Hi @Roi_Aranyi,
You will want to make use of a Timer Node and a Conditional Node. The Timer Node will trigger after a set amount of time, and the Conditional Node will give you a boolean that sends you right or left for true and false. So you will go right (true) if the freezer has been above -20 degrees, and perform some actions afterwards for handling the lower temperature (email notification, text notification, alert). The On Change Node may also provide some of what you are looking for, or you could do a timestamp comparison of the last recorded value to check for those 10 minutes (details found here).
There is also a feature request here that could benefit, so be sure to vote!
Let me know how else I can help,
Julia