Digital thermometers can be found that are also NIST-traceable, meaning their absolute, not only relative values are accurate. Cooper makes their NIST-traceable DPS300, -58deg to +302deg F, or in Centigrade if you like, for about $35. The Cooper DPS300 cannot be fully immersed, just the sensor stem which is about 5" long and made of stainless steel. It has some more rather neat features.