One more thing.
Be certain that the specs are OK. Some client will ask for a -40 specs when the temp will never go that low. If the temp really goes to -40 and there is some wind on the panel, you could need an even lower temp spec.
In the end they will be in an argument with you because that the instrument does not work at -40. The specs always call for -40 static temp. The wind will blow away the little heat internally generated, this little heat being the reason why the instrument can operate at -40.
A -40 + wind is more like a -60 !!!!! Its even thrueer if this is in an intrinsicly safe environment. A lot less power on the loops generate a lot less internal heat in every devices connected.