Identifying a constant slope using

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I need to identify the beginning of a period of constant slope in a set of data - Any general suggestions for where to start or any specific ideas are welcome. Thanks everyone.

I have a set of data that can be represented as a curve. The X axis is time and The Y axis is weight. What always holds true for any set of data i will be looking at is there is a maximum weight value followed by a period of steep negative slope that is constantly flattening out until it reaches a constant slope and holds that constant slope until the end of the set of data.

What i am trying to do is identify where the period of constant slope begins. I would like to have a formula or function that would identify the cell where the weight data is held that represents the very first point of the contstant slope that follows this maximum value. Note it is not the only period of constant slope in this set of data, what makes it unique is that it occurs after the maximum value.

In my spreadsheet, Time values are represented in column A and Weight values are represented in column B and are sorted by time chronologically.

Example: This is just a snapshot of what a portion of the data looks like, the period of constant slope is from 90 to 70 and I want to be able to get the cell address for 90. The difference between this example and my spreadsheet is im working with over points of data.

30	250
31	200
32	160
34	125
35	100
36	90
37	85
38	81
39	76
40	70

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The way I've been going about it is by creating a column of slopes that represent the slope between about 200 data points and I would somehow defining an appropriate standard deviation that could be allowed for a the set of recurring slopes to be considered constant. Having trouble deciding the best way to tackle it. - appreciate anybody willing to talk through it!