Nitrox stick O2 sensor placement

[tmassey]

Is everyone upsizing because you’re adding more mixing media?

No, I would be willing to put real money behind the opinion that most people are upsizing because “bigger is better“ or fear, uncertainty and doubt. And if you only need a few feet of pipe, who the heck cares? It’s like pennies per dollar that you already spent on the compressor, so why not?

Of course, that’s the same logic that gave us deep stops and gradient factors like 20/70…

(Now, why did I rebuild my trimix stick recently? To make the mixing section longer: my oxygen was mixing well, but I wasn’t certain my helium was, so I went ahead and rebuilt both sections, making the mixing section of both longer. Why? Fear, uncertainty and doubt. But at least it seems to be mixing very well now. Even if 24 inches of mixing media for each gas is ridiculous, it doesn’t seem to be causing a problem for my compressor, and like I said, it was cheap, so I did it. )

Like I said previously, I’m measuring 4 1/2 or so inches of water column vacuum pressure at the end of my stick. I don’t think I mentioned that that is including the resistance of not only my 6-foot trimix stick but also another 20 or so feet of pipe, with three 90° elbows. There’s 27 or so inches of water column per each PSI. So that’s roughly 1/6 of a PSI, or approximately one percent of atmospheric pressure. I’m not sure the first stage of my compressor, which is designed to generate 80 psi output, is even going to notice 1/6 of a psi.

So I’m not actually worried about it. Otherwise, I’d be working a lot harder to find an answer than merely throwing a “dear lazyweb“ out to scuba board. But if somebody brought it up in conversation, I thought I’d see if I can get some more detail. @tbone1004 doesn’t seem interested in our conversation, so I don’t know if I’ll get a more specific answer… (Or heck, maybe he actually has a non-Scuba life: I doubt it, but it’s possible… )

But apologies to the OP of this thread. You were asking about flow across an oxygen sensor. In my defense, I did answer your specific question, and it was somebody else who brought up the (related!) back pressure that I latched onto… But it’s your thread. I’ve done enough derailing. If anybody has any more specific information, though, I’d love to hear it.

 

[tbone1004]

yeah yeah, sometimes I have to actually work for a living, it's horribly overrated

@tmassey the question is really what the manifold pressure is at the actual intake, but you are correct in thinking that a few inches of water is perfectly adequate. With the smaller <5cfm pumps I have seen several where the 1/2" intake that was supposed to be a few feet long all of a sudden turned into a 30ft monster with hard elbows and nitrox sticks packed with whiffle balls and whatever else and had a vacuum more similar to a car which is a very different situation to be in and you see that with low output pressure from the first stage since you're effectively not filling the piston all the way by the time it starts to come back up. Definitely not something to "over think" and oversizing certainly has downfalls but for the stick portion itself it is important to not cause a restriction in gas flow and that's where going up a size or two isn't a bad thing, most especially when using whiffle balls to mix, much less of a concern if you have a KoFlo or DIY variants.

 

[TravisD]

I have to admit, a big part of the reason my stick uses 4" pipe is because that's what my plumber friend, who happens to be a diver (I don't know if he does underwater plumbing, just hull cleaning) brought me when I asked if he had some PVC I could buy. He was nice enough to bring some 4" reducers and such too. It worked out well since I could use some 4" drop-in PVC drains to contain the bio balls (with a little sanding the drains fit inside the reducer fittings nicely).

 

[Diver-Drex]

Simple T mixers work very well with low viscosity fluids, like air. As long as velocity is high the mixing is happening. The larger the diameter the mixing chamber, the lower the velocity for a given cfm, hence the need for baffles. More baffles means more restriction…

2” pipe, minimal baffles to keep velocity high and restriction low. Or larger diameter pipe, lower velocity, creating turbulence, and restriction, through baffles. Either method has been proven to work.