The manufacturer does not recommend the sensor be placed directly in any liquid. Instead a thermal well should be used to protect the sensor.
A thermal well (or bulb well) is simply a tube sealed on one end in which the sensor is placed for protection and the well is place in the liquid. The sensor should make good contact with the inside of the well and the well itself should either be thin walled and/or made of a conductive material so as to not insulate the probe and delay temperature readings. Thermal wells may be made of copper, stainless steel and even plastic.
Johnson makes a copper thermal well (WEL11A-601R) with a threaded application that is suitable for water tanks, boilers and other applications. For corrosive liquids stainless steel or other material that is compatible with the liquid may be used.
A copper thermal well is not recommended for freshwater or salt water aquariums because the copper can be toxic to fish, corals and other aquatic life. Instead, thin walled PVC or vinyl tubing is recommended. We do offer 3/8" clear PVC tubing for this purpose that will shrink to form a tight fit. Some hobbyist will instead simply coat the sensor in silicone to protect it.