Troubleshooting
Freeze dryer will not pull vacuum: what to check, in order
Nine times out of ten it is the drain valve, the door gasket or tired pump oil. Work through them in that order before you suspect the pump.
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Troubleshooting
Case hardening: why dried fruit is sticky but still wet inside
The surface dried faster than the water could travel out from the middle, and sealed itself. It is called case hardening, and four settings cause it.