Need VPD help

I still don’t quite understand VPD. From various google searches including the pulse forum, I’m reading that the VPD at my current stage should be approx 8.0 - 11.0 but I usually average 1.5 - 2.2. Both temps and RH are exactly where they need to be.
Lights on -77F 50%RH
Lights off - 73F 45% RH
Passive air intake and 6” inline exhaust achieving good negative pressure.
What am I doing wrong??

U need to add moisture to the air to get it down to 8,11. 15 2.2 are way to high

Sorry I’m still lost. I’m not trying to get my VPD down to 8. I’m currently at 1.9 and trying to get it up to 8, I think (from what i’m reading. Im current about two weeks away from switching to flower and reading that VPD should be around 15. Is this correct??

I think you read something wrong along the line. You’ll never want your VPD that high. You want it between 1 and 1.5 optimally. I start sweating if my tent hits 2. Check out this article on the Pulse website -

If your at 1.8, then your not too far off. You’ll either need to increase your humidity or lower your temp to get it DOWN to 1.5 or lower depending on your stage of growth.

Hope that clears it up. Be sure to check out that article.

Vpd for flower I would be happy with 14,16 vpd. But now u need a vpd of 11,13 ish u sound like u are to far down the road now to be running a vpd of 0.8. 0.8 is for baby’s as more water in the air for them.
I run the tent on 0.8 or 0.9 for a week or so when doing seedlings. And move the vpd up quite quickly after. I am working with autos mind so things move quick with them little buggers.

Are you guys forgetting a decimal point? I don’t understand why you would want a VPD of 14 or 11. Those are crazy high VPD numbers. Sorry if I’m totally missing something here. It’s almost as if you are using a different scale?

That clears it up perfectly. Thanks for your quick response and link. Still not sure why others are trying to achieve the 12 - 15 range. Maybe they’re just missing the decimals

Cool. Yeah, perhaps missing decimal point. Not sure. Good luck.

You’re probably confusing your units. The 8 & 11 vpd values are probably in hPa (hectopascal, 1 hPa = 100 Pa), while we provide VPD in kPa (kilopascal, 1 kPa = 1000 Pa). So 8hPa & 11hPa = 0.8kPa & 1.1kPa.


Interesting. I figured it had to be something like that.

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