Walter Stark said:

Yes, I agree. But the transpiration rate also decreases at night. This is the value I am seeking. Looks like Hydro-Guy has done some real data gathering which reveals an 85%/15% split in day/night transpiration rates. Any other collaboration would be appreciated.
Thank you all

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Wouldnt you need to figure in evaporation too?And it would all be different in different rooms with different size plants and methods. I assume open pots will evaporate more than a closed res with bubble buckets for example.

I thought you just compensate for the max. 100% of the input and contained water will come back out. Then the controllers are set for well, control.

Plants don't transpire at a static rate. Different each stage, size health. Wattage temps. Etc.

You are correct. We need to account for Evapotranspiration. Essentially all of the water for irrigation ends up in the air and this is why we focus on irrigation rates.

The dehumidifier must be sized for worst case. Would this be at the very end of the grow cycle?

Yea it's definitely relative to temperature ,plant size ,air movement ,strain , medium type and grow method . I run a sealed room and all dehumidifier/ac condensate collects in one reservoir. In my room it collects an equal amount day and night. I only run dehumidifiers during dark cycle except near the end of bloom to mitigate rot risk. My ac removes humidity during the day and at times I need to ADD humidity to maintain correct vpd. If it's warm and windy plants will transpire ALOT at night. This can be used to your advantage to control stretch and nutrient uptake.....

Walter Stark said:

You are correct. We need to account for Evapotranspiration. Essentially all of the water for irrigation ends up in the air and this is why we focus on irrigation rates.

The dehumidifier must be sized for worst case. Would this be at the very end of the grow cycle?

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More than likely during transition (stretch) in a full room. Or if long veg. There is the most foliage produced as and after they mature. So depending on lighting maybe 45 days from seed and 30 from clone until the end of veg. The second month.

Any large room growers know for sure?

I think it's split in my room 50/50 but my dehuey (quest105) doesn't run alot during lights on at all but it runs alot at night.I guess the only way to really tell would be to monitor how much condensate my mini is dumping outside during lights on and compare the two.

fishwhistle said:

at lights out with a/c off on the other hand it spikes and dehuey comes on.This is sealed room of course.

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I have seen this with CO2 enhancement levels ( 1200-1500ppm ) in my sealed room data, a distinct 'vapor dump' spike that needs cleanup at lights off. CO2 Transpiration ceases ( if you monitor the CO2 level, you can see it stop promptly ) however there seems to be a reserve left in the vascular system that continues. There also is the CO2 molecule to water molecule ratio, so perhaps a reduction in CO2 before lights off would head off the vapor dump. Our artificial light sources are rather digital ( on or off ) and that is probably a factor. No ramp-down (sunset) to wind down activity - so some anomalous curve should be expected on plant response in transpiration when the light snaps off.

Carbon dioxide entry: When a plant is transpiring, its stomata are open, allowing gas exchange between the atmosphere and the leaf. Open stomata allow water vapor to leave the leaf but also allow carbon dioxide (CO2) to enter. Carbon dioxide is needed for photosynthesis to operate. Unfortunately, much more water leaves the leaf than CO2 enters for three reasons:

1. H2O molecules are smaller than CO2 molecules and so they move to their destination faster.
   
2. CO2 is only about 0.036% of the atmosphere (and rising!) so the gradient for its entry into the plant is much smaller than the gradient for H2O moving from a hydrated leaf into a dry atmosphere.
   
3. CO2 has a much longer distance to travel to reach its destination in the chloroplast from the atmosphere compared to H2O which only has to move from the leaf cell surface to the atmosphere.

This disproportionate exchange of CO2 and H2O leads to a paradox. The larger the stomatal opening, the easier it is for carbon dioxide to enter the leaf to drive photosynthesis; however, this large opening will also allow the leaf to lose large quantities of water and face the risk of dehydration or water-deficit stress. Plants that are able to keep their stomata slightly open, will lose fewer water molecules for every CO2 molecule that enters and thus will have greater water use efficiency (water lost/CO2 gained). Plants with greater water use efficiencies are better able to withstand periods when water in the soil is low.

Water uptake: Although only less than 5% of the water taken up by roots remains in the plant, that water is vital for plant structure and function. The water is important for driving biochemical processes, but also it creates turgor so that the plant can stand without bones.