I've thought about how to have a discussion with Delta-T devices about the SPN1 morning "dip". Steve and I thought that reorienting the sensor would better inform such a discussion, so we did that 2 days ago. (It was oriented with the analog cable side facing north, now it is the dessicant plug facing north.) Since that time, the dips have not gone as low as zero, but the dip still is clearly present. However, both of the past mornings have had a tiny amount of haze, likely associated with the recent controlled burn to the south of the site. (This haze is clear in the ceilometer and lidar returns.) Looking at past data, obscured skies increase diffuse radiation (as expected) and can be of a magnitude to mask the "dip".
Thus, I believe we have to wait for another perfectly clear morning in order make a definitive comparison between the "dip" for different SPN1 orientations. Ideally, we would acquire data with the digital cable connector oriented north as well, but there likely won't be time during the rest of the experiment.