It is a very cold day in January as I write this. The temperature has dropped below zero (Fahrenheit) and there is snow on the ground.
Though most (Northern Hemisphere) researchers have put away their recorders for the season and are snug in their offices analyzing last year’s recordings, others are out there in the field deploying recorders for various purposes. We know because we get a number of calls each autumn from people who want to deploy in very cold situations.
"What are the limits? Is it safe to do this?"
Our SM2 and SM3 line of recorders will be fine down to -4f (-20c).
Temperatures below that can harm the display and other components so should be avoided. It is important to note that the display will become sluggish, have low contrast or go out altogether at temperatures below freezing.
As for ways to protect the recorders against the cold, we have had customers put their recorder in a cooler to mitigate short temp swings such as cold nights. We even had one researcher bury the units when they were deployed in Antarctica!