I suggest an i-Blue 747 logger.

I did some research on chipsets etc when I bought one and it's about ideal for your usage. Cheap & accurate with a fairly long lasting battery.

It has bluetooth too so you can connect phones/laptops etc to it.

There is an oss downloader which supports lots of filters and conversions too. http://www.bt747.org/

HTH
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LittleBlueThing Running twin 30's