I need some help please. I am measuring a board 20-50mm thick on a conveyor system to determine it's thickness. I plan on using a 4-20ma sensor back to a analog ampilifer for this. Then using this measurement "in a mathematic instruction" multiplied by a set number by a set number to determine a pack of boards height

20mm board X 10 boards per pack = 200mm pack height

How can I set a tolerance + or - 5mm to switch an output alarm in the event of the pack being a board short

20mm board X 9 boards = 180mm = fail and alarm.

Thanks