Airspeed indicator oscilating with sensor disconnected

I’m having some issues when flying with the airspeed sensor. As the airspeed sensor is located at the wing, and I leave the wings off the plane in the workshop, the airspeed stays disconnected from the APM 2.5
The strange thing is that in the MP, there is a constantly oscilating reading of airspeed, who knows from where? The oscilation is between 0 and 3, usually.
I’m concerned there is a problem in the APM that could be causing an error on airspeed reading.

Thanks for the help.


From what I read Eric that is not a problem, even with the airspeed sensor connected you get small deviations so the value seems very sensitive at best

I understand the high sensitivity for the airspeed readings, but is it normal to have sensitivity when there is no sensor connected? Would it be sensitivity to the electrical resistance of the air sorounding the connectors, or any magnetic field?

When there is no termination on the APM Analog inputs, they can pickup stray electromagnetic field noise in the surrounding environment and cause random pressure sensor readings without the sensor being attached to the Analog input.
Example: When I short the leads on my bench top DMM, the reading is 0.0vdc, however if I un-short the leads and hold them in the free air, the meter can display ~250mvdc and higher.
TCIII Developer

Unterminated inputs on an AD converter float so the signal reported is not guaranteed.
On Single ended A/D there may be a very weak pull down, but frequently not and they can simply float.
Even then the pull down must by design be so weak that any signal can easily overcome it (for it to be able to do it’s job).
Basically an A/D without a signal being fed to it is not at all guaranteed for what reading will be seen.
Pretty much what TCIII said above.
Pretty much pay no attention at all to an unconnected, unterminated A/D.
Among other things that is why the Airspeed sensor has to be enabled / disabled in Mission Planner.

Ok, got it!
thanks a lot for your replies, Coyote, TCIII and Gary!