VPD calculations. Why is this chart SO far off?

Advanced Techniques
#21

Been using one for a while, for more than just leaf surface temps! lol…

I found a module for infrared temp that I can use on an esp32 board. Going to make a “leaf temp thermometer”, and incorporate it within home assistant providing accurate numbers to calculate VPD :slight_smile:

Once thats automated (calculating the REAL VPD to worry about), I can then program some logic to control the environment based on stage (seedling, veg, flower), and adjust what needs to be adjusted for the current optimum VPD. Since temp is harder for me to control (mainly cooling), but RH is easier to adjust, I will adjust humidity for the current temp at the optimum VPD. I can program it to lean towards adjusting one or the other, or both depending on a checkbox or two.

I agree VPD is often overlooked. Never thought I would add a humidifier for growing, but for young plants, it really can make a difference! That started me down this rabbit hole. Ordered the parts I need, will play mad scientist and design my LST sensor this week once I have them in hand, then program and see how it works :smiley:

I am currently flowering at 50% rh. But my temps are mid to high 70s. Current VPD is 1.2 according to the dashboard with a programed (and actual) LST difference of 4° at the moment :smiley: Automating that LST reading will be the last part of the equation I need mwuhahahahaha (mad scientist laugh)

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#22

What’s crazy is vpd was a thing 10 years ago. It was a thing 20 years ago. But I always over looked it not giving a “@&$. I read my ancient high times mags from time and time on. And a lot of those interviewed farmers worry about vpd!!! Maybe it was just me. But my excessive and extensive studying of the category leads me to believe I’m not the only person in the boat. Especially after seeing a lot of great growers maintain a kPa level of 1.5 or even higher thru an entire grow. Not knowing any better. Like I said earlier. Intimidated by high humidity. I’m sure mine goes up and down from me opening and closing doors. But my average always remains the same. But then again. Some focus to much on a one thing, forget another and shit hits the deep the end quick. So it’s better to slowly but gradually graft yourself into maintaining a lower than normal vpd. Imo

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#23

What formula do you use to calculate VPD that doesn’t use ambient temps to calculate the VPair to subtract from the VPsat? The one I learned was:
VPair = [610.7x10^(7.5T)/(237.3+T)/1000] x RH/100 where T = ambient temps and RH = relative humidity
VPsat = [610.7x10^(7.5T)/(237.3+T)/1000 where T = leaf temperature

finally,

VPD = VPair - VPsat

Although I know there’s other ways to calculate it, unless I misunderstood what you meant when you said


McKushin

12h

Idk why they even use the room temps. Leaf surface temp and rh are the only things that matter for your true kPa’s.

Blockquote

Other then that I fully agree with everything you’ve said, VPD is one of those commonly understood things in horticulture that was rarely mentioned among growers until far too recently. It’s a wonderful tool and really drives home how interdependent temp, humidity, watering and fertilizing are, which enables a lot more control when you understand that dynamic instead of just at the commonly suggested 40-50% and 24C range often parroted. The nuanced understanding of how to grow in different climates without fighting the ambient conditions by learning to balance and work with the conditions you have is often lost. Although I still like to drop the humidity at the end of flowering based off the UoG paper on optimal crop irrigation strategies which suggest slight water stress during flowering increased trichome/cannabinoid production.

It’s an exciting time to grow as the increased scientific literature on cannabis specific cultivation is long over due, I can’t help but be jealous when I look at how dialed in potato farmers have things around growing/harvesting/storing them for making potato chips where the starch content of the potato is crucial as it affects how evenly they fry, which affects their price.

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#24

Op if cost isn’t too bad. you really should take a few readings and use the average . Plant directly under light will be different than partially shaded one . And bottoms will be lower than tops

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#25

Oh no. That’s not what I ment. If you see what I had said in the end. Your ambient temp reflect your rh. And the other way around. Same goes for leaf surface temp reflecting a higher humidity during higher transpirations. They all revolve around each other. I was just saying idk why they use ambient and rh only on those calculations. When the leaf surface temp (since it’s reflected based off your ambient) and rh are what truly determine your vpd. And imo are more important to pay attention. Because lst of 78 won’t be in an ambient 99. That’s what I mean. Sorry I should of been more clear. I kinda lead that in dumb direction

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#26

I’m not sure if we’re saying the same thing in different ways or we have a different understanding of the mechanics of VPD. The ambient temp and RH determine the vapour pressure of the ambient air (VPair), ie how strong or weak evaporation is due to the potential to hold more or less water at that temperature point. Which is why they’re used to determine the first part of the equation. The vapour pressure of the saturated enviro (VPsat) in this case is measuring the pressure exerted on the leaf and how it affects the ability to transpire to cool itself and maintain optimal metabolic rates (assuming adequate nutrients and lights of course). Leaf temps tell us how well the plant is functioning by showing if it is able to adequately cool itself through transpiration, which is why it’s crucial. Combined with brix measurements you can dial in the nutrients easily to match the transpiration needs of your plants as like you previously mentioned by controlling the VPD you can equally effect what EC levels the plants will thrive at.

