"That small just above the fill port looks like a guage can go there through a valve.
The bleeder valve is designed so that during the filling process, when the propane going into the tank reaches 80%, liquid will come out of the opened valve. This lets the delivery driver know that the tank has reached its maximum filling capacity. The valve is connected to what is called a dip tube that goes into the container. The dip tube is fixed and set at length equal to 80% liquid level tank capacity. In other words, the tip of the dip tube is at the level equal to the container being 80% full. The dip tube itself should always be located in the vapor space and never submerged in liquid propane.
