If you're using the native data range of the control to coerce the values (i.e., you configured the Data Range through the control properties), then you will never get a value greater than 80 reported to you, either through local variables or value change events. In order to accomplish what you're looking for, you'll need to allow a -Inf to Inf range on the control, but have its visible scale be from 0 to 80. On the diagram, whenever you get a value change event on the control, check the new value to see if it's greater than 80 or less than 0. If so, you can pop up a dialog, and also use a local variable to reassign the value of the control to be within the proper range.
I hope this made sense. Let me know if you need a sample VI.
-D
