I attached a Prime 9 worksheet which fixes some of the errors you made.
First error is about the temperature unit. You type ° and C and so you defined and angle in degree multiplied by unit Coulomb.
You have to use the unit °C from the unit menu.
Later you calculate the temperature difference and even with correctly defined temperatures you probably would be surprised by the result you get.
Temperatures are internally stored in basic unit Kelvin.
So we have
But 725 Kelvin converted to degree Celsius is
Sure not what you expect as temperature difference. That's the reason we also have 'unit' Delta°C (which basically is an alias for Kelvin).
But the wrong definition of the temperatures had nothing to do with the calculation of TH.
Here you made an error when defining Ump. You used "g" for grams, but actually "g" is the predefined constant of gravity. You can tell that its a constant and not a unit by the color - its green and not blue. In Prime you have to use "gm" for unit gram.
Feel free to define a unit "g:=gm" somewhere at the top of the sheet if you want use "g" for gram. Actually "g" is the only unit symbol for gram that is recognised by the SI (International System of Units). "gm" is not allowed by SI standards to avoid confusion with gram-metre g*m.
Now at last you used a factor m when you defined TH. Guess it should be omitted to get the desired result:
Prime 9 sheet attached
