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Michael L. Reed said on 13 November 2008

"Incubators part three - relative humidity.

You want maximum humidity if you are doing embryo culture, even if you have oil overlays for your dishes, tubes and so forth. If you are doing open culture, you really - stress really - need to maintain maximum humidity.

First, older incubators, and a number of the new ones, rely on CO2 sensors that require maximum humidity for accuracy.

Second, anything placed into a dry incubator will be subject to dehydration at 37C - for example, open cultures (no oil overlay) will dehydrate quickly, raising osmolality to extremes and killing cells quite readily. And with oil overlay, I still worry about humidity - I've seen the character of the oil change under dry, heated conditions, and that may translate to problems in the microdrops.

As for measuring humidity, why spend money on a tool to do this - 98%, 99%, etc.? The easy answer is to maintain at least one large pan of water in the incubator at all times, and if you are in a very dry environment, a second pan does not hurt.

I don't trust the manufacturer's recommendations when it comes to water pans - I have two newer incubators where the single pan provided might hold about 1 liter - so I purchased large rectangular glass pyrex baking pans that hold about 4 times the water, and use one of these in addition to the little pan supplied with the incubator.

Change the water every week to two weeks, but keep the pan or pans full. If your incubator has a door heating unit, you may not see condensation on the glass, but often you will. Condensation is a good sign that you are maximizing humidity."

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