Gas Thermometers and the Kelvin Scale

A constant-volume gas thermometer measures temperature by monitoring pressure changes in a sealed gas-filled vessel. As temperature increases, the gas pressure rises proportionally while volume remains fixed. This direct relationship between pressure and temperature allows precise temperature measurement and calibration across known reference points.

Absolute zero is the lowest possible temperature where molecular kinetic energy is theoretically zero, representing the complete absence of thermal energy. The Kelvin scale is an absolute temperature scale with its zero point defined at absolute zero. This makes the Kelvin scale ideal for science because physical quantities like ideal gas volume are directly proportional to absolute temperature.

Gas thermometer calibration involves measuring pressure at two known temperatures: 0 and 100 degrees Celsius. These pressure values are plotted on a graph and extrapolated as a straight line to find where pressure reaches zero. This zero-pressure point defines absolute zero and establishes the zero point of the Kelvin scale.

The Celsius and Kelvin scales have identical temperature intervals; the difference between freezing and boiling points of water is 100 degrees on both scales. However, their zero points differ: water freezes at 273.15 K and boils at 373.15 K. Converting between scales requires only adding or subtracting this constant offset.

The constant-volume gas thermometer is the standard because its pressure readings correlate directly and linearly with temperature. When results are extrapolated to zero gas density, different gases produce consistent measurements that agree with absolute zero. This reliability and reproducibility make it ideal for calibrating other thermometers, despite being large and slow to reach thermal equilibrium.

The triple point is the unique temperature and pressure condition where ice, liquid water, and water vapor coexist stably. Unlike freezing and boiling points, which vary with pressure, the triple point is constant at 273.16 K. This fixed reference point provides a more precise definition for the Kelvin scale than water's phase transitions.

Kelvin is the SI unit of temperature because it is an absolute scale with zero at absolute zero, making it ideal for scientific calculations. Many physical relationships, such as ideal gas behavior and thermal radiation, depend directly on absolute temperature. This fundamental connection between Kelvin and physical phenomena makes it essential for accurate scientific work.