Eric,
Thanks for the reply - I got lost on page 13
Using the PDF I posted together with a spreadsheet I can calculate the slope and the intercept so I know what model to use, but after that I get lost.
I have always worked with digital circuits (when I worked in electronics) and by boss did the analogue stuff! Now I want to learn more, so I can do more, but I just don't understand the math.
I guess I am just too old and stupid so will have to keep doing digital and pray nobody ever asks for anything analogue - perhaps I should go to the library and get 'Maths for Morons'
Thanks for trying to help anyway.
Rupert