Units of Temperature
Most modern-day thermometers and instruments use either the Celsius or Fahrenheit scale. The difference between the freezing and boiling points on the Fahrenheit scale is 180 degrees while for the centigrade scale it is 100 degrees. As we can see, temperature has to be represented on some scale, and without any calibrations it is not possible to measure temperature accurately. The size of the temperature unit and the zero point of the scale is necessary for the thermometer to give correct readings
Read more at Buzzle: http://www.buzzle.com/articles/farenheit-to-celsius-conversion.html