Nitrox stick controls question- kinda a follow up to the my last thread

[jlcnuke]

Ended up getting a pretty good deal on a portable compressor that a number of sailors have on their boats, so I'm confident it will work fine for me. That'll arrive in a couple weeks or so.

In the meantime, I'm looking into Nitrox sticks. Anyone found "good" examples of complete systems? I'm talking parts list and assembly from the O2 side to the air suction to the compressor? Reading over the oxygen hackers manual I'm seeing 3 main parts:
1. Intake (the stick itself) with connection to the compressor. This is the PVC or similar piping, baffles, O2 connection at the top/start of the intake, piping and associated tubing.
2. O2 supply. This can be as simple as a regulator, valve, and tubing going to the nitrox stick.
3. Controls/safety. These seem to range from reading an O2 sensor on the end of the stick and making manual adjustments with no automatic safety system all the way to digital control systems regulating O2 flow.

1 and 2 seem pretty straight forward (assuming you follow the basic safety recommendations from Harlow's book). For 3, I'm thinking a DIY O2 sensor with solenoid trip valves in the O2 line. Ideally, a compressor trip AND/OR a high O2 reading would trip the O2 solenoid. However, I haven't found plans for something like that specifically. Has anyone developed that and have a walk-through or suggestions?

 

[rsingler]

Here's my 2 cents.
Lots of DIY examples here on this board for sticks. Money to be saved there.
Your #3 comes at no small expense if you engineer automatic cutoff controls.
A proven stick and safety controls are all good, but for the output/filling speed of a boat-sized compressor, you're better off manual.
Why? Because for a fixed lpm input of O2, your Nitrox % changes with decreasing output volume from the compressor as your receiver pressure increases. This is more marked with small compressors.

In other words, you'll have to be there monitoring anyway. Or at least checking frequently. Even filling a "bank" of three cylinders.

And if you catch an error? Wait until the Nitrox mixes, then use partial pressure blending tools to figure out what mix to add to the last 500 psi to fix it, and top it up.

Like I said. Just my 2 cents.

 

[tbone1004]

if you want a commercial stick system, then the trihunter from Amigos is about the only one. It's really nice, but it's $3-4k...
TriHunter 6000 – Advanced gas mixing made easy

to answer the questions specifically
1. koflo mix sticks are the best, but there are cheaper DIY solutions.
2. You need a barb fitting to connect to a normally closed solenoid and then connect that to the outlet of the O2 regulator. The solenoid is typically the same voltage as your motor and wired in the same circuit so when the motor has power, the solenoid is open and when the motor kicks off, the solenoid closes.
3. typically it is just the solenoid mentioned above to cut the supply off when the motor is not running. You can get creative with an arduino or rpi based monitor, but most people just have an O2 analyzer at the bottom of the stick and adjust accordingly. Programming the pi wouldn't be that complicated, but you would have to put calibration processes in there since O2 analyzers are horribly unstable and all it would do is act as a cutoff for you, it wouldn't be able to act as a metering valve.

 

[BoltSnap]

if you want a commercial stick system, then the trihunter from Amigos is about the only one. It's really nice, but it's $3-4k...
TriHunter 6000 – Advanced gas mixing made easy

So if I want to buy a nitrox stick, this is the one you recommend in place of the others available, the best?

 

[tbone1004]

So if I want to buy a nitrox stick, this is the one you recommend in place of the others available, the best?

yes, it is used in cave country at many dive shops and there is no place in the world that pumps more nitrox every day than those shops. If you talk to Wayne and are only doing it on a small pump with only O2 the price will likely come down since it is designed for 20cfm+ compressors with helium.

 

[rddvet]

On the trihunter, don't waste your time. Build one. I had a trihunter that I got second hand for a decent price. It was wilidly iinaccurate for trimix, and the accuracy was ok for nitrox. I'm pretty sure extreme exposure in HS has stopped using there's because they always tell me theirs is wildly in accurate as well.
As for the stick:
2-3 inch pvc with fittings appropriately coordinated at the top and bottom to allow for a lawn mower filter at the top and the hose to the compressor intake at the bottom. I find these parts are the most widely varied, because it depends on what parts you can find to put together. I put a blue scouring pad (for dishes) at the bottom of my stick, then fiilled with bioballs (used in aquarium filtration). You can use golf practice wiffle balls instead, but I thiink bioballs circulate better. Then I put another scouring pad on the top. I initially wasn't going to use pads, just balls, but my compressor tech says he's found the scouriing pads keep everything in place and allow for more gas movement in odd directions for mixing. I think they can be skipped personally. Buy an oxygen analyzer wiith a remote sensor (such as the oxycheq expedition). The sensor will come with that little nipple that attaches to the sensor. Drill an appropriiately sized hole to slide the nipple into the stick at the bottom(it was an odd sized bit). Then attach the sensor. For the oxygen, I use a good quality adjustable welding reg, that then goes to a dwyer adjustable flow meter. That allows for large adjiustments at the reg and micro adjustments after the reg with the flowmeter. on the backside of the dwyer I have a solenoid that's always shut when power is off. It's wired to the compressor, and only allows oxygen flow when the compressor is runninig. i also have an oxygen sensor and a CO sensor plumbed in after the compressor. Those are unnecessary,but nice. You can just analyze for both by hand instead.
II'm out of the country, but could get photos when I return

 

[prid1279]

I'd stick with the advice from Tbone, Rslinger and rddvet from above.

I basically followed Vance Harlow's advice when I built my stick a few years back, minimal outlay and it has run really well ever since. I fill half a dozen nitrox cylinders most weeks. I have really appreciated the normally closed solenoid on the couple of occasions that my power has gone off while filling.
From the output of the welding reg on the O2 cylinder I did the filed down thread trick on a screw to help restrict the output which with a little trial and error has worked well. Start light with the file and take off more as necessary, I was able to get the output so that the reading on the gauge of the reg pretty much matches my desired O2 %. As a bonus I was easily able to file more off the screw to compensate for extra flow when I added a second compressor to the system.
My O2 sensor is in a t-piece in the hose on the outlet side of the stick, easy to keep a rough eye on and adjust flow as the compressor slows.
Bio balls in the stick were also my preference, I also like the idea of the scouring pad as an added extra but my system is missing that tweak.

 

[Wibble]

Ended up getting a pretty good deal on a portable compressor that a number of sailors have on their boats, so I'm confident it will work fine for me. That'll arrive in a couple weeks or so.

Which compressor is this?

 

[jlcnuke]

Which compressor is this?

Coltri MCH-6. Not quite as great as the Bauer Jr. II, but thousands less even new and seems to be working very well for lots of people.

 

[jlcnuke]

Tbone, Rslinger rddvet, and prid1279 thanks for your input. I think I'm going to build one with PVC and the wiffle golf balls (since I already have a bunch of them). I think I'll just monitor it and shut down manually if there is an issue with O2%, but I think I'll still wire a solenoid to trip closed if the compressor goes offline (just to be safe). So now I'm looking for adjustable flowmeters, but it looks like to get 32% I'll be at ~15 lpm (plus or minus a bit), so the basic medical ones that only go to 15 lpm will be a bit low range. The others seem to all have rubber o-rings involved, so may have to rebuild those with viton if I can't find one rated for O2 that can handle a bit higher flow rate (ideally 0-30 lpm or equivalent seems like it would be best).

I'll look for something like that later today.

For monitoring, I think I'd be fine to connect my divesoft solo analyzer to the outlet (I'll 3d print an adapter to connect the pro limiter valve to the outlet from the stick and just run that to the solo analyzer).

Thanks for the help everyone