VPD Offset question

Troubleshooting & Bugs
#1

Hi Everyone!

I’m glad to be here! Thank you for creating this product!

Quick question regarding VPD offset… The Niwa grow hub manual says [average environmental temp - average plant temp] = Offset.

In the manual, this is followed by “please note, this offset will usually be a negative value, around -3 degrees Fahrenheit.”

I’m using LED lights. My leaf temp is 3-4 degrees below the ambient temp (as measured against hanging water bottle at same height as canopy by IR thermometer).

My understanding was that leaf temps should always be lower than the ambient air surrounding it, otherwise the leaf is not transpiring. In fact, when I check leaf temps, I can tell a plant is running out of water simply by noticing the leaf temp is within one degree of the ambient air next to the leaf. (Of course, I’ll check the soil and lift to check for weight of the plant, and it all lines up with a plant that is about to start dropping due to lack of water.)

I’m only a year into growing, so I have a lot to learn still, but if I follow the equation in the manual, then I should enter in an offset of +3 or +4.

Is that right? Because other VPD charts aren’t even available for positive numbers (they all seem to assume l leaf temp is lower than ambient temp).

Thanks!

#2

I think the equation in the Niwa GH manual might be backwards, and should read:

VPD offset = leaf temp - room temp

In my case, this results in a negative number, which then lines up with various VPD chart conditions I was using prior to getting the Niwa Grow Hub. I just want to make sure I’m actually providing the optimal conditions for my grow.

Thanks!

1 Like
#3

Yes @Learner you are right, we have updated our online manuals but old versions have the formula backwards, the correct formula is, as you said:

VPD offset = leaf temp - room temp