Nitrox stick flow meter rate?

[diverman2013]

I have a 5cfm Bauer capitano compressor. I've built a nitrox stick going by the oxyhacker book. I'm ready to buy my flow meter to mount on the oxygen tank. My question is will a medical grade 15lpm flow meter give me adequate flow to achieve 32% nitrox? I've looked back at the book several times and can't seem to find an answer. Thanks

 

[tbone1004]

You have a 5cfm compressor, to get EAN32, 14% of the volume has to be oxygen. 5*.14=.7cfm. .7cfm is about 20 liters. Realistically it will be pulling about 16lpm so if you slow down your compressor a bit you can probably get away with a 15lpm flow meter but it will be all the way open

 

[diverman2013]

Thanks for the info. Next question. I can put a standard welding reg on the tank and feed a flow meter. Where are y'all getting 20-25lpm flow meters?

 

[tbone1004]

a lot of the big fill stations use O2 solenoids, but 25lpm flow meters are available on Amazon or any of the major supply companies. grainger etc.

 

[Brandon]

I use the Dwyer VF series flowmeters (VFB-67-BV-PF-SPCL-VIT). $95 for an oxygen clean, viton o-ring version, and they make them in a bunch of different sizes. Supply it with a welding regulator and you can flow as much oxygen as you want with a great deal of control: Series VFA & VFB Visi-Float® Flowmeters Product Configurator | Dwyer Instruments

Keep in mind that flowmeters are calibrated assuming that you're flowing into the atmosphere and the displayed LPM is only ballpark. You would need to be able to measure the output temperature and the actual pressure to get an exact figure. The bigger challenge to accuracy is that the compressor's input isn't flat and you'll need to adjust the flow slightly over the course of a fill to maintain your desired nitrox percentage. Or you can "set it and forget it", depending on how much your care about your fills hitting your desired nitrox percentage. I aim for within 2/10 of a percent because I geek out on the whole process. Many (including agency standards) are quite content if the final blend is within 1-2%.

Anyhoo...

At 5 CFM input, you need roughly 15 LPM for 32% and 27 LPM for 40%.
At 3.5 CFM input, you need roughly 11 LPM for 32% and 19 LPM for 40%.

Dywer makes flowmeters with 3-25 LPM scales, 6-50 LPM scales and 10-100 LPM scales. Most of us garage guys are going to want the 3-25 LPM version.

You can also order the Dwyer flowmeters with an arbitrary scale and not worry about the LPM display. It's more important that the bore is correctly sized to your compressor input. If you tried to use the 10-100 LPM flowmeter on a 5 CFM compressor, you'd have use a combination of low input pressure and low displayed LPM to get your desired 15 LPM flow rate to produce 32%. Fine adjustment would be very difficult, and your fine control would come almost entirely from the regulator pressure instead of the flowmeter. Better to use a 3-25LPM flow meter and gain a bit more accuracy.

On my compressor, 25 PSI on the welding regulator = 32% at 11 LPM displayed, and 40% at 20 LPM displayed. Pretty close to the expected flowrates, but more importantly, in the middle of the flowmeter's range and the welding regulator's most stable output pressures (15-40 PSI, per manufacturer). Adjustments are easy.

Have fun and don't blow yourself up.

-B

 

[diverman2013]

Thanks for the great info.

 

[BRT]

Remember that the flowmeters are calibrated for a specific pressure. Mine is 50 psi. As the tank empties my pressure goes up slightly which raises the O2%. I fed one HP regulator into another outputting 100 psi from the first and 50 psi from the second which holds things a lot closer. I still will have to adjust the second one slightly from full to empty,

 

[clownfishsydney]

You do not need a flowmeter, an oxygen welding regulator can be used to control the flow. You will need to make the orifice smaller so that you can use the outgoing pressure to control the flow. See my web site article on this Michael McFadyen's Scuba Diving Web Site

 

[RayfromTX]

The flow meter sure makes it easier. Mine is a max 3 cfm and it allows me to make 32% when the pressure is down to less than 10psi in the tank. Using it at 25 psi and adjusted to around 1/2 cfm, I do not have to adjust the dial much or at all from a full to an empty tank. a reading of 33% at the stick gives me a 32% final mix =/- .3%

 

[diverman2013]

Yes I'm going with a flow meter. I've now got me a welding oxygen regulator and a 0-25lpm flow meter on the way. Thanks guys