After the food is frozen down to the initial freeze temperature in your settings (default is -10 degrees Fahrenheit), your freeze dryer will move into the vacuum freezing process. During this process, the vacuum pump turns on as signaled by the freeze dryer. Be sure that your vacuum pump power cord is plugged into the back of the freeze dryer (not the wall) and that your vacuum pump power switch located on the back of the pump is in the (I) or ‘on’ position. In order for your pump to pull a deep vacuum, you need to ensure that you have a closed system. This means that the drain valve needs to be closed tightly, the door needs to be closed tightly (turn the latch twice), and the vacuum hose needs to be connected tightly at both ends.
As long as you have a closed system and your vacuum pump has been well maintained (clean/transparent oil in the sight glass for oil pumps), the pressure in your vacuum chamber should start to drop. As the pressure starts dropping, the process of sublimation starts to occur. Sublimation is the process of water going from a solid (ice) to a gas without passing through the liquid state. The frozen water in your food will start turning into a gas and will come out of your food and freeze onto the chamber walls. This sublimation process starts to happen while your vacuum pressure is in the thousands, and will happen more and more as the pressure continues to drop. During this time, the freeze dryer is sensing the rate that the vacuum pressure is dropping and monitoring that rate to make sure your batch will be successful.
One thing that can cause the vacuum pressure not to drop very well, is if there is unfrozen water still in the core of your food. If this is the case, that water will be getting sucked into the vacuum pump. The pump is only supposed to be vacuuming air (not water) and if it is vacuuming water, the pressure won’t drop as well. If the freeze dryer recognizes a slow pressure drop rate, it will assume that this is happening and that the food needs more freeze time to ensure that unfrozen water does not continue to go into your pump. This is a safety feature that will add years to the life of your pump by not putting excess water into it. If this happens, your pump will temporarily turn off and the freeze dryer will go back into the freezing cycle for 30 minutes to an hour to ensure everything is frozen properly. After doing this, the freeze dryer will go into vacuum freeze and turn the pump back on and try again. If you find your freeze dryer is repeatedly doing this, it would be wise to check to be sure that you have a closed system. It’s possible that the reason the pressure isn’t dropping is because there is air getting in through the drain valve or the vacuum hose, etc.
For reference on making sure each of these things is done correctly and that there are no air leaks, check out this short video:
How to Look for Leaks When the Pressure is High
