If your air is cooler and you have a humidifier you can achieve some decent temp and rh control by turning on the humidifier when RH is low and venting when your temp is high or RH is high.
As soon as my lights go out not only the air temp drops. The leaf temp drops even further… and this is used to calculate the VPD, right? Therefore it’s almost impossible for me too keep up the VPD like on daytime…
One way you might be able to counteract this is by ramping set points. You can prime your plants for the night period by bringing the light, and perhaps also temp and humidity down, if your controller allows for it. By ramping the lights down, there will be less leaf heating from the lights, and my guess is that the mechanisms they use to shed water in order to keep themselves cool will ramp down as well.
Also, as the plants get further into the night period, they transpire much less and you’ll see less leaf cooling so the effect you notice is transient, but I’m not sure by how much (maybe an hour?)
Another idea is to warm the room for the first hour or so of the night period, assuming your dehus can’t keep up with the spike of demand at the transition. Peak dehumidifier load is the first hour or so of lights out before the plants “shut down” and the AC is no longer participating in dehumidification. Dehus are significantly more efficient at warmer temperatures, so it will remove more pints, but also the warmer temperature will result in a higher VPD even if the humidity doesn’t change.
But I don’t think night vpd matters very much. I get good results by lowering humidity to 50 at night and this seems to give good results regardless of the night temperature. The studies referenced in the Ultimate VPD guide mainly warned against major and frequent fluctuations in vpd and if I remember correctly none of them were specifically cannabis studies. A single day/night fluctuation from 1.0 to 1.5 sounds like a lot, but is pretty minor compared to most of the studies. Gas exchange also isn’t everything; I use the night as an opportunity to get a deep dryback so I like a higher vpd. I want O2 to get into the roots, which is more important to me than getting O2 into the leaves.
If you look at CO2 rise during the night in a sealed room, you can see that after about 4-6 hours the rate goes way down. I don’t think cannabis has enough reserves to take advantage of high gas exchange all night anyway.
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