Do I need a 1.5” or 2” Alice gauge?
Use a ruler to measure the diameter of the circular fitting on your tank. If you measure it while it is still on the tank, note that the measurement will be slightly larger than 1.5” or 2”. If it is unclear what size you need from the top, take the fitting off the tank and measure the opening underneath.
These are both examples of a 1.5” fitting measurement:
These are both examples of a 2” fitting measurement: