Back on topic... Is the relative salinity of the water going to affect this?? Say you get a heap of fresh between sets, your not only going to catch no crabs but you might not get your pot when you need to? Alternatively it might pot up a little too early - as your friendly pro drives past.
Could be a problem in places where access is tidal and you need to only have a short window to check. Additionally it would also be interesting to know if you could set the 'timer' to a number of different durations.
Very novel idea though - down to the patents office I go . I would deffinately use it. Catching no crabs when you know your pots have been ratted is worse that going home fishless - at least you have someone else to blame.