Scatter Plot Analysis, how to raise an alarm.....

[dinosaur_uk]

HI there,

I need a method od detecting a change in levels in a system. I have made a confidence test to find the upper and lower limits of the unit. The X-Axis is time and the y axis is deviation from the mean. Basically i want a test to determine if it has shifted away from this limit. However, with this limit, a certain amount of data points will be out of these limits. Can anyone suggest a good mathematical solution to this?

I am running this test online, so it needs to collect data and process it.

I am using a normal distribution to determine upper and lower limits, should i use another ?

Image 1 is of a typical normal behaviour
Image 2 is a failure of the unit.

How do i spot it mathematically?