After much troubleshooting, including a visual inspection of the fuel float, and manually activating the float, I was able to determine that the sender was working correctly. But each time I added fuel, there was no reading from the sensor. After several attempts, the thought finally occurred to me that the float may be stuck down on the bottom of the tank. With 15 gallons of fuel in the tank, I gently tapped the bottom of the tank. I was very relieved to hear the arm suddenly rotate up, and to see the display register a significant change in resistance. I filled the tank up, then removed gas 5 gallons at a time, calibrating each increment.
In the event that the tank is completely drained in the future, it might be necessary to repeat the procedure of tapping the bottom of the tank near the wing root, after several gallons of gas have been added. I was unable to determine the root cause of the stuck arm/float, but my investigation did confirm that the arm has full freedom of movement.