Gas gauge calibration unit

You are all welcome for the kind words. I like to solve problems, design things and work with my hands. I have slowed down some the past few years, keeping at it keeps me going. I have many things to do on my own stuff, but multi-tasking is good too. I often learn things thing that can be used on other projects.

Back to the project, thinking about a few details:
I am thinking I can get by without an actual gauge. It would be helpful to know approximate gauge resistance. I am thinking it will be fairly low. If anyone has spare gauges, please measure resistance and report.

Del reported the sender resistance values as:
L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full)
I can estimate gauge current for full scale with sender of 10.2 ohms. It would be nice to have the unit setup with default calibration values. Since the drive circuit eliminates the 10 Ohms resistance, it has the capability to over drive the gauge, however the PWM duty-cycle is used to control the average current. The over drive will be helpful for tired gauges that require more current to reach full scale.

The sender level will be measured using a current source to supply the sender. I plan to use a current source of 20mA. The 20mA is considered a safe value, and is likely less than the OEM circuit. If I know the gauge resistance, I can determine that. The OEM current varies with sender value, worst case at full tank.

With 20mA the sender will put out about 0.2V full and 1.5V at empty. The measurement circuit range is 0 to 2.56V at 10bits, or 1023 for maximum. The resolution at 0.2V is about 1 part in 80, so that should be good. The empty resolution will be excellent at about 1 part per 600.