What is the VPD eq used by the pulse programing? Is air pressure a factor?

The equations I look at never include air pressure, but I see correlations in my VPD swing that seem related to pressure changes when my RH and Temp remained constant.

Appreciate any insight.


I would like to bump this question since I’m having trouble answering it myself. I hope someone from Pulse responds.

I used to think that VPD was atmospheric pressure dependent but I can’t find a single reference to that effect. It seems like VPD should be independent of atmospheric pressure. I’ll attempt to provide reasoning.

VPD is just the difference between the known saturated vapor pressure and the current actual vapor pressure. Saturated vapor pressure appears to be constant at any given temperature and independent of pressure. In a way, it represents the volatility of water. The actual vapor pressure is related to how much water is in the air; if the air is 10% water then 10% of the pressure of the atmosphere is exerted by water molecules.

The thing is, the “actual vapor pressure” is a matter of current water content of air and seems independent of any external factors, like air pressure. And SVP like I mentioned is constant at a given temperature. So neither of the two components that allow you to calculate VPD depend on atmospheric pressure as far as I can tell.

That said, my intuition tells me that with reduced atmospheric pressure there is a increased rate of evaporation because the air is simply not pushing back on the water as hard, allowing more to escape from the liquid phase. This means that if the atmospheric pressure dropped at your location, due to weather, your grow room would experience a higher load on the dehumidifiers because water is simply more volatile.

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Man thank you for making me feel less crazy.

You need to make a YouTube channel! I read another comment you had with someone explaining vpd, the link with your e.c. And the difference between running at home and st your facility. Extremely interesting and well put. Do you post anywhere else? Want to read all your stuff :grin::+1:t2:

Those are kind words, thanks! I’ve considered starting a youtube channel but don’t have one yet.

I would never claim to have all the answers, but it would be awesome to use something like youtube to share my thoughts and experiments and start a discourse with the viewers.

I’m building a new legal facility right now, I think when it’s in operation that might be a good time to start :slight_smile:

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Noah, you seem to be well versed with Pulse and I’ve seen your posts about coding. Is the pulse programing available anywhere? I’m just gunna have to look at the hampster wheel drivin this thing.

Air pressure isn’t used as a factor in the VPD calculation.

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Good question.

VPD is based on SVP calculations, and we use the Tetens equation (Tetens equation - Wikipedia) for those. That equation ignores atmospheric pressure.

I believe that in most cases, vapor pressure is mostly driven by temperature, and the influence of atmospheric pressure can be ignored. In fact, errors are more likely to come from the approximate fits of the various equations used, and from the measurement itself.

See more here:

  1. Does atmospheric pressure affect the vapor pressure of a liquid? | Socratic.

  2. Is the vapor pressure of a liquid dependent on the pressure of the atmosphere in which it is measured? - Chemistry Stack Exchange

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I actually did some digging when I saw this posted. I didn’t reply cause honestly I wasn’t confident in any answer. The way I understood it was that it is factored into the VPD equation already as a part of a defined constant and since you are calculating the difference between two things at the same pressure, that it didn’t matter. Yes pressure is different between sea level and Denver, but we are calculating the difference between to things, not the value of a thing.

Happy to be incorrect, but thats how I understood it.

Could you inform me what the air pressure on the pulse is used for? What would you amount of air pressure would you consider negative pressure?