Comparing Rsw last night from the Kipp&Zonen, Quantum, and SPN-1 at 7, I notice that the Quantum reads very close to zero, the K&Z read an average of -1.5 W/m^2 (range -3 to 0), and the SPN-1 an average of +4 W/m^2 (range +2 to +7). Apparently, the nighttime error is proportional to the cost of the sensor??