 32% mix...

londonsean69
17-08-2015, 01:44 PM
Dangerous Dave needs a 232 bar cylinder and a 32% mix...

Maybe Dave doesn't care for standard gases

DangerousDave
17-08-2015, 01:47 PM
Dangerous Dave needs a 232 bar cylinder and a 32% mix...

The name is perhaps a clue - ps didn't mean to dislike your post - pressed the wrong button :)

JPTaylor
17-08-2015, 03:01 PM
I've never bothered with "ideal" - when it comes to the practial stuff it's a bit voodoo anyway.


Found that "real vs ideal" makes a big difference with trimix/heliar, tend to add less helium. If you follow "standard" mixing methods tend to end up with mixes lower in oxygen & higher in helium than you wanted. Not such a big difference with Nitrox mixes.

As you can get good mixing software on your phone for peanuts, may as well us it....

Also DangerousDave appers to have a 207bar cylinder (suprised if it was 204bar, more likely 207Bar/3000psi) so the 232bar mixing not much use!!

matt
17-08-2015, 03:53 PM
Found that "real vs ideal" makes a big difference with trimix/heliar, tend to add less helium. If you follow "standard" mixing methods tend to end up with mixes lower in oxygen & higher in helium than you wanted. Not such a big difference with Nitrox mixes.

As you can get good mixing software on your phone for peanuts, may as well us it....

Also DangerousDave appers to have a 207bar cylinder (suprised if it was 204bar, more likely 207Bar/3000psi) so the 232bar mixing not much use!!

I also suspect it is a 207. Maths is the same, I guess the outcome is too.

I mixed all the gas for our Britannic trip and using a pre-cut table and my regular voodoo everything was within 1%.

For CCR I typically put more He as I prefer a 6/72 over a 7/67.

Matt.

gobfish1
17-08-2015, 09:25 PM
2 bar for ideal gas laws, 1 bar if real gas laws... but when adding this amount, more trouble than it's worth, you'll most probably add to much & end up with a richer mix! As others have said, just add air.

Yes with you on that I take my O2 in a pony and pig out on it later :D