An absolute scale is a measurement system starting at a true zero point. In thermodynamics, the Kelvin scale is the absolute temperature scale. It starts at absolute zero. Engineers use this scale in thermodynamic equations because it makes gas volume and pressure calculations direct and proportional.
Rankine is another absolute scale. It uses Fahrenheit degrees instead of Celsius. It is commonly used in aerospace and steam engineering in the United States, though Kelvin is the global scientific standard.