How We Partial Pressure Blending Nitrox

[@chrisonash]

Great video by the way!

[@LakeHickoryScuba]

Glad you liked the video @chrisonash.

[@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.

[@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.

[@Evan-gj3gw]

Is it fair to say that the valve is no longer 100% oxygen clean once it comes in contact with the air fill whip that has been in contact with every tank that's previously been filled since the whips last cleaning? And that as a fill station unless you just cleaned the tank/valve it is not 100% oxygen cleaned and will contaminate your oxygen service equipment. It seems at the 40% and below level its more of importance that the design and material of dive equipment are oxygen compatible than being oxygen cleaned.

[@LakeHickoryScuba]

Hello @Evan-gj3gw, this is a tricky answer. The short answer is, it doesn't take much to contaminate an oxygen cleaned cylinder. My official response would be, dive shops must abide by the rules set forth by their insurance company. The compressed gas association has regulations set forth that dictate what oxygen cleaned cylinders are as well. I always ask this question to my Gas Blender students, but I don't ask it for a response, but more so for them to think about it over time. If you O2 clean a cylinder in a non sterile environment, and then use non O2 cleaned atmospheric air to dry the cylinder, is it truly O2 cleaned. Though we no longer recommend the 40% rule as an indicator of what needs to be O2 cleaned, a lot of shops will still fill pre-blended Nitrox into an non O2 cleaned cylinder. I will leave you with this, there are things I will do to my personal cylinders that I won't do with a customers. Maybe that will help answer your question. We actually have a video dropping on January 3rd that goes over O2 Cleaning and why we do it. You may find it interesting.

[@Evan-gj3gw]

@LakeHickoryScuba I look forward to the new video. At nitrox fills above 40%, I would require an initial 100% 02 cleaning. Seems like most shops drain tanks completely for tec fills anyway, so a quick visual can ensure no contamination in the tank. With the valve off, it would be easier to do a light clean and degrease before filling. If there is any greater contamination than what a light preparation can solve, a full 02 cleaning should be recommended. The increased cost for a nitrox fill probably doesn't come down to the added cost of oxygen as much as it should be the extra care and handling. Pre banked nitrox fills would probably be just fine with an annual 02 cleaning so long as the owner knows the tank hasn't been contaminated. Is it fair to say now days the quality standard for fills from dive shops would make it rare for a sport nitrox tank to become contaminated by filling at any dive shop? The only way I could imagine a tank could really become contaminated would be to fill at a firehouse or SCBA station.

[@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%.

[@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.