Ridgekid,
   I bought one of those meters a few days ago and I'm very impressed with the results. It is the third one I've used and so far the cheapest and most accurate. In the past, I've dried down fresh cut sawdust at work and tested the mc using the same formula we use to check the mc of our feed stocks. The other meters were usually off by several points, and the results were sometimes not repeatable within a few points. The other meters were bought at Graingers, and they are way more than the $12.99 HF gets for theirs. I ran 2 samples yesterday at work using a 100 gram sample of fresh cut sawdust from a six inch round(oak) cut just over a year ago. I got 2 consistent readings of 30%. Using the HF meter I got a 31% and a 32%. I was very impressed.
   The method we use for testing mc in firewood is as follows: Wet sample - dry sample = %mc. In my case, we always start out with a 100 gram sample, so it was: 100 - 70 = 30% mc.
   There are other methods out there for determining mc in wood, but they make very little sense to me if your species of wood is an inherently wet one. The most common one would be (using my results) :  100 -70 = 30 divided by 70 x 100 = 42.86% mc. Now suppose your 100 gram sample dried down to 45. I would say that sample has a 55% mc (100-45=55) The other method would be : 100 - 45 = 55 divided by 45 x 100 = 122.222%.  A good share of these articles are rated at the end, and there are apparently quite a few people out there that don't buy into their methodology either.