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Wednesday, November 24, 2021

Triple Point Vacuum Achieved

After the boiling test, I decided to up the ante.  The ultimate test of a vacuum pump for a freeze dryer is to prove it can reach the triple point of water.

To prove this, I made some ice water.  Ice water will reach a temperature of 32 degrees F (0 degrees C.)  If I can draw a hard enough vacuum on ice water to make it boil at 32F, I can reasonably assume the vacuum pump can reach the triple point.

I was able to boil ice water in my home lab using a 2-stage oil-diffusion vacuum pump.  

A few things to keep in mind during this experiment:  

The boiling of ice water will not be as vigorous as you might expect.  That is because at that vacuum level, it takes very little water vapor to raise the pressure above the triple point.  You will likely just see a simmer.

The triple point, while reached, does NOT indicate the vacuum pump cannot go lower in pressure.  The boiling of the water continuously replaces evacuated water vapor from the chamber.  Essentially, the pressure will not go lower than the pressure that causes the water to boil until all of the water is gone.

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