How We Partial Pressure Blending Nitrox

[@kevindavison6019]

Nice setup with the booster. I use a stick when I need nitrox at home but use it very rarely. I dive mostly nitrox on vacation because it offers advantages there, but I rarely do repetative or deep dives at home. The only problem I see as a shop is that you trust the customer to only use nitrox or OCA in that cylinder if they have it filled somewhere else. If you Continuous blend you have fewer worries about contamination.

[@LakeHickoryScuba]

Hello @kevindavison6019, you make a very good point about trust. We have a future video planned where we discuss the trust between the customer and the dive shop. A lot of divers don't comprehend that the shops give out more trust than the customer does. You are very correct on the continuous blending as well. Most shops today, ours included fill with O2 compatible air, or some call it oxygen cleaned air, but its always good to ask the fill operator before having your cylinder topped off.

[@chrisonash]

Great video by the way!

[@LakeHickoryScuba]

Glad you liked the video @chrisonash.

[@MikeDodds]

So, do you have to shake the tank up after you fill it to mix it up really good?!?!?! 😜

[@LakeHickoryScuba]

Hello MikeDodds, in the words of Ricky Bobby, "SHAKE AND BAKE." And for the record, I have never shaking or rolled a bottle to get the mixture of gas I wanted. Now there was this one time where I unscrewed the valve and poured out the gas mixture into a electric mixing bowl from my kitchen, and stirred it until it was well blended, then I put the gas back into cylinder and screwed the valve back on, and it tasted perfect while diving. My wife thought I was crazy though. LOL! Sorry, this question was too good not to poke fun at. NO, NO, NO, you do not have to shake or roll a bottle to get the gases to mix. In my 36 years of diving, I have no clue where that myth even started.

[@DavidMaruca_]

Without a calculator: (11/79) * 3000psi (or substitute your own tank’s working pressure) 11/79 is found out by 32%-21% (how much you are enriching) and 100%-21% (the range of concentration) Fun maths!

[@chrisonash]

That formula, (11/79)*3000 equals 417.72, not 402. Why did the computer app spit out 402?

[@LakeHickoryScuba]

That is one of the great things about diving @DavidMaruca_, it makes the math more fun.

[@chrisonash]

I’m confused about the partial pressure of O2 that was put into the empty tank. The computer calc called for 402 psi of O2 to get a final mix of 32% O2 (960 psi ppO2 for a 3000 psi fill). My calculations (I tried basic algebra and I also used the partial pressure formula from Bryan’s “Partial Pressure Formula Explained” video, kzbin.info/www/bejne/jpzNZqupgrRoosk) yield a pure O2 fill of 417.72 psi. Can anyone explain the discrepancy to me? Thanks.

[@LakeHickoryScuba]

Hello @chrisonash, we get this question all the time from our Gas Blender students. What really throws our students for a loop is when we exceed the O2 partial pressure even by 50psi, and the mix still comes out perfect (you will see this in a future video, hint, hint). In our video Partial Pressure Formula Explained (kzbin.info/www/bejne/pn69g5drn7B5q68), we are calculating how much O2 we need to mix to 32% from an empty cylinder. The computer program is doing the same thing, however, it is assuming a perfect system where the cylinder is at a constant temperature. We understand through Charles's Law, temperature can effect pressure, thus, effecting the mix as well. The fill rate alone can effect the final mix too. Mixing gas is an art form that takes practice. Thankfully, we have a small margin of error as no two fill stations are the same. Ambient Temperature, Bottle Temperature, Digital Gauge vs Analog Gauge, etc. etc., can all effect the final mix. Thankfully, we have a margin of error where things will still calculate out. Hopefully this answers your question.

[@chrisonash]

Thanks for the response. I agree with most everything that you have said. What I’m trying to figure out though is why there is a disconnect between the partial pressure calculation formula from your previous video and the computer-based calculation. If the assumptions are the same between both the computer generated partial pressures and the hand calc partial pressures (based on the partial pressure formula from your previous video) the O2 percentages will be different. For a ratio of partial pressures of 402psi ppO2 and 2598psi of air the final mix would yield 31.6% O2. The computer and the partial pressure formula don’t yield the same results and I’m wondering what else is baked into the computer calcs. One way to account for the difference would be that the computer assumes a temperature rise great enough that the “hot” pressure of the tank is 3000psi and when cool it would reduce to 2900psi. That would account for the difference. The calculation would be 402psi + .21*2498psi = 926.6psi pp02. 926psi ppO2/2900psi = 31.95% O2.

[@LakeHickoryScuba]

@@chrisonash The end result of 31.95% is 32% Nitrox. 32% Nitrox is a range between 31.5% and 32.4%.