I've looked at a DIY comp as well, for various reasons. I'd like integrated O2, diluent and PO2 receivers for a homebrew rebreather. I want it to track all of the important variables and help me to remember to surface when I'm getting near the limits on tank press, N2 load, O2 toxicity, etc. The two top-of-the-line comps do mostly what I want, but... $5K is a helluva lot of money when I have to trust someone else's programming! Jeez.
I don't want it to go brain-dead just 'cos I'm still running the same Nitrox mix that I ran last dive if I'm on tanks, and I don't want it to sulk if I have a serious problem and short-cut a deco stop. I want to be able to upload a dive log easily to my Palm Pilot via IR, optionally to a laptop.
It's probably a little more complicated than you guys are interested in, but we might be able to help each other out. Mine's gonna take a while....
I'll need to experiment with transmitters and recievers. From people I've watched, RF may not be ideal. I've seen 'em go into contortions trying to get their wrist unit to read the tank transmitter behind their head. Hmmm. Looked like they needed a lower frequency transmitter (a LOT lower). I may just give up and run a single unsealed or minimally sealed cable from the counterlung (where my electronics will sit) to a wrist/console module, but I'd really like to lose the extra line if it's reasonable. I hate all of the dangly little hoses, controls and monitors I see on some rebreathers.
You can find Erik Baker's VPM source code in both C and Fortran (what's that, Grampa?) out on the 'net. That ought to much of what you need for the N2 loading. Save your cash and don't buy Bruce's books or papers unless you find a Master's Thesis in math exciting. Erik & Eric can make a complicated subject simple enough that *I* can figure out the algorithm for it. Thanks, guys! It's refreshing to see that at least a couple of the folks in the field can write well.
). The real issue is switches. 
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