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Earl · #3 (**permalink**) 11-12-2007, 05:32 AM

I have gone into my grow room and been puzzled by a phenomena that I didn't understand. There would be water standing on a leaf that was under HID light and I was very bewildered why this was happening. I knew I had not allowed any water to spill on the plant, and I knew that my roof wasn't leaking, hell it wasn't even raining outside. So, where did this mystery water come from? Obviously the plant was transpiring water through the leaf and puddling on the top. But why? The answer is due to high vapor pressure deficit in my grow room. If we grew in soil, this condition would be very detrimental to our plants, but since hydro roots can get all the water they need, we just see "water pooling" on the leaves due to heavy transpiration.

If your interested in increasing your plants ability to grow in an enclosed environment, and you want to avoid diseases, and provide the best growing environment for your girls, then you need to be concerned about VPD.

So what is VPD?

Vapor pressure deficit, in ecology, is the difference between the actual water vapor pressure and the saturation of water vapor pressure at a particular temperature.

Unlike relative humidity, vapor pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation.

Plants lose moisture by transpiration from their leaves into the surrounding atmosphere.

The less moisture they lose, the more they like it.

We tend to think that the higher the relative humidity, the moister the air, the better it is for our plants, but that is only true up to a certain point.

What I am trying to show here is that relative humidity does NOT relate directly to the rate at which transpiration of water from the plant occurs.

Changes in relative humidity are not proportional to the rate of plant moisture loss.

How come?

The moisture holding capacity of air is measured in units of pressure, and there are two important measurements concerned with figuring out how much moisture a given block of air can potentially absorb.

First is the saturation vapor pressure (SVP): think of this as the maximum amount of water vapor a given block of air can hold.

Second measurement is the difference between the amount of water vapor actually in a given block of air and its SVP (i.e., the maximum amount of water it could absorb).

This difference is called the vapor pressure deficit, or VPD. Think of VPD as the water sucking power of the air, because it is actually the VPD that interests your plants, not the relative humidity.

At face value, VPD (sucking power) seems to be the same as relative humidity - because relative humidity is the ratio of the actual vapor pressure in the air to the SVP.

Its not the same, because the SVP of a given block of air increases exponentially as the air temperature rises - the higher the temperature, the greater the amount of water vapor that air can hold.

Rather than giving a physical explanation of why humidity and VPD are different measurements, because I'll get out of my depth in about two seconds, just look at how the VPD (sucking power) changes at various temperatures if the relative humidity stays the same at 75%:

Cpa represents how much water the atmosphere can absorb

VPD calculation is an improvement over relative humidity (RH) measurement alone because VPD takes into account the effect of temperature on the water holding capacity of the air, which roughly doubles with every 20°F increase in temperature.

Rather than giving a relative measure of the water content of the air, VPD gives an absolute measure of how much more water the air can hold, and how close it is to saturation.

For example, a typical 100' long x 30' wide x 10' high greenhouse with 80% relative humidity has about 14 lb of water in the air at 50°F, while 70°F air holds about 28 lb of water at the same RH.

This is reflected in the VPD values of 0.036 psi (0.25 kPa) and 0.072 psi (0.50 kPa), for the lower and higher temperature conditions, respectively (see Figure 2). Thus, VPD can be used to identify healthy air moisture conditions for plant production, while taking into account different temperature levels.

How does VPD compare to relative humidity?

Figure 1 shows how VPD relates to the customary thinking about humidity. Higher VPD means that the air has a higher capacity to hold water, stimulating water vapor transfer (transpiration) into the air in this low humidity condition. Lower VPD, on the other hand, means the air is at or near saturation, so the air cannot accept moisture from the leaf in this high humidity condition.

Figure 1. Vapor Pressure Deficit (VPD) enhances or inhibits the crop’s ability to transpire.
Therefore, vapor pressure deficit is a useful way to express the vapor flow in the system, both for condensation and transpiration.

Higher VPD increases the transpirational demand, influencing how much moisture from plant tissues is transferred into the greenhouse air. Consequently, VPD is being used to predict crop water needs in some commercial irrigation systems.

In contrast, very low VPD indicates closer proximity to the dew point, meaning harmful condensation can begin to develop.

Using the canopy temperature to determine VPD gives the best indication of condensation risk, showing particularly how close the canopy is to the dew point.

Looking at the temperature and vpd on a graph, you can see how the vpd is increasing exponentially as the temperature rises, while the relative humidity remains constant:

Here is another good article with a graph. VPD

VPD values run in the opposite way to RH values so when RH is high VPD is low.

If humidity is too low (i.e. high VPD), the stomata on the leaves tend to close in order to limit transpiration and prevent wilting. This closing of the stomata will also limit the rate of CO2 uptake and hence limit photosynthesis and consequently plant growth. Low humidity also reduces turgidity (water pressure within the plant cells) and this in turn also restricts growth. Blossom end rot in tomatoes and capsicum can also be attributed to low humidity (high VPD).

