To dynamically format a thermometer, use the SET FORMAT command. Make sure that all of the settings go into SET FORMAT as a single string, like this:
SET FORMAT(myTherm;"0;100;10;10;18;### ##0")
The trick is to put ALL of the arguments between quotes in a single parameter to SET FORMAT. You can use this approach to dynamically format any 4D object that has format settings. Use the Object Properties Palette to see the correct arguments for the object you are working with.