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May be a basic question, but I'm a technician, not a physicist (or physics student.) I came across triple point cells in some research on practical thermometer calibration, and I'm having a hard time wrapping my brain around the idea of a triple point.

This might be a more complicated question than I realize, and I may not understand the answer, but I'm picturing a triple point of water cell like :

https://us.flukecal.com/products/temperature-calibration/its-90-temperature-standards/its-90-fixed-point-cells/5901-triple-p?quicktabs_product_details=2

My question is, looking at the PT graph of water...

https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=imgres&cd=&ved=2ahUKEwiT4KLNgo7aAhVX0WMKHbyhBkkQjRx6BAgAEAU&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FTriple_point&psig=AOvVaw04Mu1DS0pB9CE7vvco7HAk&ust=1522292227660057

What would happen (and is it physically impossible for some reason I'm not seeing) if, starting from the triple point ( 0.01 Deg. C at ~611 Pa , contents 100% water ) , the internal pressure of the cell was raised, either by the introduction of additional liquid water at ~the same temperature, or by reduction of the volume of the cell (as with a moveable piston).

I'm hung up on the fact that both liquid and solid water are mostly incompressible, but the phase diagram seems to indicate that the vapor phase would be excluded if the pressure increased. How would the sample behave, what would happen to the volume in the cell previously occupied by the vapor, or would there be a consequent change in temperature to keep the contents in one of the vapor-allowed regions (like the ice melting to keep the contents on the vapor/liquid transition line?)

Thanks in advance

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The point on the phase diagram represents the stable equilibrium phase at that point. It doesn't mean that other phases can't exist there temporarily.

You're correct that as you increase the pressure, the equilibrium shifts away from the gas phase. After that, some of the water leaves the vapor phase and either freezes or condenses. But since pressure is dependent on the quantity of water in vapor phase, the pressure goes back down afterward.

If you maintain the elevated pressure continuously (say with a constant force on the piston), you'll drive more and more of the water from the vapor phase into liquid or solid.

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