In short: Clear the unknown variable first: Clear(T.2)=

The equation you are solving for is evaluated in the current context (all known definitions are substituted). In your FindRoot-Example, the variable T.2 which you are solving for, already has a value (just type T.2 to see it). Thus, after substitution, your equation doesn't contain the variable any more.

This is a typical mistake in equation solving. Always make sure that your unknowns are indeed unknown. This is also explained in my example upstream the topic.

Wrote

OK, going back to sanity check some values.

As it looks: A US standard of mild temperature.
1srt thing to: give the link to the standard.

Wrote

In short: Clear the unknown variable first: Clear(T.2)=