We’re using external temperature sensors, always insulated from the outside. The Analog Devices / Maxim / Dallas Semiconductor DS18B20 is perfect for this, but the problem is that most ‘DS18B20’ sensors on the market are fakes; clones marked as the real thing. These work well for the most part, but they are susceptible to issues. Working around issues from these clone / fake sensors has been a major headache. Our software tries to detect them, and we allow the user to enter a bias offset in case they read high or low.
It’s always best to buy genuine items from DigiKey, RS, Farnell, Mouser, etc. but we know that many people will not.
However, the DS18B20 does not have a sealed package and cannot be submerged in water. If you need to submerge the DS18B20 in a liquid, several third parties manufacture sealed temperature probes that use the DS18B20. Maxim does not endorse any particular manufacturer of these probes nor maintain a list of these probe manufacturers. Most probes that we found do not have genuine chips in them.
Lots of information here.