VPD, vapor pressure deficit explained,...

[Death The Cultivator]

@Waira -- should have known it would be you that would post about VPD.
I was searching for info and found this, now I have a question:

I always knew that humidity was an important factor in buds getting you-know-what.
But I was still just thinking in terms of RH % numbers.
I see a lot of people maybe getting RH of 70% during flowering, and getting all worried.
Then, for ex. a couple of years ago, I had RH of literally 85% every day for a month,
and it was only after 3 weeks that the you-know-what appeared.
So I thought why are people worried about just 70%.

But, if I get this VPD thing right, it's not just the RH %, right??
If I had a low temp during flowering, then maybe 70% could be really bad,
but at higher temps, it would actually need a higher RH % to have the same problem.
E.g. from the chart at the top, 15C at RH 75% gives VPD 4.2,
and this would be equivalent to about, 30C, RH 90%, VPD 4.2.
Is that correct?

This seems to make sense, because while I get high RH, higher than many here, easily up to 80% at night,
my temps are also hotter than most, maybe 28C-30C at night, so that 80% RH isn't so dangerous.

Vapor Pressure Deficit VPD Calculator - Dimlux Lighting - The Best Grow Lights

Vapor Pressure DeficitCalculator Calculate both Room VPD and Leaf VPD automatically Just enter the value below to generate the VPD values for your room and your plants. You’ll need to know the room’s temperature, relative humidity, and the plant’s temperature. We recommend calibrating your...

www.dimluxlighting.com

It's a combination of humidity, ambient temperature, and leaf surface temperature. You need an IR thermometer at the least to measure leaf surface temperature to give you your temperature offset.

You're looking at the kpa which is 0.42kpa for what you are talking about. The

 

[Shiva's Mistress]

Vapor Pressure Deficit VPD Calculator - Dimlux Lighting - The Best Grow Lights

Vapor Pressure DeficitCalculator Calculate both Room VPD and Leaf VPD automatically Just enter the value below to generate the VPD values for your room and your plants. You’ll need to know the room’s temperature, relative humidity, and the plant’s temperature. We recommend calibrating your...

www.dimluxlighting.com

It's a combination of humidity, ambient temperature, and leaf surface temperature. You need an IR thermometer at the least to measure leaf surface temperature to give you your temperature offset.

You're looking at the kpa which is 0.42kpa for what you are talking about. The

Interesting. Though I have no way at present to measure that.

 

[DummyGrower]

Such an important thread, thanks for sharing @Waira and the others.. I am slowly starting to realize that it is what made the difference between big buds and my popcorn. Sure, some common sense and a min of average measuring will help to get approximately in the right range, but accurate measuring will really boost harvests.
I've been looking for IR thermometers but I'm struggling to find good info on them. Medical thermometers seem to work in a different way so they won't cut it.
If anyone knows of any accurate enough IR thermometers, I'd appreciate if you could share the link... cheers