I was looking at the chart I use compared to another and it seems like they don’t always agree on best values or even a good range of values. This was a common theme as I looked through a few of them earlier today.
I also noticed some are based on ambient temps and others based on leaf surface temps.
Which do you use and why?
Some thoughts on VPD calculations.
Both ambient and leaf temperatures are used to calculate VPD.
A simplification can be made by assuming the temperatures between the two are equal. It should get you close enough under most circumstances since VPD is something of a guidance as opposed to the rule.
Also interesting, when the calculation are made we assume that the leaf is 100% humid and able to transpire (e.g. alive).
Pictures? I’m new to vpd charts as of 1 minute ago
VPD is a great thing to learn for optimizing your environment. Chart examples: Check out @SuperiorBuds integration of VPD calculations into his set-up, over here:
@plant also did something in Javascript:
Formulas for doing the calculation yourself are there as well:
This is quite interesting, im assuming the formulas are calculated on different people’s tests and data. It seems like we need to investigate data and also strains used it’s definitely a good ballpark thing to just pickup but there’s a million variables. You may have to collect data and make your own for a strain for it to be a proper guide.
Yes, you could dial it in.
There are general guidelines for different stages in the plants life, too. Seedling, vegetative, flower, etc. Don’t really recall the guidance off the top of my head but there are many suggestions of what are the most productive ranges are under differing conditions around and about.
Has anyone tried pulse? I’m interested but waiting to see if the next version also measures CO2. Not sure about the light accuracy either, most cheap sensors only do visible light.
Yes, we have a couple. They are not very accurate imo.
So. After some thought my question is how much would running the plant at 90-95% leaf humidity (under watered) effect the graph
I don’t believe a leaf would ever be considered less than 100% humid unless it was severely under-watered such that the leaves were essentially drying out.
The vapor pressure of the leaf is always considered to be the saturation vapor pressure because we’re evaluating the conditions just inside the leaf stomates. Due to the proximity to unprotected internal leaf tissue, the small air volumes inside of the stomatal cavities are considered to be saturated. So once we measure the leaf temperature, we can use an equation to determine the saturation vapor pressure of the air inside the stomatal cavities.[1]
It’s an interesting question though. I’ll look around for any references where the leaf vapor pressure is considered less than the vapor saturation pressure. There are conditions when things go wrong such as too much light where the leaf temperature ends up something higher than the ambient (typically it’s slightly less if the leaf is has normal transpiration – think evaporative cooling).
Typically your target is to adjust the environmental conditions to achieve the desired VPD to optimize the plant transpiration rates (and hence optimize the nutrient uptake).
Maybe it’s coincidence, maybe not, but it really seems to me when plants are in the proper VPD it helps considerably with pests & pathogen problems.
I much appreciate all the help explaining vpd to me so Thank You everyone! I’m not sure which to use, maybe I’ll use @George 's one because it’s got the broadest area hahaha
I actually found an android app the I like called simply “VPD Calculator”
It takes ambient temp, leaf temp and RH and then spits out a kPa number.
On the bottom it lists good ranges for each stage.
At least this chart helps against PM. More in line with 50% RH.
