VPD chart, which one.

[EP3]

Hello AFN. Thanks for looking.

Just wanted to ask which VPD chart is accurate? I am trying to get my environment dialed in and noticed that the VPD charts I have been looking at some have different values for temp v RH.

Thank you,
EP3

 

[Bruce Campbell]

Which charts are you talking about / can you give a link?

VPD depends on air temperature, relative humidity, and leaf offset temperature - so most sites will usually give a few charts for different leaf offset temperatures, and some sites may just give a single chart for zero leaf offset.

The optimal VPD range also varies with growth stage - seedling, vegetative, flowering - the values themselves wouldn't be different, but some sites will color code the optimal range based on the growth stage.

If you're wondering what leaf offset temperature to use, you can measure that with an infrared thermometer. Otherwise, if you don't know your leaf offset, -1C/-2F is usually recommended for small plants and -2C/-4F is usually recommended for large plants.

 

[tronN00dles]

I use www.vpdchart.com because it removes any ambiguity associated with leaf temp and air temp. You have to enter both. I typically assume leaves are 2 deg C cooler than air, which I have verified with an IR gun.

It also shows you targets/ranges for each stage of growth, and associated danger low/over-transpiration zones.

 

[DCLXVI]

I was having the same issue of feeling like the charts weren't consistent. @WildBill and a couple other people recommended the Pulse One calculator. Punch in your numbers and it will give you the VPD kPa value. So you still need a chart or to know what ranges to be in at what stage.

VPD Calculator

Solve For: Temp RH VPD Temp (F) Temp (C) RH (%) VPD Air (kPa) Leaf Offset (F) Leaf Offset (C) VPD Leaf (kPa)

pulsegrow.com

I have been eyeballing the Scyence VPD chart a bit my last couple grows. Rough ranges and all. However, I was pointed at some other ones as well and they do seem to have different numbers. Hard to know.

Scyence chart: (Yeah it's a bit small and you have to zoom in. Well I do with everything anyways.)

 

[Olderfart]

I use vpdchart.com because it removes any ambiguity associated with leaf temp and air temp. You have to enter both. I typically assume leaves are 2 deg C cooler than air, which I have verified with an IR gun.

It also shows you targets/ranges for each stage of growth, and associated danger low/over-transpiration zones.

Very nice. That will be my new source - dead easy, no ambiguity, and geared specifically to cannabis. Thanks for posting.

Now if I could just find an equally effective app for my phone that does not need internet...

 

[Bruce Campbell]

Does anyone know where the "target VPD" numbers actually come from?

Every site I've seen just lists them as "this is the VPD/range you should aim for...trust us." But I've never actually seen any of them reference or cite a source (e.g., study, textbook, etc)...

 

[EP3]

Thank you folks!! @DCLXVI check out @tronN00dles recommendation, it's easy as pie to use. Have it bookmarked now.

Thank you to you guys for answering my question.

Hopefully can dial my environment in to get a better result. Looking forward to seeing what difference it makes to my buds.

Thanks again,
EP3

 

[St. Tom]

Thank you folks!! @DCLXVI check out @tronN00dles recommendation, it's easy as pie to use. Have it bookmarked now.

Thank you to you guys for answering my question.

Hopefully can dial my environment in to get a better result. Looking forward to seeing what difference it makes to my buds.

Thanks again,
EP3

here you go bro that's one I've always had I'm not sure how accurate it is and if I'm honest as long as my temps aren't too high and humidity isn't over 50 I'm not to concerned but I'm just a home grower

 

[Delps8]

Does anyone know where the "target VPD" numbers actually come from?

Every site I've seen just lists them as "this is the VPD/range you should aim for...trust us." But I've never actually seen any of them reference or cite a source (e.g., study, textbook, etc)...

Heh, good question.

Before I drank the Koolaid and bought a PulseOne, I poked around the internet and there's buckets of research on VPD. I haven't seen specific research on how cannabis, specifically, reacts vis a vis VPD but transpiration is a pretty basic function of plants so extrapolating from one plant species/"type" to another would be a pretty safe bet. Can't say for sure though.

This paper at Pulse has some good info but nothing cannabis-specific. Again, I suspect that's because that's simply how plants function.

 

[THE BEAR]

Hello AFN. Thanks for looking.

Just wanted to ask which VPD chart is accurate? I am trying to get my environment dialed in and noticed that the VPD charts I have been looking at some have different values for temp v RH.

Thank you,
EP3

Hey mate I found this one a while back

VPD Chart

Vapor pressure deficit (VPD) chart & calculator for horticulture in controlled environment. Cannabis, tomatoes, leafy greens, cucumber.

vpdchart.com