Maybe start with a Condition interface for your models.

Create models and tables for boolean operations: not, and, or. Not will have one polymorphic condition, the others many.

Have 'value providers'. For example a temperature provider class.

Implement comparators implementing the condition interface. You can have an Integer comparator. When saving it to the database you save the comparison method '<=', the value to compare agains (10), the polymorphic device it refers to (device id 2) and the name of the value provider class.

In a similar fashon you can define actions to be executed when the conditions are met.