Sorry if I over explained things like VPair/VPsat, that was more for anyone following along who may not understand what we’re talking about to try and make it more accessible as you clearly understand the issue despite the miscommunication issue.

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#27

Lol. Again. All I’m saying is your lst will never be 76 in a 99 ambient.

#28

As my code sits now, I read air temp, and have a user-input for the LST difference. Thats static. I took a few readings with the laser thermometer, over the course of day / night, and LST varied from 4-8 degrees depending on time, and only 1-2 degrees depending on where (top of canopy, middle, bottom). Granted, averaging top / middle / bottom would be the most accurate way, but I think even adding just one sensor pointing towards the bulk of the mass will be better than the current static reading, as it will become dynamic, adjusting according to that sensor at that moment in time (not when I change the LST value manually).

Cost wouldnt be terribly bad, the sensor runs like $16, and an ESP32 costs $6 ea. But in the interest of simplicity I wont be making 3 sensors (2 more power wires to run / 2 more sensors to place differently). This isn’t THAT precise (considering most people don’t bother to monitor VPD and do ok, I think even doing a little with VPD will help)

Sensor arrives today, then I’ll design and print some kind of housing / aim-able mount for the sensor and ESP32, and report back :smiley:

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#29

Of course not but the difference between a LST that’s 2C lower vs 8C lower tells us a lot about the health of the plant and it’s ability to photosynthesize, which is why it’s equally important to know and measure if using VPD instead of assuming it’s an offset of x. The three metrics combined are what create a clear picture of the grow and what the plants doing, looking at them separately or ignoring one would be like looking at pH and EC independently when they are intrinsically linked.

#30

You hit the nail on the head, by automating the LST readings your VPD is going to be good enough that it won’t be the biggest limiting factor in your grow, assuming your lighting is already sorted (usually the easiest aspect) that only leaves watering and fertilizing to dial in. Perfection isn’t as important as achieving balance in a grow. Can’t wait to see how it all comes together in the end.

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#31

Would something like this work for tracking leaf temp? https://www.walmart.com/ip/AMG8833-IR-8-8-Infrared-Thermal-Imager-Array-Temperature-Sensor-7M-Farthest-Detection-Distance-with-Housing/556361610?wmlspartner=wlpa&selectedSellerId=101103649&&adid=22222222228341489475&wl0=&wl1=g&wl2=m&wl3=427863558897&wl4=pla-892853935159&wl5=9009687&wl6=&wl7=&wl8=&wl9=pla&wl10=498870200&wl11=online&wl12=556361610&veh=sem&gclid=Cj0KCQjwyt-ZBhCNARIsAKH1176pQLcQVb52_BPXQ5iuAVqBHjVqfq1bpU4wVPlz3zmcIb4Kg6fu8BkaAjZyEALw_wcB&gclsrc=aw.ds

Apparently I’ve also been doing vpd wrong :sweat_smile:

#32

Nice looking little unit. Not sure if it shows the actual temp in the crosshairs? As long as you can see that, it can be used to calculate it. Not sure if it would integrate with my HA setup (I know ESP32s will), but for others, that might be a nice option. REALLY like the little display at that price!

#33

Again man. My apologies. My main point was simply saying your leaf temps reflects everything. Just forget I said anything about ambient air not needing to be checked. I run my leaf temps. That’s it. Everything else clicks in my head naturally. Not so much for anyone else.

If your new to advanced environmental controls. I agree with you 100%. That all falls in a very important part of learning how to calculate everything accurately. For the op. Just make sure you include leaf temps in your equation or whatever you input your numbers into. Make sure it’s a 3 way read. Ambient, rh, and lst.

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#34

I think it’s understood, but keep in mind the VPD targets are estimations. Individual environments, nutrient concentration, light intensity, etc may mean that you don’t need to fall into some of the suggested ranges. In the end, it’s figuring out the ranges that work the best for the situation and endeavoring to keep it more or less in your target range(s).

