If your Mars Soiling Sensor reads NAN for either Soiling Ratio or Transmission Loss, please update your firmware to the latest version. 

After updating your firmware to the latest version, please allow the Mars unit to operate outdoors in its installation location overnight through at least one sunset, as directed in the User Guide. In the morning the unit should read valid results.

If the unit continues to read NAN after updating the firmware version and allowing outdoor overnight operation, please contact Atonometrics for assistance.