Old Strains - New LED Light!

[Delps8]

For those who have had positive results using VPD, what calculator or chart did you use? What strain were you growing? Every calculator and chart seems to give different final values. 79.5F at 85RH has given a PVD of .49 to 1.45 on different calculators. Charts range around .8 to 1.2. Is there an offset range to accommodate for the needs of arid vs tropical strains?

I just look for the strains ancestral origin and then replicate the natural climate conditions. Generally this puts me around 30RH over the average recommended PVD with equatorial strains. F/RH; 80/80 seedlings, 80/75 veg, 75/70 flower. Reducing RH causes the plants to stop praying and wilt. Just curious if anyone has further insight into this.

VPD is nothing more than one number that covers a range of temperature and RH values. It's like the "feels like" temperature for human beings.

The reason for it describing a range of temperature and RH values is because it's measurement of the difference in vapor (water) pressure between the plant and the atmosphere. In plant terms, that's "transpiration".

If the temp is 77 and the RH is 62 that's a VPD of 1.0. A plant in veg will transpire "normally" at that temp+RH (VPD). If the temp is 64 and RH is 62 that's "cold and clammy" to a human and, if you check the chart, the vapor pressure deficit is lower at 0.64 because it's not as warm as before (77°).

Take that same 77° room and run a dehumidifier full throttle and the RH will get down to, say, 34%. That's the same temperature as before but the RH is waaay down so the plant will give off more water/transpire more.

That's all that VPD is about. If your VPD is high, you're going to need to water more because your plants are transpiring more. The issue is that if you don't adjust the strength of your nutrients, you're going to get more nutes in your plants than they can handle.

If VPD is too low, that's increases the chances of getting disease in your grow.

 

[Sativa1970]

I understand pressure gradients, what and how VPD works. All plant bio chemistry functions on pressure gradients. My question was why is it, that the charts and calculators, have so much variation, from one to another. I know that there are different charts for different stages of growth. Plugging the same temp, RH and stage into several calculators or charts tends to yield different results. Calculators do seem to be far less accurate. This is just the first chart from a google search result. Notice they don't exactly match. 75F 60RH is a perfect VPD 1 on your veg chart but that would be a 1.19 on this chart. Perfect for late flower. Good science is repeatable. Why isn't this repeating?