VPD calculations. Why is this chart SO far off?

[Nagel420]

Ok, I’ve had this chart pinned to my grow room wall forever.

I’ve generally followed it, and just looked at my temp / RH, cross referenced and knew where I was.

OR SO I THOUGHT

On the same page that chart can be found:

Vapor Pressure Deficit - The Ultimate Guide - Dimlux Lighting - The Best Grow Lights

You ALSO get a nice little calculator where you can punch in your own numbers.

Well, I wanted to integrate a VPD reading in my home assistant controller, and found someone had already put together the code for it using my sensors. But GAH! it didn’t match the chart!

Current temp 77.63°F / RH 49.3%
CHART VPD 1.09 kPa
Calculated VPD via code 1.64 kPa
Calculated VPD via the application on the page 1.64 kPa

I now feel like I’ve been following the wrong / an incorrect VPD for a LONG time. Mind you the code writer showed his calculations, and it makes sense that the calculated values are correct, and the chart is actually incorrect.

Thoughts?

[Northern_Loki]

Hard to know without examining the underlying source code between the two. There are multiple equations (they are estimates only and for specific use cases) along with multiple variables that may or may not be accounted for. Differential leaf temperature for example.

See this thread for some detail:

Source code: C++ Temperature Conversion and Vapor Pressure Deficit Classes

I’ve made several updates to the temperature conversion classes: Added Rankine units. Set the base class to Celcius such that any mathematical operations using the temperature types are always performed in Celcius and then converted back to the target temperature unit. Math with temperature, in some cases, is nonsensical since we should be talking temperature deltas. Though, as a class, mathematical formulas can utilize the conversions allowing for flexibility. Just be aware that additions be…

And, this post in particular:

Source code: C++ Temperature Conversion and Vapor Pressure Deficit Classes

There are several VPD algorithms that are essentially estimations applicable to specific circumstances. Here are a couple that are most applicable (with reasonable error) for the temperature ranges we’d typically consider. It’s been awhile since I looked at this but from what I recall, three steps, here: Saturation pressure (aka partial pressure of water) Vapor pressure Vapor pressure deficit (1) Saturation pressure These calculate the “saturation pressure”: Goff Gratch (1984): [image] whe…

[Somatek]

The 1.64kPa is the rooms VPD, not the plants VPD as you need to enter the leaf’s temperature to find that, I’d guess the chart you’ve been referencing makes an assumption about leaf temp based on averages to calculate their VPD range. Which explains the difference as your 1.64kPa is the VPD of the room as the calculation hasn’t been finished. Of course the numbers probably won’t line up perfectly as the chart relies on an assumption whereas using the formula/apps you’re theoretically inputting accurate info to give a more accurate result.

[Somatek]

It’s pretty simple to sort out if you go to the link provided and look at the VPD calculator and punch in the numbers provided to find out how the OP got the 1.64 kPa which only calculates the room VPD instead of finishing the equation. VPD = VPSat (vapour pressure of saturated enviro) -VPair (vapour pressure of air) and the OP only worked through the VPair as they didn’t add the leaf temperature which is critical in determining the VPsat and needed to finish the equation. Simple user error due to not understanding VPD or how to calculate it.

[Nagel420]

Chart uses a leaf temp offset of 5 degrees.

My actual here is 4 degrees measured with a laser thermometer (need to find one of them to integrate to HA to TOTALLY automate VPD calculations LOL)

The original code writer for the integration had a mistake on his variable inclusion. He was adding the difference, not subtracting. Fixed the code, and matches the table a bit closer now, not exactly, but thats because I am measuring exact and the table is approx. Close enough to make me happy and realize I’ve been on track (phew!).

And now I have it on my HA dashboard. SWEET!

[Gpaw]

In my setup I’m getting a larger delta on leaf temperature, more like 8F but that is dependent on water consumption. In later flower when transpiration slows that will drop to 4F. (consider it a variable not a constant)

Cheers
G

[Nagel420]

If I can find a laser thermometer that Home Assistant would recognize. I could automate that too

I wonder if I can do it on ESP 32 boards…
edit: Yes! infrared thermometer MLX90614 module for ESP32.

Takin this to the next level!

