Actually I've had the same problem -- three thermometers, all within 2 full degrees of each other, but as much as 4 degrees between the top and bottom ranges. These are all darkroom dedicated thermometers. A while ago I realized that my negs did not look right (probably due to underdevelopment), got a couple of thermometers to test against the current one, went with the one that matched my wall thermometers, and have had more luck since. That being said, I may try the ice water method to see if that confirms which one is the most accurate.