Air conditioning vs Dehumidifier vs Galileo's square cube law; and a wee bit of Wild Turkey 101.
As far as operation and bits and pieces go there is very little difference between an air conditioner and a dehumidifier, an air conditioner has the evaporator cooling coil inside the conditioned space and the condenser heat removing coil located outside that space. Its main purpose is to move heat from one location to another, the fact that it also dehumidifies is secondary and is only a function for the want of cooling - once the cooling level is satisfied there is no more dehumidification. Note: you can put a $100 window AC on a chair without venting out a window and it becomes a dehumidifier albeit a battle ensues.
The dehumidifier has both the evaporator and condenser inside the same conditioned space, the humid air is drawn over the chilled evaporator which if below the dew point will extract moisture from the air, that dry cool air is then passed through the hot condenser to cool the refrigerant on its way back to the compressor to start the cycle over again - the heat given off is only secondary to its function to dehumidify which does not monitor temperature.
Now herein lies the problem, it seems we are infatuated with the percentage of relative humidity regardless of temperature even though they are somewhat proportional inversely or otherwise. After reading this article https://www.growweedeasy.com/humidity and its subsequent links I made a quick chart and plagiarized some pertinent information.
"According to Wikipedia: “the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 to 0.95 kPa.”
It seems our plants tend to favor varying VPD's depending on what stage of life they are in, and the high humidity for clones make sense if you read the below super blurb I pirated.
The below has been plagiarised but is very informational - the site is listed at the end so I may be vindicated.
Vapor pressure deficit (VPD) is more accurate when determining water loss from the plant. VPD is simply the difference between the vapor pressure inside the leaf compared to the vapor pressure of the air. If the VDP is high, meaning that the vapor pressure inside the plant is higher than the outside air, then more water vapor escapes out through the stomates (pores in the bottom of leaves). This process of water loss through the leaves is called "transpiration". If the VPD is low, the stomatal openings close and little water and fertilizer is taken up by the plant from the growing medium. VPD is important to know because it is used to schedule irrigations, to determine if air exchanges are needed and if air temperature needs to be increased in order to hold more moisture.
Role of humidity in plant growth: Plants are always adjusting their leaf stomatal openings based on the VPD and the humidity in the air. As seen above, high humidity is a problem because water usage by the plant is too slow and compromises quality, even though the stomates are constantly open. Likewise, if humidity is very low and subsequent transpiration is too high, the plant closes its stomatal openings to minimize water loss and wilting. Unfortunately, this also means photosynthesis is slowed and subsequently, so is plant growth.
As alluded to above, the two major functions of the plant that ties in closely with the humidity in the air and affect crop performance are transpiration and photosynthesis.
This is the process where plants absorb water through the roots and then give off water vapor through pores in their leaves. The drier or the hotter the air temperature, the faster the transpiration rate from the plant. However, the moisture deficit and transpiration rate are not directly related. This means that in Transpiration: very dry air, the increased rate of transpiration can only go so high in the plant and then it begins to wilt. For example, if the air is extremely dry, but the growing medium has enough water, the plant may wilt and, unless the humidity increases, the plant could die.
On the other hand, if the air is very humid the plant does not take up much water from the growing medium, which also means there is little uptake of fertilizer elements. This is a problem for some elements, particularly calcium, as inadequate uptake can lead to nutrient deficiencies. Also, low water usage from the growing medium often correlates with climbing growing medium pH, which makes micronutrients such as iron unavailable to the plant. Typically these problems are seen in the winter and early spring, when air temperatures in the greenhouse are low and transpiration is inadequate, or during the hot, humid summer months.
Photosynthesis: This is the process of fixing carbon dioxide and water in the plant leaves to produce sugars that are used for energy and growth. When the temperature is high and humidity is normal, more stomata will open, letting in carbon dioxide for active photosynthesis. If the air is excessively dry and the plant is wilting, the stomatal openings close, thereby reducing photosynthetic activity and ultimately plant growth. The quality of the crop is dependent on the conditions that promote optimal photosynthesis and humidity plays a role in this process.
https://www.pthorticulture.com/en/tr...-crop-quality/
As far as operation and bits and pieces go there is very little difference between an air conditioner and a dehumidifier, an air conditioner has the evaporator cooling coil inside the conditioned space and the condenser heat removing coil located outside that space. Its main purpose is to move heat from one location to another, the fact that it also dehumidifies is secondary and is only a function for the want of cooling - once the cooling level is satisfied there is no more dehumidification. Note: you can put a $100 window AC on a chair without venting out a window and it becomes a dehumidifier albeit a battle ensues.
