G gnome said:

I thought it was vapor pressure deficit...?

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Yep....you have it right. What it exactly means i dont quite get but it is a pressure value cause it is expressed in kilopascals.

Dee

I guess it is the difference between dew point and the and the humidity measured value. Between dew point and the less than saturated value. There must be a measurable pressure difference.

Dee

Call me ol fashioned but im shooting for 75deg and 45%

I usually run at 75 - 80 f with a RH of between 50 - 60 % during veg and lower RH to about 40 % when in flower. A dehumidifier/humidifier is the perfect tool for the job. In my opinion they run about the same cost as an average aircon unit. Im looking at puchasing a fairly economical all in one. Right now I place black bin bags loosely over pots and this helps bring down humidity somewhat. ( I use fabric pots that release a lot of moisture) I also adjust fan controller so a costant airflow keeps dry air coming in. This is dependant on outside conditons. If humidity is high outside it wil draw moist air in. So occasionally have a struggle. I also have lights on at night during summer months taking advantage of low temps. Hope this is helpfull in some way.

G gnome said:

Call me ol fashioned but im shooting for 75deg and 45%

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Thats not allowed ...chart says so. That would put you at 19.3.... And plants cant grow at 19.3

According to chart I am at 17.8 which is in the red :-( we need transpiration ahhhh !!! All seems allright thouh.