I use a Polar RCX5 which has been a very good performer. I always wet the strap and my chest; in any event my sweat builds up well before 50 miles. I can look at my data from the DD and see where I was after the rest stops, where I took on lots of carbs and electrolyte drinks. I'm pretty sure my intake is sufficient. Over the course of 125 miles I had 10 Gu gels, 8 bottles of Gu Roctane, multiple bottles of water & camelbacks, 4 PB&J's, many pretzel sticks, Gorp and Endurolytes. When done I felt great. I did consciously try to keep a lid on my HR as I was in new territory past 10,000' of vertical.
I am willing to go with the experience of others who find the same thing. It was more from an abundance of caution I brought this up to make sure there were no red flags.