[DQDick]

As Carl said it's a matter of learning how much propane you use depending on the weather and what you are using. The rig will pull from the tank the arrow on the valve is pointing to until it turns red then you should switch the arrow to pointing toward the other tank and fill the empty one. It's just a matter of checking the color of the arrow when you feel you may be getting empty. Lot's of products out there to tell you how much propane is left. I've never had any of them work all that well for me or others I've observed. Not a big deal to open the door and look at the arrow.