Problem: There was no dry air supply going into the CSI during a pre-flight ground test of the CSI. However, the CSI was receiving dry air during the following flight.

Cause: The power to the CSI is split between the two main instrument DC power lines and the mass flow controllers are on the DC line that contains the 2DS and CDP. During ground tests, the 2DS/CDP breaker typically is not pushed in, thus the CSI mass flow controller was not being powered on.

Solution: Push in both main instrument DC power breakers (along with the two dedicated CSI breakers) when ground testing the CSI.