Kelvin and Celsius can both be used interchangeably and you can always get the same answer every time using either; they are equally as precise. So is fehrenheit for that matter, although the conversion would get even more complicated.
It’s just usually using the one with zero offset makes the math easier, which is why it tends to be the one used for scientific calculations.
When the measurement being used is ∆T, change in temperature, this is correct. Occasionally, like in the ideal gas law equation, the measurement is T, or absolute temperature, which requires zero offset. In these cases, Celsius will give the wrong answer.
As I said
It’s just usually using the one with zero offset makes the math easier
You can use Celsius in the ideal gas law. You just have to make sure to include the offset in your calculation. There is no loss of precision by using Celsius, and it isn’t wrong. It’s just the math is easier if you use kelvin, because as you point out (in this case) it’s the ratio of the absolute T that’s important, and a delta T is not enough.
By including the offset in the calculation, you have converted to kelvin.
