I'm going to be brewing my first AG batch soon and wanted to know which is the best way to check for starch conversion at the end of the mash, a hydrometer or a refractometer? I've got a small hydrometer that can remain in the mash tun. The guy at the HB shop recommended it, and it certainly sounded better than the iodine test. I have a refractometer as well, but didn't know if that would be more accurate.
You've got a misconception here, as was noted. Neither of those will tell you about conversion, and you certainly don't want to leave a hydrometer in your mash tun! You really don't need to worry about conversion....if you mash anywhere from 148-165 degrees for 60 min. or more, you will have conversion.
If you read in Palmer's book the section on mashing you'll see that one can determine the mash conversion of starch to sugar based on the grist ratio using a hydrometer.
"extraction efficiency is determined by measuring the amount of sugar extracted from the grain after lautering and comparing it to the theoretical maximum yield. In an optimum mash, all the available starch is converted to sugar. This amount varies depending on the malt, but it is generally 35-ish points per pound per gallon for a 2 row barley base malt. This means that if 1 pound of this malt is crushed and mashed in 1 gallon of water, the wort would have a specific gravity of 1.035. Most brewers would get something closer to 1.031. This difference represents an extraction efficiency of 88%, and the difference could be attributed to poor conversion in the mash, but it can often be explained by lautering inefficiency."
As Palmer has stated one can determine the lautering efficiency which is relative to the actual conversion of sugar in the mash. I find using a refractometer is easier to use than a hydrometer because you only use a few drops of liquor and it's faster. Good Luck.