Conversely, if humidity is too high (i.e. low VPD) the stomata will fully open but even so the plants will be unable to evaporate enough water to carry minerals into the plant and so again, growth will be impeded and mineral deficiencies (particularly calcium) may occur. In addition, the plants may exhibit soft growth, fungal problems and mineral deficiency symptoms.

It is frequently stated that VPD more closely matches what the plant "feels" in relation to temperature and humidity and therefore forms a better basis for environment control. Unfortunately, VPD is extremely difficult to determine accurately as it is necessary to know the leaf tissue temperature. Attempts to measure leaf temperature reliably on an ongoing basis have often ended in disaster. One of the problems is that the plants leaves are in differing amounts of sun with some leaves in full sun, some in partial sun and others in full shade. This makes the concept of "leaf tissue temperature" particularly complex.
VPD can be used to identify disease-causing climate conditions. For example, several studies that explore disease pathogen survival at different climate levels reveal two critical values of VPD.

Studies show that fungal pathogens survive best below VPD (<0.43 kPa).

Furthermore, VPD is most damaging below (0.20 kPa).

Thus, the greenhouse climate should be kept above (0.20 kPa), to prevent disease and damage to crops.

How do we calculate VPD?

Well this is a problem. You need to know several temperature values and use a formula to calculate VPD. This is probably not something you are going to do everyday. There is a chart but I am not able to copy it into the post, so I will try to extrapolate it here.

Relative humidity thresholds for disease prevention, correspond to (0.20 kPa) VPD.

50ºf 0.2Kpa = 100% rh
60ºf 0.2Kpa = 80% rh
70ºf 0.2Kpa = 55% rh
75ºf 0.2Kpa = 45% rh
80ºf 0.2Kpa = 40% rh
85ºf 0.2Kpa = 27% rh

minimum rh for 75ºf = 60% rh
IDEAL rh for 75ºf = 73% rh
Max rh for 75ºf = 86% rh

Minimum rh for 86ºf = 70% rh
IDEAL rh for 86ºf = 80% rh
Max rh for 86ºf = 89%rh

Here is a VPD calculator if you have all the temps to plug into it. http://www.hydro.co.nz/vpd_calc.php

Sometimes, when you think you have a nute deficiency, you are really just experiecning VPD.

