In the documentation of the Hydrocarbons Parser, we have explained why temperature relative errors should be expressed in the Kelvin scale.
According to a Wikipedia article, a relative error only makes sense when measured on a ratio scale, (i.e. a scale which has a true meaningful zero), otherwise it would be sensitive to the measurement units (https://en.wikipedia.org/wiki/Approximation_error). The reason is that Celsius is an interval scale, whereas Kelvin is a ratio scale (with a true zero). As temperature relative errors vary across scales (Kelvin, Celsius, and Fahrenheit), these should always be computed in degrees Kelvin.
The above is easy to prove. Because oC = K – 273.15, the relative error in the Celsius scale is ΔoC/oC = ΔK/(K – 273.15); i.e., without a true zero.
Moreover, ΔoC/oC is undefined for ΔK ≠ 0 and K = 273.15, and indeterminate for ΔK = 0 and K = 273.15.