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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.

Boiling Water at 70 Degrees F

I was able to boil water at 70 degrees F today.  This is the first test to see if the pump can reach appropriate levels of vacuum. 

Once a few bubbles rose to the top, the water broke into a vigorous boil within seconds.

It is important to note that the chamber pressure will not go lower than the pressure required to boil the water.  The boiling of the water replaces water vapor being evacuated form the chamber.

Saturday, November 20, 2021

Vacuum Pump Testing Started

 I started testing the vacuum pump to see if it could draw down lower than previous tests.

I was only able to draw about 25 inHg in previous tests.  So today, I started tracking down issues with the vacuum system.  

What I found was encouraging.  See, to be used in a freeze dryer, the pump has to draw down to 600 Pa or lower.  A rough calculation tells us the pump will need to draw down to less than 29.2 inHg.  

Sealing hoses and fittings for vacuum use can be a challenge.  A leak does not manifest itself as a drip, for instance.  It cannot be observed from outside.  Basically, everything is backwards.  

I found the vacuum canister had a design flaw in its fitting port.  I fixed that flaw.  It took some doing, but the seal is now reliable and vacuum tight.

I fixed a few hose leaks and a leaky valve packing.

Sealing most vacuum leaks can be performed like any other leak.  The difficult part is finding the leak.

I used Vacuum Grease to seal threads and hose fittings.  It is designed to do just that.  

I was eventually able to draw down to the limit of the gauge on the vacuum canister.  But it is way better than it was in previous test.  I will have to hook the vacuum sensor up to the canister to get a more accurate reading.

Monday, November 8, 2021

Water State Library Progress

I am going to develop the water-state library in two stages.  I have decided to abandon the complex state for now.  It is taking more time than I want to spend right now.  I will only focus on the simple-state aspects so the library doesn't delay the FreezeDryer project any more.