Answer: There are many ways to address this issue. Our current recommendation is to fill the casing with a water + propylene glycol solution. Propylene glycol is a non-toxic antifreeze fluid. Although this solution should be harmless, always wash off the probe and cable after use.
Here is a table showing the freezing point for various solutions. The table is provided by engineeringtoolbox.com.
|FREEZING POINT - PROPYLENE GLYCOL AND WATER SOLUTIONS|
|% Solution by mass||0||10||20||30||40||50||60|