nickdigger - Good suggestions. I'd definitely go for decimal values rather than hex, as you suggest. Putting the column names in the first row is a good idea too. For sample rate it may need to be slightly less so it doesn't slow everything else down: storage is cheap, but the bandwidth / CPU time to write to it isn't. I have a friend who designed a motorcycle datalogging system (
Dyna Pro) who put me onto the VDIP module, and he's logging several dozen parameters 100 times / second but to do that he's cranking the module along at a much higher baud rate and running a really optimized main loop.
Timestamps may or may not be a problem. Right now I'm getting timestamp data from the GPS module but for some reason they're all offset by 2 hours, and I can't figure out why. People who don't have GPS would need some alternative way to get the current time but that could be done with an RTC chip. I've done some coding for a DS1307 in a rocket telemetry system and it's a piece of cake to work with so that's a possibility.
cfg83 - I like the idea of using directory structures to indicate the date. That's how things like iPhoto work too. I'd prefer not to use hex values in the filenames though, that just makes it too obscure for human processing.
Cheers :-)
Jonathan Oxer