Why you should build a Nitrox stick..

I’ve helped a bunch of people build simple nitrox sticks in the past few years so I wanted to do a blog post about it. I really don’t understand why most dive shops don’t use nitrox sticks. I mean I do but I’ll get into that later.

Nitrox sticks (continuous gas blending) are not exactly anything revolutionary nor are they new technology. I learned about them in Van Harlow’s Oxygen Hacker Guide which I think was first published in the mid-1990s. That’s at least 25-30 years ago. I’m pretty sure people were building them earlier than that too. This was also in the era where PADI and other training agencies disavowed anyone who taught Nitrox. The devil’s gas! Voodoo gas! You’ll die!

DEMA (Diving Equipment & Marketing Association) even banned nitrox training providers in 1991. My my how things have certainly changed. Now you can do $99 basic Nitrox eLearning course online and you never even have to get in the water. What the fuck? I’m serious.. It’s literally the most popular course taken and offered after basic open water.

Sadly the four closest dive shops near my home are strictly air-only. They don’t even blend nitrox to begin with so it’s a moot point. Honestly if either of the shops closest to my house offered banked nitrox I would probably go in there weekly as an excuse to buy things. It absolutely astonishes me but that’s the reality of gas fills in New England. The irony is the majority of New England dive sites via boats would be much safer using banked 32%.

I would get into another rant about how most dive in New England run on archaic business models and are stuck in the 1990s but it would be lost on most people. There are actually a few shops that do properly but that involves me sitting in over an hour of traffic each way and still having to go into a dive shop. Why should I do that when I never have to leave my house?

Why Partial Pressure Blending is Less Efficient
Before I explain how to build a nitrox stick let me rant a little bit more..

Most dive shops can’t afford or don’t want to spend the money on a membrane systems. They don’t see the return on investment. That leaves two other options for making nitrox – partial pressure filling or a nitrox stick. A lot of people are sold the lie that partial pressure blending is the easiest way to blend nitrox. It’s bullshit. I highly disagree and I will literally argue this to everyone.

-Partial pressure blending is FAR more dangerous because you’re literally introducing pure oxygen to tanks that you have no idea the O2 cleanliness. Are you sure you REALLY trust the O2 clean sticker for Joe Blow’s Dive Shop in bumfuck nowhere?

-Risk of adiabatic heating occurring in the valve. This can be very dangerous with pure O2. See points above. Ever see a valve damaged from an O2 fire?

-I can bet you Joe Blow’s highly trained $9/hr employee or intern put huge globs of silicone grease on the valve. There was (actually still is..) a local dive shop in the area whose employees would punch out that they did an Eddy Current test on every visual inspection sticker, even steel tanks. The problem is you don’t do that test on steel cylinders. Why would I trust them to do a proper O2 cleaning then they blindly stamp “eddy current test” on their visual stickers when it’s fucking obvious they didn’t do one. Ok let’s keep going..

-You’re adding pure oxygen via scuba valve (the majority which are rarely certified for use with pure O2). This is another little “dirty secret” in the dive industry. No scuba valves are designed or certified for pure O2. Scuba valves have far too many sharp bends and often use materials (o-rings/ nylon seats) that are not O2 compatible.

-In addition to your tanks needing to be O2 clean, all your whips, DIN/yoke fittings, valves should also be O2-clean. Basically everything on your fill panel needs to be O2 clean. Did I mention everything should be O2 clean?

-You need to make sure you’re adding the right amount/pressure of O2 and account for how much gas you need to add when you top off with air. This introduces many variables and ways to make errors. Partial pressure blending (with inexperienced hands or when someone rushes or is not paying attention) can be very inaccurate. Obviously experienced gas blenders can do this very easily and accurately but let’s think about this in terms of our $9/hr dive shop employee.

-Say you want to make 32% in a high pressure tank (call it ~3500psi). Most people know you need to add ~480psi of pure O2 and top off with air. What happens if you add too much O2 or too little O2? What happens if your air bank can’t do 3500psi or you fill too quickly and it cools down. Answer: Your mix is fucked up. You then need to spend time fixing it either by topping off with air or boosting high pressure O2 on top.. If you are a busy dive shop and trying to do this to multiple tanks, good luck. It’s an incredible waste of time.

-This brings me to another point. Unless you want to return O2 bottles with leftover gas you’ll probably want a booster. Once a T bottle gets below ~400psi it’s fairly useless for partial pressure blending. Sure you can cascade but that requires multiple O2 bottles to do efficiently and takes more time. You’re still going to return T bottles with unused gas. You’re wasting gas and you’re wasting money. Get ready to shell out $7000-$10,000 for a Haskel booster. Oh yeah, you also need to regularly service this and make sure it stays O2 clean. Now I should point out that most “tech friendly” dive shops will invariably own a booster for deco gases and trimix blending.

