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D4i, 10 seconds. The more data, the better. Especially in the event of "an event", God forbid. With 30 seconds, (and this is just my 2 cents), you can have a radical "bounce", and the sampled data would be none the wiser. More granular with 10 seconds. Room for the "extra data", has never been an issue. Download your dives to your PC (Mac) often.