Thanks for the advice on the temperature variation. Tonight I am going to place another sensor outside and see what happens and will let you know.
Good move. That gives you some sense of trust in your equipment.
Try to mount it right on the 5n1 if possible. There can sometimes be a substantial difference in a surprisingly short distance.
When I do a verification like this, I often use the $8 Acurite "Tower" sensor. I use a little hook to hang it from the lip of the 5n1's rain cup.
If things go well, they should agree nicely at night. In direct sunlight, though, you could see major differences as the Tower sensor has no fan aspiration. I've seen as much as 10F degrees. Typically the Tower sensor should be mounted in the shade.
The published error specification for Acurite temperature sensors is plus/minus 2F degrees at 32F. So if you're comparing 2 of their sensors, in the worst possible case you might see a 4F degree difference between them. Usually it should be with a degree or two, though. As you get down to around -40, the possible error range grows to plus/minus 4F for each sensor.
Relative Humidity readings are much more variable. Their published error range is plus/minus 5 percentage points in the 20% to 80% range. The readings in the upper and lower 20% get really iffy. I just think of them as "really dry" and "really humid".
For completeness, I might as well say that if your temperatures are expected to drop below -4F, you should be using lithium (e.g. Energizer Lithium) batteries in the sensors. Otherwise, you can stick with regular alkaline AA batteries. Rechargables and other battery chemistries (like "heavy duty") aren't recommended.