-Another reason. You need to typically drain any leftover mix in your tanks or re-do the math for blending and/or use a booster. It’s fucking stupid.

I could go on and on and on about why partial pressure blending is bullshit but people are fed the lie that “all you need is an O2 bottle and a transfill whip.” Sure ok..

Nitrox Stick
Enter the lowly nitrox stick which you can build for under $150. Actually you can build one for less if you already own a nitrox analyzer. Alternatively you can buy a commercial nitrox stick offered by many vendors for a few thousand bucks.

A nitrox stick is magical and mystical device that allows you to “dial-in” the mix you want and feed that mix into your compressor’s air intake. It’s magic! Ok I lied but it’s pretty simple. I’m sure I will explain it poorly but I will try.

A nitrox stick a basically a mixing chamber where a small amount of oxygen is added and mixed with air which is at ambient pressure. Inside the nitrox stick there are several baffles (wiffle balls, bio balls) that allow thorough mixing of the oxygen and air inside this chamber. You need just enough agitation that allows for a homogenous mix but not anything that will severely restrict the flow of gas.

Before running any O2 into the nitrox stick, do a “dry run” with your nitrox stick connected to your compressor intake and O2 analyzer on and sampling the gas flow.  Your O2 sensor should remain stable at 21% or close to it. If it starts trending down or jumpy (e.g, dropping to 19%) you have a vacuum in the air intake hose/stick which means there is too much resistance. Shorten intake hose or remove baffles / widen PVC section. 

The top of the nitrox stick is typically open to allow normal air to flow inside. Usually you stick a filter on top to prevent things like bugs, leaves or dirt from getting inside the nitrox stick. I started with cheap clamp-on cold air intake filter but I replaced this with Solberg inlet silencer filter.

After this typically O2 is added somewhere at the top of the nitrox stick fed by an oxygen regulator. This oxygen welding regulator is either hooked up to a needle valve or adjustable flowmeter that allows you to adjust the flowrate of oxygen into the chamber (nitrox stick). Many will argue a needle valve is really all you need but I like using a flow meter so I can see how much oxygen is going into the nitrox stick. There is actually math involved here (I’ll get to that shortly) but to dumb it done even further the more oxygen that flows the higher your nitrox mix will be. You simply “dial-in” the correct mix by watching the nitrox analyzer at the end of your nitrox stick.

The middle section is a mixing chamber that allows the oxygen and ambient air to mix. There are many different ways to do this. I am using a static mixing chamber. Mainly because it looks cool. 🙂 https://en.wikipedia.org/wiki/Static_mixer

Finally at the bottom of your nitrox stick (hopefully after all the mixing has occurred) there is a nitrox analyzer that reads the Nitrox mix (partial pressure of oxygen) inside your nitrox blending stick. This part of the nitrox stick is then attached to your compressor’s air intake hose.

What this means is that if you have 32% nitrox flowing out of your nitrox stick and there are no leaks downstream before your compressor air intake then this is same mix that will flow into the air intake of your compressor, pass through your compressor, get compressed (I’m really dumbing this down..), pass through your air filter stacks and eventually show up at your scuba fill whips. Viola! Nitrox on-tap. Dial-a-mix!

The real beauty here is the only thing that really needs to be O2 clean is your oxygen regulator. These are typically welding regulators, you know DESIGNED for oxygen. All the mixing is happening at ambient pressure before it goes into your air intake. It’s (hopefully) pre-mixed BEFORE it enters your compressor. This means that your tanks, scuba whips, scuba valves never see pure O2. The only things that sees pure O2 are your O2 welding regulator and your needle valve or adjustable flow meter.

Now there are some caveats and concerns here. First one and it’s a BIG one that worries most people.

You do not ever want to pump more than 40% of oxygen into your compressor intake. This is the arbitrary but prudent standard that most compressors that are “certified” for nitrox blending say you should limit yourself to. The risk here is very real. You do not want to accidently pump high percentages of oxygen or pure oxygen directly into your compressor. Your compressor could explode and/or could be severely damaged. This scares a lot of people off. While the risk here is real there perhaps thousands of people and dive shops that pump nitrox through their compressors every day, especially in places like north Florida (Cave Country). I believe the benefit outweighs the risk involved but there some people that vehemently disagree.

