### 14.2.4. Using Expressions

The interaction with CEL consists of two phases:

- a definition phase, and,

- a use phase.

The definition phase consists of creating a set of values and expressions of valid syntax. The purpose of the Expression details view is to help you to do this.

#### 14.2.4.1. Use of Offset Temperature

When using temperature values in expressions, it is generally safer to use units of [K] only. When units are used that posses an offset (for example, [C]), they are converted internally to [K]. For terms that have temperature to the power of unity, any unit conversion will include the offset between temperature scales. However, in all other cases the offset is ignored because this is usually the most appropriate behavior. You should therefore take care when specifying an expression involving non-unit powers of temperature. For example, each of the expressions below is equivalent:

    Temperature = 30 [C]
    Temperature = 303.15 [K]
    Temperature = 0 [C] + 30 [K]
    Temperature = 273.15 [K] + 30 [K]

These are only equivalent because all units are to the power of unity and units other than [K] appear no more than once in each expression. The following expression will not produce the expected result:

    Temperature = 0 [C] + 30 [C]

This is equivalent to `576.30 [K]` because each value is converted to [K] and then summed. The two expression below are equivalent (as expected) because the offset in scales is ignored for non-unit powers of temperature:

    Specific Heat = 4200 [J kg^-1 C^-1]
    Specific Heat = 4200 [J kg^-1 K^-1]

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