- The Rayleigh refractometer utilises the refractive index of a gas to calculate its concentration.
- Thermal conductivity is used in katharometers. In these devices, the cooling of a wire causes a change in resistance proportional to gas concentration.
- Solubility is employed in devices such as rubber strips when an increase in length accompanies gas absorption.
- Light emission features in the Raman light scattering measurement device.
- Both infrared and ultraviolet absorption are used in gas concentration measurement.
The Lay Medical Man 
I find it fascinating how diverse methods like refractive index, thermal conductivity, solubility, and even light emissions can be utilized to measure gas concentrations.
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