A lot of commercial nitrox stick solutions have a solenoid that will automatically turn the compressor off if the oxygen percentage exceeds a certain valve. For a lot of home blenders we simply watch the nitrox analyzer like a hawk and make sure the oxygen percentage doesn’t exceed 40%. You should NEVER walk away from a nitrox stick when the compressor is running! In practice a lot of people with smaller compressors still need to keep an eye on pressure and manually open condensation drains but I typically do not walk away from my compressor when the nitrox stick is running.

I typically limit my mix to 32% but will occasionally do 36%. Honestly 99% of the time I only mix 32%. I might as well never touch the oxygen flow rate going into my nitrox stick. If I need to make 28% or 30% I can simply lower the flow rate of oxygen.

I also have a couple other “safety” features built into my nitrox stick. I have an over-pressure valve (OPV) set to 40psi right before my flow meter. This ensures that if the IP were to creep on my oxygen welding regulator it can never send more than 40psi of oxygen into my adjustable flow meter. I typically leave my welding regulator set to something like 30psi. We’re talking low pressure here. Remember it’s the flow rate of oxygen that’s actually important. Pressure != Volume.

I also have a shutoff solenoid tied to my compressor motor starter. If my compressor were to accidently shut off the solenoid that feeds oxygen into my compressor is automatically closed. This is to ensure that pure oxygen is not flowing into my nitrox stick without my compressor running. What you don’t want a slug of pure oxygen going into your compressor intake on startup.

Some other benefits of a nitrox stick..
Your mixes are incredibly consistent provided your analyzer is calibrated correctly and there are no downstream leaks. I’m never more than .5 off my mixes.

You can drain an O2 bottle down to nothing. You don’t need a booster and you don’t need to cascade O2 bottles. You basically get to use all the O2 that you paid for. I never return T bottles with any gas left in them.

None of my scuba tanks, whips, hoses, valves need to be O2 clean because nothing in this system ever sees more than 40% O2. This saves time and money.

I can make mixes without any sort of math or worrying about having to re-top off with air or boost high pressure O2 on top of an existing mix. This also saves you time.

Mike’s Ghetto DIY Nitrox Stick

Here’s an incredibly basic Nitrox stick design. I can’t take credit for it as I simply borrowed some ideas from other people who have been doing this far longer than me. It’s nothing revolutionary.

The one difference is I am using a static mixing tube for the mixing chamber. It takes the place of various baffles or wiffle balls that people use to get a homogeneous mix in their nitrox stick. It’s a clear static mixing tube from TAH Bell Static Mixers – StatiFlow Clear Mixers You can buy static mixers in various places but some get pretty expensive. I admit It’s total overkill for this application but it looks “cool.”  It’s about $80 plus shipping
Static Mixer 2″ ID “clear static mixer” PVC process mixer biodiesel | eBay

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 ~30psi. I also have ~40psi 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.

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.

Nitrox Stick Math
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 = 14.3lpm so you can see where I got 14litres per a minute from on my flowmeter.

Coincidentally this is exactly what my cheap Chinese flowmeter reads when pumping 32% on my Alkin.

That is pretty much it. This setup has been working flawlessly for past few years. I can’t claim to be an expert on anything here but all my mixes are accurate to within less than .5%. I pretty much dive and bank 32% for every open circuit dive. The only few things I’ve changed in my above setup is I’ve removed the generic cold air intake and added Solberg silence filter. I also swapped out my oxygen regulator for a much higher quality Harris one.

I never return leased O2 bottles with any gas in them and having banked 32% on hand makes blending standard nitrox mixes (21/35, 18/45, 15/55, 10/70) extremely easy and accurate.

Some Tips
If you’re getting 1-2% variation in mixture check for leaks in nitrox sticks or near air intake. Everything should be sealed after the O2 injection on the nitrox stick. Make sure hose connections are very snug on air intake.  

The filter in your compressor removes humidity and concentrates oxygen so if you have high humidity then 31.5% on nitrox stick might actually be 32% on final destination. When humidity is removed, oxygen is concentrated. It’s very minor but try running the stick around 31.5%. and see how your results are. See humidity worksheet below but honestly you don’t really need to do anything special. Your mixes should come out within .5% if there are no leaks and everything is calibrated correctly but you might have to experiment a little bit.

Here are a couple excel sheets (I can’t take credit for them but I believe they were both made by Derek at Rubber Ducky Designs.
https://docs.google.com/spreadsheets/d/1SuAVbFe8fvD4rQJ7Mt8DQ1yKd_yDS-87/edit?usp=sharing&ouid=115878126833965574586&rtpof=true&sd=true

tps://docs.google.com/spreadsheets/d/1_xGYusagFlZ7edJa5cUtCX7NIkS5n2fT/edit?usp=sharing&ouid=115878126833965574586&rtpof=true&sd=true

Thank you for coming to my TED talk.