Most any should do for firewood, e.g. http://www.woodcraft.com/Search2/Search.aspx?query=moisture meter. I have always used the pin type simply because there were no non-invasive types (that I knew about) when I started xxx years ago, most non-invasive ones I have seen since have been wicked expensive, and I have not been doing in recently so have not checked out the new (probably within the last 5 - 10 years) inexpensive models. That said, for firewood I would get non-invasive deep-sense type if possible. The problem with firewood is that a pin measurement gets only the surface and even the ends seal over time, giving a false (low) reading for larger logs. Water moisture is only part of it if you also want to avoid sap flare-ups. Of course, you'll probably never completely eliminate the latter, so a pin-type may be good enough.
That's a good point Don regarding reading what is below the depth in case of firewood. I was just worried that unlike lumber, the firewood is very irregular so the non-contact type may not read it correctly. But I am just guessing.
I like to recycle trimmings from our trees. And since it rains so much here, getting wood dried is a bit more challenging than in California. I had a pile drying under the roof of our deck, only to see that there was a leak and the logs had gotten wet. I let them dry but when I tried to burn them, I noticed that they were not dry enough. Fortunately, a Pizza oven makes a great Kiln . I put the logs on the side as I am starting the fire and that gets them going. Still, you want a smoke free fire and quality of the logs matters in this regard.
Surprisingly, the last moisture meter survey by FWW was back in 2002. They're typically examining S2S or S3S boards vs. logs, but if you're interested in what they had to say, Amir, let me know and I'll get the back issue out of my hoard and summarize for you.
Steve -- The main reason for owning a moisture meter is to check the seasoning of wood, and make sure the wood is (a) enough and (b) equally dry among all the pieces in a project . Otherwise, you risk warping after assembly, as pieces of wood with higher moisture content will shrink more, leading to cracks and/or poor joints. It is not uncommon to get a supply of boards from the store and find various moisture content, plus it needs to reach its nominal level for the place you are using it.
Truth be told, except maybe just for fun, I have never bothered to check the moisture of my firewood. If it burns, it's dry enough.
Amir, I do not know that much about the non-invasive designs to know if irregular surfaces are a big issue. Could well be, point taken. Maybe you should build a drying shack... If nothing else, the project may take your mind off your case of chainsaw lust for a while!
There are more options for moisture meters online, if you are not able to find what you're looking for then a couple ways to tell if wood is dry enough to burn is if it is especially heavy then it's probably too damp. Also if you hit two pieces of wood together and the sound is hollow then it is dry enough to burn, but if the sound is heavy and solid, then it's too wet.
The idea of a moisture meter for firewood really makes no sense . As don pointed out the wood is too thick and can be wet inside and dry out. The next point is if you are using trimmings then it needs to cure for a season or year. So either way this is fruitless . Make a firewood shed and call it a day. Also do you have an indoor stash pile this is where you could put your trimmings to dry out faster.