Question Continuous blending O2, do I really need to 'fly' the O2 valve?

[JohnN]

After some fussing about, I finally started to fill my tanks with Nitrox. The compressor is a RIX-SA3 and I'm having to continually adjust the O2 needle valve to hold things close to my desired 32% O2.

My O2 regulator only provides a PSI gauge on the inlet to the blending stick, and when O2 is flowing it only shows 0 PSI (not a surprise since there isn't any backpressure on the compressor intake). I have an Argon/Nitrogen valve on my MIG welder that I can use to control the volume of gas (cu ft/min) but I've not been able to find an O2 rated regulator that does this.

Is this just the way things are, or should I keep looking for a better O2 regulator?

Many thanks!!

 

[rjack321]

You will have to turn down the o2 as the O2 pressure goes up, the blowby is increasing as pressure rises so your output is declining.

I suspect with an SA-3 you need a much smaller regulator in the medical size range (max ~20Lmin). That will provide you with a much more stable flow rate and each touch of the dial won't be such a big jump. You can generally take the CGA 870 pin adapter off a medical valve and put on a CGA 540 nipple. They're both 1/4"NPT

 

[grf88]

Something like this will help:

CNBTR Multicolor Acrylic 3-30LPM LZQ-7 Oxygen Air Gas Flowmeter with Control Valve for Oxygen Adjustable​

It will save you from trying to chase your O2 meter.
I also was able to find a balanced O2 regulator but the flowmeter will make the biggest change. With the SA3 you could probably use a lesser capacity one than the 30LM I use on my SA6
Was unable to post the Amazon link, just look it up.

 

[rob.mwpropane]

I am in the process of getting where you are. I have 2 O2 regulators running in series and then a liter meter to dial it in.

I think @Tracy gave me recommendations of what psi to set each at.

 

[Tracy]

You need a double regulator to maintain consistent output from your oxygen tank. You can buy them but unless you find a used one on ebay, they are typically prohibitively expensive. You can make your own by putting a second regulator inline with your first regulator and then a needle valve for control.

I think I have my first reg set to 90 psi, the second goes from 90 to 15 psi, and then the needle valve controls the flow.

This won't fix the issue of the compressor slowing down as pressures increase, but it will give you a much more stable output for longer.

 

[Curious_George]

Why are two regulators needed; wondering what I'm doing wrong?

I use a single Victor regulator set at ~18psi and a 0-30 SCFH Dwyer that seem to work fine. Typically adjust the flow once or sometimes twice from about 21 to 18 over the course of filling a tank, after I pass the PMV pressure level. Setup has over 40-50 hours of filling Nitrox.

Regulator may have been for argon, I don’t remember. It was rebuilt and O2 cleaned as much as possible. I occasionally check it as the O2 tank runs down but rarely have to do anything to keep it rock solid on 18psi.

I have a precursor to the Bauer Junior, called Utilus 10 at a little under 3cfm.

P.S. I don’t ever close either of the butterfly valves. I heard its bad to flow 100% O2 into a closed valve.

 

[Tracy]

Why are two regulators needed; wondering what I'm doing wrong?

I use a single Victor regulator set at ~18psi and a 0-30 SCFH Dwyer that seem to work fine. Typically adjust the flow once or sometimes twice from about 21 to 18 over the course of filling a tank, after I pass the PMV pressure level. Setup has over 40-50 hours of filling Nitrox.

Regulator may have been for argon, I don’t remember. It was rebuilt and O2 cleaned as much as possible. I occasionally check it as the O2 tank runs down but rarely have to do anything to keep it rock solid on 18psi.

I have a precursor to the Bauer Junior, called Utilus 10 at a little under 3cfm.

Two regulators give you better consistency over the course of the fill.
Running a 3cfm compressor, you are most likely filling scuba tanks and not going through any real volume of oxygen. What you are doing will work just fine, you just have to tweak it more often.
When you are filling banks and running a large compressor, you will go through multiple bottles of oxygen in the course of one run cycle. Most cheap welding regulators will swing 5-7 psi across the working range of the tank. A standard welding regulator is an unbalanced reg, the output pressure will rise as the inlet pressure drops. A second regulator inline is a cheap way to build a balanced regulator and have a stable output without spending the money for a factory double ended regulator.

 

[tbone1004]

@Curious_George the second regulator is effectively creating a 2-stage regulator, not dissimilar to what we have with scuba regulators. Getting the balancing right is really hard with a single stage regulator. If you think about it, if we set it to say 10psi for even math, we are asking it to output anywhere from 0.5% to 10% of the inlet pressure as it goes from 2000psi down to 100psi. That's a really wide spread to ask for stability from a mechanical device. The second stage allows for an intermediate pressure drop to say 100psi and then the final outlet pressure becomes much more stable. Same reason that single stage scuba regulators fell away 50 years ago, the 2-stage design provides for much better breathing stability as the tank drains.

 

[Curious_George]

Got it. Thanks @tbone1004 for chiming in with @Tracy’s answer. My mistake was assuming that earlier posters were filling a couple tanks at a time. I laughed when Tracy mentioned using multiple bottles of O2 during a filling session. I get months out of a bottle - haha. And just a little tweaking of the regulator at the very end of the O2 bottle.

On an unrelated note, did you ever finish your DPV smart controller? I’ve finished the programming for mine and waiting on hardware to arrive. It will convert my Mako to smart controller and cost under $100 all in. Send me a DM if you want to discuss.

 

[Doc Harry]

I had a needle valve between the two-stage oxygen regulator and the Nitrox stick. I turned the O2 reg to about 30 psi to keep a head on the needle valve, and then adjusted the flow to the stick using the needle valve. Set it and leave it. I could fill my 3 bank cylinders and the stick O2 analyzer varied during the fill by only +/- 0.2%. Good enough for me.

The other question is: Are you sure your Nitrox stick is mixing adequately? Perhaps the fluctuations are not from the O2 regulator flow but from uneven mixing of air and O2 in the stick.