I am trying to build a formula with the data that I have below. The formula will be embedded in a Arduino sketch.
Column A is the time stamp and Column B is my voltage value. Column C is a threshold value, which I am pretty sure is not relevant to my question here.
Column E give me the slope between two points. The Excel formula for column E is E1=(B2-B1)/(A2-A1) This formula was based on slope 'm' = (y2-y1)/(x2-x1)
Column G gives me the percent change between 2 values in column E. G1=((E2-E1)/E1)*100 This formula for percent was based on [ (New-Old)/Old ] x 100. So far, so good.
If you look at row 5 which has coordinates of (550, 36), column G is telling me that there has been a positive increase in slope by 2642.857%. Visually, I can see the slope increasing on the graph and the percentage change is 2642.857% according to Excel. So far so good.
Here is the issue. If you look at row 7, visually, it seems that the slope is increasing. But column ‘G’ is telling me that the value there is -1,300%. I am assuming that Excel is telling me that the slope is decreasing because of the negative sign in front of the 1300. I am obviously interpreting the values in column G incorrectly.
What formula should I use such that the values in column G will show me that the slope is increasing ( by using a positive + symbol) and also will be some numerical value?
You just need to take the absolute value of the old value in the denominator. Basically, percentage change would be given by $$\frac{\text{new value} - \text{old value}}{|\text{old value}|} \times 100 \,\%$$
So, replace E1 with ABS(E1) in the formula (denominator) and you should be fine.