What exactly is undesirable of having too low of a FG? Is it not what your recipes are designed for? Does it mean that you’re not converting the right stuff during mash rest?
If you want to leave a bit of body in the beer, having too low a FG will take that body out. It can also throw off the balance of the beer. If your recipe is designed for a 1.013 FG and you get 1.003, that’s a real difference. What happens is that the mash keeps converting long chain dextrins into shorter ones.
Be sure your hydrometer is in good condition, not just calibrated, I had the same problem and then I realized my hydrometer has a tiny crack, I changed it and problem solved.
Ok, a word from the science side of the house. What you are trying to do when you calibrate a thermometer is to make a calibration curve. Some things respond in a linear fashion and some do not. Just becaseu you have controlled for two temperatures does not mean you have good accuracy at a point distant from these temperatures. In a typical lab instrument we will use a 5 or 6 point curve such that you have better control over a larger area. Denny is quite correct. Two control points simply doesn’t get the job done. I have not invested in a super-dandy theromometer yet but I also havent had any issues to indicate that my cheaper digital cooking thermometer is severly off. If I was seeing problem like the OP discribes however…
