# 4.5.3. Universal Expressions

While User Expressions can be used in Aria materials, they are often better suited to be applied globally. For example, if you provide a user expression to convert temperature from Kelvin to Celsius it would be repetitive to have to define that in every material. The universal expression block (at the Sierra scope in the input file) allows you to define such expressions that will be valid on all blocks, regardless of their material type.

    Begin universal aria expressions
      User Expression = scalar_string_function user_tag = "Tcelsius" f = "temperature - 273.15"
      User Expression = scalar_string_function user_tag = "radius_cm" f = "sqrt(x^2 + y^2)*0.01"
      Axial Coordinates = Vector_Aligned axis = 1 0 0 origin = -0.7 0 0 ref = 0 1 0
    End

Expressions defined at this scope are available to use in all material properties, boundary conditions, sources, initial conditions, or post-processors. The only types of Expression that are allowed in the universal expression block are `user expression` and `axial coordinates`. Outside of string functions, coordinates x, y, z referenced in models should use coordinates_x, coordinates_y, coordinates_z instead.