A hydrometer takes all the guess work out of bringing the maple syrup to the correct density. The rule of thumb is to boil the syrup to seven degrees above that of water; so from 212 to 219 degrees Fahrenheit. After we got the hydrometer the correct density was achieved at the final temperature of 221.
Those few degrees make the world of difference. Before the syrup was thin; now it is, of course, thicker -- more acceptable. Ironically, both our thermometers are broken. In the photo, at this rip roaring boil one was reporting a T of 187 and the other 257. Both are heading for the trash. So for this batch we watched for the bubbling to erupt and then checked the finished product with the hydrometer.