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DIY Hypoxicator MK2

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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Well-Known Member
Jan 21, 2004
For those following the "Get high and get on down" thread you would have seen the MK1 DIY hypoxicator that I made. I've just rebuilt it and the new one was again cheap to make but is a big improvement on the previous unit. I'm up to day 7 of a 20 day cycle of 90mins per day (5mins on/5mins off repeats) I forgot to take pics of the analyser that I made which plugs into the top of the unit (I'll add those pics tomorrow)

Here are some pics




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Hello ADR
How did you make this hypoxicator,did you get any prof. advice or plan??
I will be very glad if you can describe a little bit.
And how many days are you planning to make these exercises??

Before freediving I was a tech diver and essentially the design is the same as a rebreather. If you do a search on DIY rebreathers there is a lot of info available. I just did away with the O2 tank and have a tap that mixes air to control the FO2. I'll try to do a rough schematic for you this weekend.

Here is a good link describing how rebreathers work. http://www.bishopmuseum.org/research/treks/palautz97/rb.html

Also attached is a pic of the oxygen analyzer I made ($20 for the console $135 for the sensor- aussie dollars that is :) ) Here is the link to the page with full instructions for how to make one:



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Thanks for the quick reply ADR.
Very nice work.We will be waiting for your information about your training with your hypoxicator.Very good work
It's only day 11 but already some intersting things are happening.

1. My hypoxic tolerance levels are changing. I'm now stable at 8-10% FO2 which relates to 70%-80% SaO2. The sessions at this level are now more comfortable (less dizziness/lower breathing rate/lower heart rate) than the earlier sessions.

2. My freedive training at the moment has gone backwards significantly and I think this may be a result of the body being under stress and it being very easy to over train while going through this. (times and distances are shorter/contractions start early)

3. My aerobic capacity is changing significantly. I go to the Gym 3 times a week and always warm up on the eliptical trainer - 20 mins ate 120 heart rate. I've been doing this for months and I normally need to turn the thing at 65rpm to hold my heart rate at 120 BPM. I have been 3 times since starting the IHT sessions and I now have to turn the thing at 76-78RPM to get my heart rate to 120 BPM. Additionally my morning wake up heartrate has dropped from 55 - 58 BPM to 42-46 BPM.

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G'day Andy
Keep up the reports. Good stuff. I'll bet that there's a lot of us following your experiment.
Hi Bill,

Thanks for the post, I'll let you know how it goes.


How are you adjusting the 02 levels on that thing to get them down to 10%?

Do you have the analyzer plumbed into the inhalation hose, or the scrubber bag?

Any other close-up pictures of the intake valving?


Hi Jon,

There is a tap(green thing in the pics) on a tube off the main tank that I turn off to close the system and then breathe it down to 8-10%. I then fine tune how much I turn it on to maintain the 8-10% range. Effectively it mixes a little fresh air (21%) on boht the inhale and exhale cycles. It is a very crude/simple approach that works surprisingly well! :)

The sensor is on the top of the unit near the inhale tube.

I don't have any close up pics but here is a rough diagram (very rough) that may help explain it with the pics I've already posted. In the diagram I've taken a bit of licence and moved one tube to the other side of the unit so as not to confuse the drawing (essentially the same apart from that)

I hope that helps a little


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second try as a zip file instead of a thumbnail


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I build and dive my own rebreathers as well as freediving so this is quite an interesting thread for me. Well done, you've gone for a radial scrubber, probably the hardest to actually make work reliably! Quite an interesting idea for adding air -- is that a Hozelok garden hose valve? My first mouthpiece was made from one of those. It's amazing how easily you can build a RB with a visit to your local hardware store. For anyone without engineering skills or resources, you can usually pick up old firefighting rebreathers on E-Bay that could be converted for hypoxic use, a Biopak 45 or 60 for example or there are tons of RGU-FM's about (East German emergency rebreathers for tank crews) that sell cheaply. In fact, your system isn't that much different from the Halcyon RB80 which sells for about $10k.

There are plenty of resources on the web for rebreather builders. Try www.rebreatherworld.com in the homebuild section. A few off the top of my head:

www.lizardland.co.uk (my own)

I might have a few spare analysers in a couple of weeks time.


