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I have two SQM-L meters.
I bought one in 2010 and the other in 2020.
They certainly are a useful tool as trying to gauge how dark a sky is by eye alone is pretty subjective.
Like all instruments, there is no exact reading.
Generally, you won't get exactly the same number each time for repeated meter readings. If the surroundings are brighter in one direction, there will be some affect on the reading even if pointing directly up. You can test this by holding it at chest height in front of you and turning 90 degrees each time. I usually average these readings.
The newer meter consistently gives a lower value than the older one, anywhere from about 0.05 to 0.12 mags/sq arcsec difference. I guess that's close enough to the typical precision of 0.10 mags/sq arcsec given in the documentation. I prefer to use the older one because it gives me a darker sky reading!
So whether, someone's sky is darker at 21.8 or 21.9 mags/sq arcsec with two separate instruments is a moot point.
Regards, Rob
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