there was a lengthy thread a few months back similar to this one.
Here was my response.
Rebreathers are EXPENSIVE, training is EXPENSIVE, and you will be putting your life in your own hands as if you are standing on train tracks.
As others have noted you will find plenty of used units for sale all the time, the more you look, the more you will find. and if you get in the habit they will spread like flies (partly how I ended up with 12...)
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If you are NEW to rebreathers, you should FIRST ask WHY you want a rebreather, THEN look in your direct vicinity and see WHO is diving WHAT, and who is offering TRAINING.
Its all too common to see keyboard warriors going online to find the "best CCR ever!" getting caught up in spec sheets, capabilities, redundancies etc. Doing this often leaves behind the WHY you are getting into CCR to begin with, if you truly do not have a specific reason for going CCR that isn't "I wanna be like the cool kids" then DONT buy a rebreather. end conversation
So you have a real reason for a CCR, now look at who around you is diving CCR and find out what rigs they are diving, who did their training, and their genuine feelings about their course and if they felt it or their instructor was lacking.
If most people come back with more or less the same answer for those questions it should be a pretty clear fit, the people around you doing similar styles of dives to you, utilizing the same rig or style of rig, and they haven't voted out the instructor. It may be the best option to follow their path. This will provide you with hopefully more than one character to lean on and learn from, help with service and parts, and an instructor that will make you enjoy the rig.
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You've settled on a rig, an instructor, and timeline of events to get yourself diving silent. Now and only now can you start perusing the forums looking for that rig and asking your instructor if they feel the rig is in appropriate condition. At the end of the day you will find many of these rigs found online may have been modified from their stock configs, ask for the stock parts if they still have them as most GOOD instructors will not train you on a non-stock rig for liability purposes.
At the end of the day we all have to eat. Your instructor will more than likely charge you more to train you on a used rig than a new one as they will have to take extra care servicing the rig prior to use but YRMV.
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Don't be that guy that buys a random rig off the internet and kills themself trying to DIY learning CCR.
If you want to read that whole thread it has some interesting bits in it.
Here.
I'm seeing a lot of second hand rebeathers sale recently, something going I'm not aware of, as I'm tempted to get one.