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The MK1 (the biscuit tin version) took me 4hrs to make and had an axial scrubber but I wasn't impressed with amount of soda lime I was using so I thought I'd have a go at a radial version which is working really well for me. I can't remember the brand but the tap and fittings are garden supplies as you have worked out. The one way valves are from an Inspiration and cost me far too much for what I'm using it for. I should have used snorkel flapper valves at $5 each instead.

I had a look at your site - very cool! !

Draeger flapper valves are cheaper (I think) and can be bought online at www.danshop.com or www.tecme.de (cheaper still) both in Germany unfortunately :( In the UK you can't buy Inspiration parts unless you are a registered owner.

I really like the idea, nice and simple. I'd never thought of using my RBs for freedive training but now I've seen it...
So, I could just pick up a used rebreather and run it without the drive bottle to get the same effect- assuming it has an 02 guage on it to monitor what I am breathing?

You could do that however I think you will find it cheaper to make your own. I can't find the link but there is a site for making a rebreather for $100. It uses a hot water bottle for the bladder and has a simple plumbing pipe with caps scrubber design.

Jon said:
So, I could just pick up a used rebreather and run it without the drive bottle to get the same effect- assuming it has an 02 guage on it to monitor what I am breathing?


Absolutely not!!! The loop would eventually go hypoxic because there is nothing to replenish the oxygen content. It is a closed system, the oxygen has to come in from somewhere. With the above DIY model the air comes in through a valve. With a closed-circuit rebreather pure oxygen is added from an external supply with the bulk of the breathing gas being air or a helium based gas (the diluent). This is usually done by an electronically controlled solenoid valve which will inject oxygen when it is needed (as in the Inspiration, Mk15, Megalodon, Prism, etc) or there is a tiny constant flow of oxygen getting bled into the loop through a precision orifice to replenish it (the Jetsam KISS).

This kind of rebreather is very expensive, even buying used. A used Inspiration doesn't sell for much less than a new one. They also don't come up for sale that often. You'd be looking in the region of $4000(US) at least. It would also be very difficult to get the O2% down as low as 8%, the units usually have a failsafe in their control systems to prevent this ever happening. It would be possible with the Jetsam KISS as this is manually controlled and has no electronics.

Oxygen rebreathers work differently. They ONLY have oxygen (no diluent) in the loop. As oxygen is consumed then the loop volume decreases and a valve is triggered to inject more oxygen. There is also a manual pushbutton over-ride to add oxygen. These are usually ex-military technology or used in firefighting or working in contaminated environments. Oxygen rebreathers can be picked up very cheaply on e-bay now and again (the Biopak 45 often goes for about $150). It would need a bit of work, you would have to add an oxygen monitor (diving rebreathers usually use 3 not just 1) and you would need to disable the automatic oxygen valve.

In practice you would inflate the rebreather with air by blowing in it. Then you would breathe it down to the required %O2 and use the button to put a bit of oxygen in. It all seems a lot of hassle though, and you still need oxygen to drive it. The DIY model doesn't.

Jeff Bozanic's Mastering Rebreathers is an excellent read for anyone interested in rebreather diving.


I was thinking more along the lines of using a Dolphin, or Ray, and just running it with air, instead of nitrox, to get a lower 02 mix. Or, breath the loop down, crack the valve, repeat- while watching the P02 guage.

I thought I'd bring this thread back to life given some recent time in the workshop. I have built a microprocessor control system for the unit and some photos are attached. The software has a number of preselect programs and a user defined option for custom programs. You can see from the display(bad pic sorry) that you can monitor the target(Tg) and actual O2(Ox) breathing percentages and the program gives total time, actual time and whether or not it is an on or off interval. The system automatically controls a valve on the unit to maintain the system to within about 0.5% of target you set. Its nice to set a program and sit in front of the TV with the display in your lap and have it buzz, flash and beep each time you need to get on or off it.

....not bad for a guy that claims not to be technical:t

For me the hard part was the crash course in electronics as the coding side was relatively simple in comparison. Lots and lots of very late nights in the workshop.

I know that atleast Wal is going to love this one and I'd be keen to hear what others think as well.


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...replying to my own post is not great I know but I forgot to say that if anyone is interested in making their own unit I'm happy to provide the code and info on the electronics to make your task easier.
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