POLL: Who is homebrewing Nitrox/Trimix and for how long now?

[roakey]

Though I have the gut-level feel of "bump up the He and drop the O2" too, I can't seem to justify it. I just have this vague feeling of unrest.

Ignoring my gut for a moment, we have a conservative PPO2 of 1.2 (actually a little less) especially considering the length of the dive and an END of 101.

There might be a "standard gasses" argument in here somewhere; the problem being that altitude really wanks out some of the calculations!

To show how weird it can get, at a 10k elevation we can actually go on pure O2 at 30' and not exceed a 1.6 PPO2!

Roak

 

[Uncle Pug]

Originally posted by roakey
Though I have the gut-level feel of "bump up the He and drop the O2" too, I can't seem to justify it. I just have this vague feeling of unrest.

The issue has nothing to do with altitude Roak...
It has to do with the compressibility of the gases...
I would leave the O2 as is but add more He...

If you had a He analyzer you would find that your FHE in the mix would come out low if you only figure things by straight ideal gas equations...

Shoot where is Omar... he's the smart guy when it comes to gases and compressibility...

 

[MikeFerrara]

roakey,
I wasn't suggesting that you change your mix. Although I think UP was dropping a hint. I was suggesting a little extra He to compensate for compressability (real gas laws) so that you actually end up with the intended mix. but I looked back through the posts and seen you were already planning on overshooting the He. I should have paid more attention.

Mike

 

[roakey]

Originally posted by Uncle Pug

The issue has nothing to do with altitude Roak...
It has to do with the compressibility of the gases...
I would leave the O2 as is but add more He...

Shoot where is Omar... he's the smart guy when it comes to gases and compressibility...

Oh, you're talking about the compressibility, I thought you were recommending a higher % of He. Yhea, the compressibility factor I understand, and this fill will be the first data point on the learning curve. Which is a nice way of saying "I'll probably be off, but I'll know more when I'm done"

Where's omar? NetDoc has stooped to telling bald-faced lies about DIR, omar and myself in http://www.scubaboard.com/t9606/s.html and omar's desperately trying to right the damage done there.

Roak

 

[roakey]

Originally posted by MikeFerrara
but I looked back through the posts and seen you were already planning on overshooting the He. I should have paid more attention.

More credit than is due. I was planning on overshooting, but I didn't state so.

Anyone got a fuzzy for overshooting at thermal equilibrium? 5%? 10%? More?

I gotta break out the DIR II tape, I think George talks about this on the tape.

Roak

 

[MikeFerrara]

I too am still perfecting my technique. I think GI recomends adding like 17% He. Part to compensate for compressability and part for temp. Some recomend adding a little He and reducing the O2 a little. I just finished mixing two batches of 20/30. Would have went higher on the He but I was finishing off a bottle. I added about 7% extra He (cool) and added about 7% less O2 (7% of the additional O2 not 7% of the total). Why 7% and not 5% or 10%? 7 seemed like a lucky number. The Mix came out to 19.7% O2 in one set of doubles and 20% on the nose in the other. I don't think one needs to get real anal about it just put in a little extra He. As long as you get what you want on the O2. When I mix I usually have the time to analize the mix after the He and O2 are in. This is probably not required but gives a sanity check and makes me feel better. Don't want to have to dump mix. I keep a log of my fills and adjustments for reference.

I wish I could get He for the price UP does with my current supplier I pay $79 for a T bottle. The last supplier charged $107.

 

[roakey]

Hadn't heard anything from the gas supplier by 3pm so I called them -- Are you sure you're delivering my order? If there's a problem tell me by 3:30 and I'll DRIVE up there and get it!

No problem, driver's got your order, he'll be there soon.

At 4pm I get a call -- "We just found out the driver swapped Tuesday and Wednesday, is delivery tomorrow OK?" He mumbled someting about a ton of irate customers from the 'springs calling the shop asking where their orders were until they finally got ahold of the driver who THEN told them (like at 4pm) that he had swapped delivery dates.

The gas shop's an hour and fifteen minutes from here and they told me this an hour before closing. Thanks a bunch, guys.

Problem is, the dive shop's cascade which is on a trailer (for on-site filling) leaves the shop Wednesday afternoon.

Cancelled the order, cancelled the trip, am currently in a deep funk.

Roak

 

[Uncle Pug]

Originally posted by roakey
Cancelled the order, cancelled the trip, am currently in a deep funk.

There is a reason for everything and deep in a funk is not a worse case scenario...

 

[omar]

Roak,

you can find a real and ideal gas mixing spread sheet (excel) that also has an equation for altitude PPO2 at:

http://www.atomox.com/mix_calc.html

Click on the <Here> the image is just an image of the spreadsheet.

I do dives on a regular basis at 5000 to 7000 feet with no problems. I do take longer between dives and make sure to limit exertion when I first get out. Sunday we were at 5298 MSL, the water temp was 38 degF, BUT we had the whole lake to ourselves! I do typicaly have a higher helium content for my dives eg 20/40 to 165'; 17/50 to 200'; 15/55 to 235. Haven't gone deeper than this.

When I dive at altitude I set decoplanner or gap for the appropriate altitude and use the profile that it generates. I do add deep stops and spend a little longer in the middle range to get the tissue gas PP down. I go by the graph and output of decoplanner to do this. I also run a scenario at sea level to get a feel of what is occuring at elevation.

I used to get all wound up about altitude diving, cross corrections etc. until a guy pointed out that maybe I was experiencing math narcosis. I have since stepped back from all of the math and use a little more experience to do the planning.

omar

 

[omar]

How do you correct for temperature???

you need to know how hot it gets which is a function of the fill rate and heat exchange between the tank and the atmosphere. You are just shooting in the dark by trying to nail this down to a consistent "fudge factor"

I fill my tanks primarily by continuous blending to get around this. When PP blending I fill, let the tank sit for an hour or more and check the pressure and top to the desired P. Let sit and then add the next gas, let sit, fill to desired pressure and let sit, than top with air.

It takes time to do it right.

omar