@MarkT I quickly plotted some historical air temperature data for the month of January at our field site in the graph below. The site is at 82.4° latitude, and it's interesting to see how much variability there can be during an Arctic winter in 24-hour darkness. While temperatures tend to stay quite cold, they can vary as much as 15°C over the span of a couple days.
It's also good to know about the concerns with epoxy under such temperatures. Conformal coating definitely sounds like the way to go.
@JohnRob Thanks very much for your input! It has really helped bring a number of things to my attention, which is exactly what I need. We will most likely be running the Arduino-based logger alongside our existing instruments in order to gauge: a) how well they operate over the course of a year and b) how reliable the resulting measurements are compared to existing commercial solutions. It's very much a proof of concept project.
The DC-DC converter idea has been put aside, and running the batteries in series/parallel now sounds like the better solution moving forward. I've been searching for an efficient step-down converter, and I'm wondering if it would be best to just bypass the Arduino's onboard regulator and supply a stable 3.3V directly. I'm also aware that the internal 1.1V bandgap can greatly vary with temperature, and a stable external reference voltage may also be handy to have. I measured a change of 1.077 to 1.073V in the bandgap of my Pro Trinket just by placing it in the freezer (+20°C to -5°C). I can only imagine how -40°C would affect it.
Thanks again everyone for your help!
