I’ve used this http://www.campmor.com/outdoor/gear/Product___87617 that I bought at Walmart. Also at Walmart they have 2 types of mercury thermometers. I picked up a 7” one so I could see the graduations better. It cost $1 and weighs 0.6oz. It reminds me of this product: http://www.gemplers.com/product/145930/Mercury-free-8-1-2quot-Plastic-Case-Traditional-Thermometer

I like thermometers. If I’m freezing at 50F in my 30F bag, this tells me it’s not the temperature hurting me but something physiological.

If I’m in the mood, I’ll bring my big do-it-all watch that graphs the temperature for the last 24 hours.

-Barry