The dehumidifier has both the evaporator and condenser inside the same conditioned space, the humid air is drawn over the chilled evaporator which if below the dew point will extract moisture from the air, that dry cool air is then passed through the hot condenser to cool the refrigerant on its way back to the compressor to start the cycle over again - the heat given off is only secondary to its function to dehumidify which does not monitor temperature.
Now herein lies the problem, it seems we are infatuated with the percentage of relative humidity regardless of temperature even though they are somewhat proportional inversely or otherwise. After reading this article https://www.growweedeasy.com/humidity and its subsequent links I made a quick chart and plagiarized some pertinent information.
"According to Wikipedia: “the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 to 0.95 kPa.”
It seems our plants tend to favor varying VPD's depending on what stage of life they are in, and the high humidity for clones make sense if you read the below super blurb I pirated.
| Temp F | Temp C | RH % | VPD kPa |
| 70 | 21 | 50 | 1.25 |
| 70 | 21 | 65 | 0.87 |
| 70 | 21 | 80 | 0.5 |
| 75 | 24 | 60 | 1.2 |
| 75 | 24 | 70 | 0.9 |
| 75 | 24 | 85 | 0.45 |
| 80 | 27 | 65 | 1.25 |
| 80 | 27 | 75 | 0.9 |
| 80 | 27 | 85 | 0.54 |
| 86 | 30 | 70 | 1.27 |
| 86 | 30 | 80 | 0.84 |
| 86 | 30 | 90 | 0.43 |
Vapor pressure deficit (VPD) is more accurate when determining water loss from the plant. VPD is simply the difference between the vapor pressure inside the leaf compared to the vapor pressure of the air. If the VDP is high, meaning that the vapor pressure inside the plant is higher than the outside air, then more water vapor escapes out through the stomates (pores in the bottom of leaves). This process of water loss through the leaves is called "transpiration". If the VPD is low, the stomatal openings close and little water and fertilizer is taken up by the plant from the growing medium. VPD is important to know because it is used to schedule irrigations, to determine if air exchanges are needed and if air temperature needs to be increased in order to hold more moisture.
Role of humidity in plant growth: Plants are always adjusting their leaf stomatal openings based on the VPD and the humidity in the air. As seen above, high humidity is a problem because water usage by the plant is too slow and compromises quality, even though the stomates are constantly open. Likewise, if humidity is very low and subsequent transpiration is too high, the plant closes its stomatal openings to minimize water loss and wilting. Unfortunately, this also means photosynthesis is slowed and subsequently, so is plant growth.
As alluded to above, the two major functions of the plant that ties in closely with the humidity in the air and affect crop performance are transpiration and photosynthesis.
This is the process where plants absorb water through the roots and then give off water vapor through pores in their leaves. The drier or the hotter the air temperature, the faster the transpiration rate from the plant. However, the moisture deficit and transpiration rate are not directly related. This means that in Transpiration: very dry air, the increased rate of transpiration can only go so high in the plant and then it begins to wilt. For example, if the air is extremely dry, but the growing medium has enough water, the plant may wilt and, unless the humidity increases, the plant could die.
On the other hand, if the air is very humid the plant does not take up much water from the growing medium, which also means there is little uptake of fertilizer elements. This is a problem for some elements, particularly calcium, as inadequate uptake can lead to nutrient deficiencies. Also, low water usage from the growing medium often correlates with climbing growing medium pH, which makes micronutrients such as iron unavailable to the plant. Typically these problems are seen in the winter and early spring, when air temperatures in the greenhouse are low and transpiration is inadequate, or during the hot, humid summer months.
Photosynthesis: This is the process of fixing carbon dioxide and water in the plant leaves to produce sugars that are used for energy and growth. When the temperature is high and humidity is normal, more stomata will open, letting in carbon dioxide for active photosynthesis. If the air is excessively dry and the plant is wilting, the stomatal openings close, thereby reducing photosynthetic activity and ultimately plant growth. The quality of the crop is dependent on the conditions that promote optimal photosynthesis and humidity plays a role in this process.
https://www.pthorticulture.com/en/tr...-crop-quality/
