Find the time at which the difference in data points starts to increase

Ok, a new conundrum...

Lets say column A is "time", and it goes in intervals of every 0.2 min or so. Column B is the reading of an ion probe vs that time, and gives readings that start at zero (or close to it) and as the concentration in the trap increases, goes us to 100 or so. At a certain point, the concentration starts increasing rapidly....I want a formula to find this point. It is not life or death but it'd be pretty great.

So basically, I need a formula that will scan column B and if the difference between row X and row X+1 in B is greater than, say, .5, return the corresponding time from column A. I want to know when the points start increasing by more than 0.5 every 0.2 minutes.

The janky formula that I came up with (which is obviously horribly wrong) was:

=IF((IMSUB(B2,B3)>.2)),A3,no)

I don't know....that's the most horrible formula ever, eh? I just figured it would be something along these lines but as far as how to implement the formula I have NO idea.....help!! Thanks so much guys!

Ok, tried that..........I get a #value error, and when I trace the evaluation steps for it I see that for the second variable in the Match function, it returns 989 on my sheet.....which cannot be true.

=INDEX($A$11:$A$1000, 989 + 2)
is the result after the first 2 arguments are evaluated stepwise, and for this particular set of data the first row to have a difference >.5 is around 172, so I'm not sure where the 989 is coming from. After only evaluating the first step I can see that it's correctly calculating the differences between the rows, but maybe the problem is that the difference is not ever EXACTly 0.5?

Thanks!