Check for used computers.
It also should be noted that, in general, the warranty on a computer only applies to the original purchaser. So, buying used means no warranty. For that reason, I personally would only buy a used computer if it was a REALLY good price compared to new. And I say that fully acknowledging that I have a used computer for sale in Classifieds and would caution anybody that asks me about buying it that the warranty does not transfer.
---------- Post added August 27th, 2015 at 10:41 AM ----------
It seems like you really want the Petrel but it is a bit outside your budget right now. I am not trying to be a DB, but have you thought of just using tables until you save enough for the Petrel? As someone mentioned, your backup computers should be the same computer or at the very least use the same algorithm so you really can't use it as a backup. Also if you are diving in the northeast you really want a computer that is easy to read. The water clarity is not the best.
I am not a vintage diver or someone that rejects modern technology, however it seems the certifying agencies and the manufacturers keep pushing additional gear which raises the barrier of entry into diving. Now they want you to have a computer and a backup computer. Then they complain that diving is not as popular as it once was.
Why does the backup computer need to be the same algorithm? Isn't it really the case that the backup computer just needs to be as liberal or more liberal? Alternatively, the backup computer just needs to have Gauge mode and it doesn't matter what algorithm it has?
Also, what agency or agencies are telling people they need (I'm inferring from "they want you to have") a backup computer (versus just telling people it's a good idea - which it is)?
A Petrel will give you the Buhlmann algorithm with Gradient Factors. A common GF setting seems to be 30/85. When I check Recreational dive plans using Multi-Deco, calculating using Buhlmann 16C w/GF30/85, the results are more conservative than what my Atom 3.0 computer produces, using the Oceanic implementation of the DSAT algorithm.
So, I don't see where there would be any problem using a Petrel and having my Atom as a backup. I.e. no requirement to have the backup use the same algorithm. It just needs to be more liberal. Otherwise, it needs to just operate in Gauge mode, which definitely is not as nice as, then, your "backup" still requires you to use tables. If I were using a Petrel for Recreational dives and it failed, I would have no problem continuing to dive using my Atom and still without having to fall back on tables.
As for readability, I'm old and my eyesight is starting to suck. I did ante up for masks with gauge reader lenses in them. I regularly dive in my local quarry where the visibility is often less than 5'. It's not unusual to be 2 - 3'. So far, I have never had any trouble reading my Atom. A big color display would be "neat", but I'm still waiting for that dive experience where I feel like I can't do what I want to do because my plain old black and white LCD display (with backlight) isn't readable enough. But, I have been diving less than a year and have less than 50 dives, so maybe the relatively few low-viz dives I've done so far just aren't very representative.
