help.......rtd

we connected RTD to reactor, and the measured temperature is indicated on indicator.

and I have question:

i don't know why the equation y=(x-325)/2.28+20 exist. I mean what is purpose to use this equation which is given  in the center of the picture.

Here, the calculated temperature "y" is displayed on indicatior like below picture.

y-axis unit is ℃ and x-axis unit is time

please help me..