I'm using a good quality O2 welding regulator and a cheap 2-20LPM Chinese flowmeter on my nitrox stick. On my O2 welding regulator I have it set to something low like ~30-40psi. I also have ~50psi OPV here so I don't accidently run higher pressure through my flowmeter. I have a Divesoft analyzer on the output side of my compressor which I compare to the analyzer on my nitrox stick. I found a cheap Maxtec O2 medical analyzer on fleeBay for like 40 bucks. I cut the connector off and soldered another one on it. That's what I am using at the bottom of my nitrox stick before it goes into my compressor intake.
For my mixing tube/chamber I'm using a clear static mixer (CLR-200-2ST-MM40 clear 2" MPT ends length 20" from
TAH Bell Static Mixers - StatiFlow Clear Mixers It's kind of overkill for this application I just bought it because I honestly like how it looks.
I also own a Dwyer flowmeter (RMB-52D-SSV Flowmeter, range 4-50 SCFH/2-23 LPM air) which is much higher quality but my little Chinese one is working fine so I haven't replaced it yet. I have several friends who had the Chinese one but weren't happy with the accuracy so they switched to the Dwyer one. If I were doing it all over again I would go with the Dwyer flowmeter. Once my Chinese one starts giving me trouble I will replace it.
Dwyer RMB-52D-SSV Flow Meter, 5" Scale, 4-50 SCFH/2-23 LPM Air, SS Valve
It's all pretty accurate for me. I'm using a Alkin W31 (~3.6cfm / 100lpm) compressor. I never really mix anything other than 32% so rarely have to adjust things too much. For example, the math for 32% is 14.03% of the gas has to be pure O2 into the nitrox stick.
3.6cfm x .1403 = 0.50508cfm
50508cfm = 14.30227308276lpm so you can see where I got 14lpm a minute from on flowmeter.
Coincidentally, this is exactly what my cheap Chinese flowmeter reads when pumping 32% on my Alkin.
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The biggest changes I made was I replaced the clamp-on cold air intake filter with a
solberg filter silencer on the top of my Nitrox stick and I added a solenoid on my welding regulator to shut off flow of O2 when compressor is not running that is wired into my accessory port on my motor starter.
EDIT: Here is hopefully a clearer picture of the nitrox stick. There are obviously many different variations of this design but mine has been working well with no real issues. Instead of using a t-piece at the bottom. I just used an M16 x 1mm tap directly into the PVC. The O2 sensor screws into place and seals with an o-ring on sensor itself. I also ended up adding a longer section of PVC to the top to that O2 was injected further into the nitrox stick. Honestly I'm not sure if it was needed since I never noticed any issue before. My compressor has a 1" intake so I am short section of 1" hose that goes directly into my nitrox stick with a 2" to 1" reducer to barb fitting on the bottom. I kept this section as small as possible to minimize resistance. It's about ~32" (81cm) from my compressor intake to the nitrox stick since my compressor sits right next to it.
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