Hi TomC:
Notice that when you have a FAT torp loaded, you will see a label that says FAT on top of the tube indicator light. If you press that label, you will be able to use it with it's FAT "qualities" and you will be able to see the dial again. If you click again on the FAT sign and the light goes off, it will be a normal torp again.
Hope this helps
CapZap