Thanks for you reply. I am trying to define the "Fire Profile" expression according to the attached graph. As far as I know, I have to insert some units to let the solver reads it correctly, unless if there is another way to do so, then I will appreciate knowing it. Just to make it clear, the message I am receiving is referring to the expressions below, which the expressions above are a part of. However, the reason why I post the " Fire Height" expression is that I started facing this warnings once I defined the "Fire Height" expression.