Working of a Constant Volume Gas Thermometer

[Ali Asadullah]

How a constant volume gas thermometer works?
In "Thermodynamics An Engineering Approach" by Cengal, author says that this type of thermometer is usually filled with Hydrogen or Helium and We use Constant volume gas thermometers for measuring temperatures near absolute zero.
Now if volume is constant and suppose we are using Hydrogen, then Hydrogen will solidify at 14.01 K. Then how can we measure the temperature when hydrogen becomes solid and in this state of matter, the postulate that Temperature is proportional to Pressure will not be valid.
Also let us suppose that we have container of constant volume filled with the gas, if we start decreasing its temperature, then its pressure and temperature will decrease but volume will remain same and it will never solidify. Where is the mistake in this statement?

 

[Chestermiller]

If the pressure is low enough, the gas won't become solid. The gas pressure has to be higher than the equilibrium vapor pressure of the substance at the temperature in question for the gas to begin to solidify.