anparkinson, You can command a “Preflight Calibration” action using the actions tab in Mission Planner. This should zero the airspeed sensor.
I have had analog airspeed sensors that would only calibrate with the static and dynamic air tubes hooked up one way on the sensor.
After preflight calibration the sensor should zero and respond with positive airspeed when you blow gently on the end of the pitot tube. If it doesn’t try reversing the tubes connecting the pitot tube to the sensor, recalibrate and check. There is a parameter in the configuration parameters that allows you to manually set how the pitot tube is connected to the sensor. If the sensor calibrates but has negative airspeed you can use this parameter to set things right.