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#35

Actually the way my code is written to use a static / manually input LST, its pretty much just swapping a little code once the sensor is up / recognized by home assistant. Swap the input value with a calculated value from the sensor.

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#36

Alrighty, sensor seems to be working properly. Everything seems to be calculated and displaying Ambient temp, Leaf Surface Temp, Air VPD. Leaf / Plant VPD and Leaf Temp Difference now…

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#37

Ok, having run this sensor for 24 hours now, you can certainly see it working dynamically. Tracking the LST difference shows anywhere from 4.6° C to 0.6°C difference based on day / night. Seeing the current VPD on my dashboard eliminates me cross-referencing on the chart, and so far it matches pretty closely (probably because I am using 2 decimal places on temp). I’d call this sensor a success, now I just gotta make a cool housing for it.

Oddly enough, I found a project where this sensor / esp32 board and a display were used to make a DIY quartz banger temp reader for $25 or less, compared to the $75 - $100 they want for a store bought model. Bookmarked that for the future :smiley:

Now that I have a working sensor and properly calculated values, I can start playing with the programming logic. Essentially choose a mode (early veg, late veg and flower) and then have the logic decide what to do based on mode and current readings, to get to the optimum VPD value (i.e. turn on/off exhaust fan, or dehumidifier, or even humidifier).

Having the ability to chart things REALLY makes some things visible.

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From 1pm when I installed it in the flower room until 10pm the lights are on. You can then see the swings with the dehumidifier turning on / off, and no heat from the lights.

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Corresponding chart for ambient air temps

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Corresponding chart for RH %

Getting a visual on the situation certainly helps with programming the logic (I can also make a chart of the on / off / on / off changes from the switches that control fans, lights, and humidifier / dehumidifier)

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#38

This is how I calculate VPD. It may help the ones how doesn’t know what it is or doesn’t like working with charts or complicated equations or apps or whatever else.

Vapor Pressure Deficit at a given temperature is the difference between current vapor pressure in the air and the saturated air with vapor pressure at that given temperature. This quantity describes how fast plant transpire and what Water Use Efficiency is going to be. VPD is not the best model, leave-to-air-VPD is better one but VPD is good enough and relatively easy to measure.

Definitions:
VPD(at a given temperature)= SVP(at air temperature) - VP(air temperature)
RH=VP/SVP

we can write:
VPD= SVP - VP = SVP - RH*SVP = (1 - RH)*SVP

So
*VPD =(1 - RH)SVP

Reletive Humidity is easy to measure. SVP depends on average air Temperature and can be calculated by some equations like this one below:

SVP= 0.6108exp[(17.28T)/(237.3 + T)]

We all like simplicity and to get to it we substitute T with 25° C to get 3.2 kpa. then we substitute T with 26° C to get 3.4 kpa. there is no need to be exact at all.

Here is the simplest way to calculate SVP. at temperature 25°C SVP is 3.2 kpa and it changes 0.2 kpa for every 1° C changes in temperature.

Now everyone should be able to calculate VPD easily using RH and air Temperature without having any charts, apps or working with unnecessary complicated equation.

If you prefer Farenheit instead of Centigrade you have to know SVP is 3.2 kpa at 77°F and changes 0.1 kpa for every 1° F changes in temperature.

finally:

Leave-To-Air-VPD=SVP(at leave temperature) - VP(at air temperature).

But leave and air temperature are almost, but not always, the same. that’s why VPD is a good quantity For WUE.

Good to know that optimal nutrients concentration depends on VPD

#39

@Nagel420 In case it’s useful or interesting to you, ledgardener on youtube has a series on garden automation and he’s “made” an IR leaf surface temp measuring “device” for his garden.

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#40

In my experience the best bud comes from people who grow the same couple strains consistently and dial them in over time. Some of these genetics come from dry arid regions with high amounts of sun exposure while others come from more humid forest type environments.
I don’t believe any one set of parameters will yield the best results across such a diverse spectrum of plant genetics.
With that being said I do believe that working within known ranges is a great place to start.
Plants grow a natural type of sunscreen to protect them selves from the high amounts of uv light outdoors. I often wonder if giving a plant to perfect of a environment actually lowers the quality due to the plant not having to produce those protective compounds that may contribute to the quality of the end product. Maybe some of those defensive compounds make it last longer in storage etc etc. There is still so much for us to learn about the cannabis plant and all of its internal mechanisms.

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