I borrowed the SQM-L meter from NOAO again to try a new thing. As per Sky & Telescope, a new iPhone6 app allows me to use the iPhone as an SQM meter! So, I wanted to a) compare readings from the two devices, and b) calibrate the iPhone as needed.
I ran seven sets of 10 readings, 3 on the SQM-L and 4 on the iPhone6. The 42% moon was still in the West and there were some clouds. Temperature was dropping rapidly.
Conclusions:
a) the SQM-L has better reproducibility. Its readings are within 0.1 units most of the time (my readings are about 20).
b) the iPhone gets the same readings but with greater variability, perhaps +/- 0.3 units. Comparing the 10 reading averages shows the iPhone statistically the same as the SQM-L. That’s good, since I don’t know where I would put my calibration value in the app. The app provides 2 corrections, one for readings less than 20 and one for readings over 20. My readings were right around 20, about half just under 20 and half just over:)
The following night when the moon went down, the sky was “pretty clear” (it had been cloudy earlier).
SQM-L showed 20.51 (5 point average, throwing out the first few) = Bortle 5.82
iPhone6 showed 20.42 = Bortle 5.76
I will try again when the sky is definitely clear.