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I would like to know if the lid should be kept down when not in use?
For a ventilated model, the advantage of keeping the lid closed is to keep the chamber clean, by not allowing ingress of particles, bacteria, solvent vapours and so on.
You should, in any case, remove the rotor of any centrifuge and clean the bowl and rotor once a month.
For a refrigerated or thermostatted centrifuge, keep the lid closed. Some models will keep trying to maintain the bowl at the programmed temperature even with the lid open, and this be trying to maintain the set temperature in the whole lab if the lid is open! If they do not try to maintain temperature when the lid is open because of the absurdity of doing so, then your centrifuge won't be ready when you need it.
Keeping the lid closed is even more important if the set temperature is low. Lab air entering the bowl will have its vapour condensed onto the bowl so you will soon have water in the bowl. Also, by keeping the lid closed, your chamber will be ready for immediate use at the required protocol temperature. You rotor / buckets / adapters should also be kept inside for the same reason.
Don't forget, however, that unless you pre-cool your samples, it will take some time for them to reach protocol temperature.
I hope this helps!