[Somatek]

The difference between leaf and ambient temps is an indicator of health with lower leaf temps indicating healthier plants, so it makes sense there’s less of a difference in late flower when senescence is kicking in and the plants pumping every last bit into trying to make a seed.

[olschool]

holy shit it aint rocket science …or is it ? points to ponder after a couple of good bong rips seriously tho looking at this chart i think im pretty well with in the good zone

[Nagel420]

@olschool

it aint rocket science

Well, no… More like an earth science Very different coursework

I like to keep an eye on things, more for flower and rot, but have noticed in the winter when my ambient RH was low in the house, a mini humidifier in the tent and watching the VPD for early veg and veg made a big difference for me in young plant health as well.

Being able to automate things with HA, just makes this a fun challenge for me, and now I can look and see my VPD on my dashboard (with a static difference for leaf temp) but with another little project, adding an ESP32 and infrared sensor, I can automate even that, and those little projects make this fun for me Plus it saves me on the electric bill by not running the big dehumidifier when not needed… I just need to add a variable now to choose between early veg / late veg / flower and I can even show when its optimal, or choose what needs to be done to get there (turn on / off fan, humidifier, dehumidifier).

The automation stuff is a rabbithole for sure… Still check on the girls daily, nothing beats eyes on, but having environmental controls that you can adjust and automate? Hell yeah!

[ReikoX]

The NIWA grow controller has two useful modes, “temp + VPD” and “RH + VPD”. The “temp + VPD” mode allows you to set a target temp and VPD. It will then adjust the RH to maintain the target VPD. The “RH + VPD” mode allows you to set a target RH and VPD and adjusts temp to maintain VPD.

Maybe this is something you would like to play around with.

[JoeCrowe]

I go with 45% RH at all times, it’s controlled via dehumidifier!

[Nagel420]

does that integrate with home assistant?

Once I can determine those numbers, I can write the logic to do either of those things, thats easy even for me, a non-coder, to do with what I have in hand. I’ve controlled RH and temp individually, but I can use this to work on a sliding scale, and tie them together.

plus, I need to challenge the brain occasionally

Or check what my current VPD is on my phone when I am halfway across the country.

One or the other ROFL

[ReikoX]

@Nagel420

does that integrate with home assistant?

I’m not sure, I think it is just the stand-alone app.

[Nagel420]

In the summertime My ambient humidity is 70% or higher. Once I turn on the heat because I have forced hot air it will drop to 30 and even 20%. So in the summer I need to dehumidify and in the winter I need to humidify.

[JoeCrowe]

Yah the humidity drops in the winter, but I have the grow isolated from everything else with doors and vapor barrier as well as insulation. It’s kind of it’s own little world. The plants are a major source of humidity! I’ve tried all sorts of RH from 25% to 90%, but it’s best to stay in the 40-50% range at 18-22C for veg and bloom.

[Somatek]

Is it monitoring the rooms VPD or does it have infrared sensors to monitor leaf temp to control the actual VPD of the grow?

[ReikoX]

Just the air temp/rh. I believe it has a leaf temp offset parameter.

[Somatek]

That’s where a lot of the VPD products/charts seem to fall flat as the vapour pressure of the room isn’t horribly useful by itself. The difference in leaf temp is kind of critical as that tells us how well the plant is able to transpire to cool itself, making an assumption on the critical part devalues the entire exercise.

[McKushin]

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

Vpd is really underestimated. I mean iv seen 4 weeks of growth in 2 weeks with a vpd kPa level under 1.0. It’s quite incredible. You can experiment with heavier feedings without burn. Just make sure you keep that rh up. I run my kPa’s at around .7-.9 during veg. Around 70-80% humidity and an ambient air temp of 84-86F. But my Lst are around 76F.

Yes. Your ambient air temps will reflect lst. But most of us maintain a relatively simple environment. Other than I see people very afraid of high humidity levels. Leaving levels low. Around 55-65%. I flower at those levels. But all that stuff runs the same factory. Humidity will reflect ambient air temps. Vice verse and air temp and humidity reflect lst. And VISE VERSE. This why humidity will rise with bushy plants. More leave more stomatas. More transparency.

So I guess keep in mind that your leaf surface temps are the key part of the equation. Amazon has lasers temp guns for the low. And you’ll use it for everything lol