HPS RH vs LED RH

I'm looking for data in 2 different grow styles.


HPS growers:
If you grow w HPS, do you have to add RH, or do you have to exaust RH? What is your RH and tent temp? If you can, give canopy LST and ambient temp. Just add like so...

Light type:
Growth week:
Veg or Flower:
RH:
T:
LST:


LED growers:
If you grow with LED, do you have to add RH, or do you have to exhaust RH? What is your RH and tent temp? If you can, give canopy LST and ambient temp. Just add like so...

Light type:
Growth week:
Veg or Flower:
RH:
T:
LST:


I'm trying to put together a data set to plot the differences in RH and T vs LED and HPS.

Its puzzling to me that we add RH to LED tents because they are transpiring too much. In my experience HPS produces more RH than LED, which would mean they transpire more, yet we seem to only experience issues with the LED. A data set may give insight to this phenomenon.

there are people who understand VPD and then there's you.

hint: what happens if your canopy gets warmer than ambient due to infrared emissions?

I don't run a humidifier for either and I grow under 77 degrees with both. I don't fudge with VPD or temps as long as my RH is between 40-60 %.
I even run my dehumidifier in my flower room 365 days a year.

ChiefRunningPhist said:

Its puzzling to me that we add RH to LED tents because they are transpiring too much. In my experience HPS produces much more RH than LED, which would mean they transpire more, yet we treat only the LED.

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I add a shit ton of humidity to my rooms with HPS. Large swamp coolers. That said the VPD chart is what you want.

In nature outdoors VPD is variable depending on time of day and season, it's not a set in stone number.

Yeah but we strive for optimal conditions all the time.

sethimus said:

there are people who understand VPD and then there's you.

hint: what happens if your canopy gets warmer than ambient due to infrared emissions?

Click to expand...

Logically, it should increase in transpiration.

Renfro said:

Yeah but we strive for optimal conditions all the time.

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But optimal conditions don't exist in nature as a whole as plants have adapted to their environments they are cultivated in.

jarvild said:

But optimal conditions don't exist in nature as a whole as plants have adapted to their environments they are cultivated in.

Click to expand...

I think the best thing about growing indoors is making the conditions as near perfect as we can, thus getting better yields and improved quality.

@sethimus I don't think you understand what VPD is. Check your logic.

If we add RH trying to reduce VPD in LED, why not HPS, at least the majority of folks? (Unless I'm wrong lol which is why I think a data set would help)

LED has crappier VPD realised, but only due to RH (if you manage your heat). If HPS and LED are the same temp, but LED is lower RH, then it has higher VPD.

If we are supposedly controlling transpiration with VPD, why is there higher RH in HPS than LED? If there's no RH in LED, yet its tanspiring too much, where is the water going? Why so low RH compared to HPS?

@sethimus do you get it now?

What's the average RH of the mountains of Afghanistan or the average RH on the equator? But yet our plants grow in both those regions and many more from the arctic circle to the equator.

Rt vs LST, seems pretty universal over the plant kingdom...


Rt vs VPD, which likewise seems pretty universal throughout the plant kingdom...


T vs VPD...


Increased LST increases Rt.
Increased VPD decreases Rt.
Increased T increases VPD.
Rt = Rate of transpiration
WUE = water usage effeciency
Pn = Rate of photosynthesis
Canopy depression = Ambient temp - LST
LST = Leaf surface temp

(Canopy depression) vs (Rt)...


So all this would leave me to believe that hotter LST, or hotter ambient are going to increase transpiration. This phenomenon can be described using VPD as well, (because they are related). The temp goes up and the air expands and VPD goes up. Leaf transpiration rates, when dealing with external temp stimuli, are being ultimately effected by how easy it is for the water to evaporate.

If we look at the initial environmental conditions provided by the 2 types of light, LED and HPS, we see it's actually HPS needing the close attention to VPD, not LED. This is contradictory to reality. Thus why I have a hard time excepting that if decreasing VPD helps with LED, that its solving a high transpiration rate problem other than something else.

@sethimus

Here's your increased LST vs Abient answer...

Notice the CPD x-axis goes " - " or higher LST than ambient. Seems transpiration is minimally effected....

I appreciate everyone's $0.02 but I was wanting the data


I've heard, and used to believe myself, that VPD was curing overtranspiration due to LEDs, and maybe it is, but there's some pretty contradictory evidence to the reasoning and so I'm trying to get some useable data to further the investigation.

So far we know that increasing RH helps in LED growing, other than that though, idk, the reasons given seem to contradict.

I've read blue light triggers transpiration, so LED transpires more, ... why then low RH?

I've read LED has lower LST, ... why then more transpiration?

I've read LED ambient is cooler, ... why then more transpiration?

I've read HPS is hotter and so stops transpiration, which is just false.

The best reasoning that I've come across is the difference in IR, between the 2 lights. That the CTD is greater in LED than HPS, but where that reasoning fails is, ... where does all that supposed high transpiration rate water go? Why isn't the RH climbing, or more than HPS?

Why is there lower RH in LED tents vs HPS if they have issues with overtranspiration?

I don't have the specific #'s you are looking for but what you are saying seems the complete opposite of my experience. I am in a very humid coastal climate and find HID lighting to have a Drying effect on the air within the grow. So much so that I'm not sure my dehu even runs during lights on

OneHitDone said:

I don't have the specific #'s you are looking for but what you are saying seems the complete opposite of my experience. I am in a very humid coastal climate and find HID lighting to have a Drying effect on the air within the grow. So much so that I'm not sure my dehu even runs during lights on

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No that helps, that's the kind of info I'm after, thanks. Do you grow with LED too, but only notice your HID drying out the air?

jarvild said:

I don't run a humidifier for either and I grow under 77 degrees with both. I don't fudge with VPD or temps as long as my RH is between 40-60 %.
I even run my dehumidifier in my flower room 365 days a year.

Click to expand...

Word thanks, before you had your nutes dialed in, did you ever notice anything different between the 2? RH, nute strength/ratio, anything?

So if hps is hotter that will reduce humidity which should make VPD worse... but because QB LED light is stronger and LST is lower it causes the plant to transpire at an abnormal rate which then makes the effects of Higher VPD worse than under HPS? buggered if I know either but I think this is what I should understand? Random has tried alot to help me get this but I am thick...

One thing I know is that plants under high QB light levels are very sensitive to nute levels compared to hps, but I am being tutored to run lower nute levels to compensate for this which is a reflection of higher VPD i.e. run lighter nutes to compensate for more water uptake. This has helped!