#3 (permalink) 11-12-2007, 05:32 AM
Earl Abuser Join Date: Aug 2007 Location: Ohpressed in the U.S. Posts: 300	Understanding VPD I have gone into my grow room and been puzzled by a phenomena that I didn't understand. There would be water standing on a leaf that was under HID light and I was very bewildered why this was happening. I knew I had not allowed any water to spill on the plant, and I knew that my roof wasn't leaking, hell it wasn't even raining outside. So, where did this mystery water come from? Obviously the plant was transpiring water through the leaf and puddling on the top. But why? The answer is due to high vapor pressure deficit in my grow room. If we grew in soil, this condition would be very detrimental to our plants, but since hydro roots can get all the water they need, we just see "water pooling" on the leaves due to heavy transpiration. If your interested in increasing your plants ability to grow in an enclosed environment, and you want to avoid diseases, and provide the best growing environment for your girls, then you need to be concerned about VPD. So what is VPD? Vapor pressure deficit, in ecology, is the difference between the actual water vapor pressure and the saturation of water vapor pressure at a particular temperature. Unlike relative humidity, vapor pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation. Plants lose moisture by transpiration from their leaves into the surrounding atmosphere. The less moisture they lose, the more they like it. We tend to think that the higher the relative humidity, the moister the air, the better it is for our plants, but that is only true up to a certain point. What I am trying to show here is that relative humidity does NOT relate directly to the rate at which transpiration of water from the plant occurs. Changes in relative humidity are not proportional to the rate of plant moisture loss. How come? The moisture holding capacity of air is measured in units of pressure, and there are two important measurements concerned with figuring out how much moisture a given block of air can potentially absorb. First is the saturation vapor pressure (SVP): think of this as the maximum amount of water vapor a given block of air can hold. Second measurement is the difference between the amount of water vapor actually in a given block of air and its SVP (i.e., the maximum amount of water it could absorb). This difference is called the vapor pressure deficit, or VPD. Think of VPD as the water sucking power of the air, because it is actually the VPD that interests your plants, not the relative humidity. At face value, VPD (sucking power) seems to be the same as relative humidity - because relative humidity is the ratio of the actual vapor pressure in the air to the SVP. Its not the same, because the SVP of a given block of air increases exponentially as the air temperature rises - the higher the temperature, the greater the amount of water vapor that air can hold. Rather than giving a physical explanation of why humidity and VPD are different measurements, because I'll get out of my depth in about two seconds, just look at how the VPD (sucking power) changes at various temperatures if the relative humidity stays the same at 75%: Cpa represents how much water the atmosphere can absorb VPD calculation is an improvement over relative humidity (RH) measurement alone because VPD takes into account the effect of temperature on the water holding capacity of the air, which roughly doubles with every 20°F increase in temperature. Rather than giving a relative measure of the water content of the air, VPD gives an absolute measure of how much more water the air can hold, and how close it is to saturation. For example, a typical 100' long x 30' wide x 10' high greenhouse with 80% relative humidity has about 14 lb of water in the air at 50°F, while 70°F air holds about 28 lb of water at the same RH. This is reflected in the VPD values of 0.036 psi (0.25 kPa) and 0.072 psi (0.50 kPa), for the lower and higher temperature conditions, respectively (see Figure 2). Thus, VPD can be used to identify healthy air moisture conditions for plant production, while taking into account different temperature levels. How does VPD compare to relative humidity? Figure 1 shows how VPD relates to the customary thinking about humidity. Higher VPD means that the air has a higher capacity to hold water, stimulating water vapor transfer (transpiration) into the air in this low humidity condition. Lower VPD, on the other hand, means the air is at or near saturation, so the air cannot accept moisture from the leaf in this high humidity condition. Figure 1. Vapor Pressure Deficit (VPD) enhances or inhibits the crop’s ability to transpire. Therefore, vapor pressure deficit is a useful way to express the vapor flow in the system, both for condensation and transpiration. Higher VPD increases the transpirational demand, influencing how much moisture from plant tissues is transferred into the greenhouse air. Consequently, VPD is being used to predict crop water needs in some commercial irrigation systems. In contrast, very low VPD indicates closer proximity to the dew point, meaning harmful condensation can begin to develop. Using the canopy temperature to determine VPD gives the best indication of condensation risk, showing particularly how close the canopy is to the dew point. Looking at the temperature and vpd on a graph, you can see how the vpd is increasing exponentially as the temperature rises, while the relative humidity remains constant: Here is another good article with a graph. VPD VPD values run in the opposite way to RH values so when RH is high VPD is low. If humidity is too low (i.e. high VPD), the stomata on the leaves tend to close in order to limit transpiration and prevent wilting. This closing of the stomata will also limit the rate of CO2 uptake and hence limit photosynthesis and consequently plant growth. Low humidity also reduces turgidity (water pressure within the plant cells) and this in turn also restricts growth. Blossom end rot in tomatoes and capsicum can also be attributed to low humidity (high VPD). Conversely, if humidity is too high (i.e. low VPD) the stomata will fully open but even so the plants will be unable to evaporate enough water to carry minerals into the plant and so again, growth will be impeded and mineral deficiencies (particularly calcium) may occur. In addition, the plants may exhibit soft growth, fungal problems and mineral deficiency symptoms. It is frequently stated that VPD more closely matches what the plant "feels" in relation to temperature and humidity and therefore forms a better basis for environment control. Unfortunately, VPD is extremely difficult to determine accurately as it is necessary to know the leaf tissue temperature. Attempts to measure leaf temperature reliably on an ongoing basis have often ended in disaster. One of the problems is that the plants leaves are in differing amounts of sun with some leaves in full sun, some in partial sun and others in full shade. This makes the concept of "leaf tissue temperature" particularly complex. VPD can be used to identify disease-causing climate conditions. For example, several studies that explore disease pathogen survival at different climate levels reveal two critical values of VPD. Studies show that fungal pathogens survive best below VPD (<0.43 kPa). Furthermore, VPD is most damaging below (0.20 kPa). Thus, the greenhouse climate should be kept above (0.20 kPa), to prevent disease and damage to crops. How do we calculate VPD? Well this is a problem. You need to know several temperature values and use a formula to calculate VPD. This is probably not something you are going to do everyday. There is a chart but I am not able to copy it into the post, so I will try to extrapolate it here. Relative humidity thresholds for disease prevention, correspond to (0.20 kPa) VPD. 50ºf 0.2Kpa = 100% rh 60ºf 0.2Kpa = 80% rh 70ºf 0.2Kpa = 55% rh 75ºf 0.2Kpa = 45% rh 80ºf 0.2Kpa = 40% rh 85ºf 0.2Kpa = 27% rh minimum rh for 75ºf = 60% rh IDEAL rh for 75ºf = 73% rh Max rh for 75ºf = 86% rh Minimum rh for 86ºf = 70% rh IDEAL rh for 86ºf = 80% rh Max rh for 86ºf = 89%rh Here is a VPD calculator if you have all the temps to plug into it. http://www.hydro.co.nz/vpd_calc.php Sometimes, when you think you have a nute deficiency, you are really just experiecning VPD.