Rating: 5 / 5
There are a few different versions of this available--the important thing is to have a sending unit, which transmits the temperature where it is placed, and a "home" unit, which reads the temperature where it's located, and also receives the temperature from the sending unit. The home unit has an accurate clock, and a button which illuminates the temperature/time read-outs.
These are too heavy to be considered ultra-lite, or even something you'd want to take on every trip, but for sorting out sleeping bag capabilities, etc., they're great gadgets, available for less than $20.
The home unit is kept close to hand, either, for example, in your sleeping bag, or out of the bag but in the tent or bivy sack, and the sending unit is kept nearby outside.
Initial setup is either difficult, or automatic, so don't even try to do it manually, but wait until the home unit receives a signal from the national atomic clock, and sets things like time, date, etc., by itself-- none of which really matter for our purposes anyway.
You'll be able to see the relationship between exterior temperature, temperature in the tent or bivy sack, temperature in the sleeping bag, and perceived comfort. For example, on a recent night spent in low single digits, I was able to measure that, when the temperature outside was 6 deg, and I was sleeping on my side, temperature in the bivy, but outside the bag, was about 10, and in the bag, about 60, and I was comfortable. When on my back, temperature in the bag was closer to 50, and subjectively I was cold (though not in any danger.)
So if my skin temperature was around 97, the clothes I was wearing were sufficient to be comfortable with about a 38 degree gradient, but not a 48 degree gradient. And the sleeping bag was able to maintain a 55 degree gradient, when the contribution from the pad was minimized (sleeping on my side), but not when I was sleeping on my back (too much heat being lost through the larger contact area with the pad.) So my bag was fine, but the pad could have been improved.
When you're not using this for camping, of course, it's an accurate indoor--outdoor thermometer. The home/receiving unit also measures relative humidity, and indicates whether a temperature (either from the home unit or the remote unit) is constant, dropping, or rising. It will also read out the maximum and minimum temperatures from the previous 24 hours.