RAM – More than just staying cool!

What is it that you first think of when you consider the RAM brand? For most of us, the thing that first comes to mind is some kind of fan. After all, that’s what RAM does, isn’t it? Rapid Air Movement.

There are several interacting factors, some atmospheric and some non-atmospheric, which need to be about right for a plant to thrive: -
Atmospheric Factors:

• Relative Humidity
• Temperature
• Air movement
• CO₂ availability

Non-atmospheric factors:

• Light availability
• Nutrient availability
• Water availability

Plants need light for photosynthesis – the process by which they create energy in the form of sugars from water and Carbon Dioxide (CO₂). They need nutrients to grow (to build more plant material). And they need water for several purposes:

• It is used in photosynthesis
• It is used to transport minerals / nutrients throughout the plant
• It keeps the plant cells turgid (distended) which supports the plant and prevents wilting and droop
• It cools the plant’s leaves as it evaporates

A continuous column of water passes through the plant. It is absorbed at the roots by osmosis and drawn upwards through the stem due to the pressure gradient caused by evaporation from the leaves as the plant transpires.
Transpiration & Vapour Pressure Deficit

Transpiration is as essential to a plant as respiration (breathing) is to humans and other animals. The transpiration process drives the passage of water through the plant and allows the plant to excrete the excess water as vapour into the atmosphere. Most of the excess water is excreted by the plant by transpiration through its stomata – microscopic openings on the leaves also used to take in CO₂ and to release oxygen during photosynthesis. The evaporation of the water before it exits the stomata cools the leaves of the plant.


The stomata of a plant will usually only open when light falls upon them, unless the temperature is high, and the Relative Humidity is low. Thus transpiration is predominantly a process which occurs during the photoperiod (daylight time).

Transpiration is reduced if the ambient temperature is low.
Transpiration is reduced if the ambient Relative Humidity is high
Transpiration is reduced if the air is still because water vapour accumulates around the Stomata, increasing the Relative Humidity locally to the leaves. This can also cause problems if the vapour condenses to form a film of water on the leaves, providing conditions in which pathogens can thrive.

There is a relationship between the temperature of the air and the Relative humidity, known as the Vapour Pressure Deficit, or VPD. VPD is a measure of how much more water vapour the air can hold, when the current Relative Humidity and Temperature are taken into consideration.

At any given temperature, there is a limit to the amount of water vapour that can be held in the air. Hotter air can hold more water vapour, and colder air can hold less. That is the reason why when the atmosphere cools during the night, dew forms as the air sheds water vapour that it can no longer hold.

For any temperature, the maximum amount of water vapour that air can hold is known as the Saturation Vapour Pressure or SVP. The amount of water vapour held by the air currently is known as the Actual Vapour Pressure, or AVP. The Vapour Pressure is the amount of air pressure that is due only to the amount of water vapour in the air, and so, although it is measured in the same units as the total air pressure, kilopascals or kPa, the Vapour Pressure will always be less than the total air pressure (Barometric Pressure).

Relative Humidity (RH) is the ratio of the current AVP to the current SVP, expressed as a percentage. Thus, if the actual vapour pressure (AVP) is the same as the air’s saturation vapour pressure, meaning that the air cannot hold any more water vapour, then the RH = 100%. If the humidity of the air tends to increase above the SVP, then water will precipitate out of the air in the form of dew, rain or snow.
When there is still capacity in the air to accept more water vapour, in other words when the AVP is less than the SVP and the RH is less than 100%, there is said to be a “Vapour Pressure Deficit” or VPD. The VPD is the difference between the AVP and the SVP, and is measured in kPa.

For plants to be able to transpire, there must be a Vapour Pressure Deficit, but for plants to thrive, the size of the VPD should ideally fall within a range between 0.45 kPa and 1.25 kPa
If the VPD is too small, then the air is too humid for plants to transpire efficiently, and there is an increased risk of condensation and wetness of the plant, creating an environment suited to pathogens, with increased danger of mildew and rot

If the VPD is too great, then the lack of humidity risks increased rates of transpiration which may dry out the plant if there is a finite amount of water available at the roots. If water is plentiful, then excessive transpiration and the associated increase in water uptake may result in greater-than-optimal concentration of nutrients in the water available, causing toxicity. To combat drying, the plant will narrow or close its stomata to conserve water, but this then limits its ability to take in CO₂ for photosynthesis. The plant is likely to become stressed.

At different stages of the plant’s development, the ideal VPD varies due to the plant’s varying requirements:

• At the propagation / seedling / cutting / early growth stage, plants have yet to develop a strong, effective root system. It is better if transpiration is slower to ensure that the plant does not lose water more rapidly that it can take it in. A VPD of 0.4 kPa – 0.8 kPa is recommended.
• During the later vegetative (growth) and early flowering stages, the plants have effective roots and plenty of leaves, and are capable of transpiring strongly, driving good water / nutrient uptake. A VPD of 0.8 kPa – 1.2 kPa is recommended.
• In the late flowering stage, when the entire plant is well established, extra water uptake is beneficial, and it is desirable to reduce the risk of bud rot. A VPD of 1.2 kPa – 1.6 kPa is ideal, possibly achieved by increased temperature and reduced humidity.

To determine the VPD for your grow, there are two possible methods:

• If you’re a plant scientist (in which case, you’re unlikely to be reading this blog!) you will prefer to measure the leaf temperature of your plants using an infrared temperature gun, then calculate the Saturated Vapour Pressure at the leaf surface and compare this to the calculated AVP of the air to arrive at the VPD.
• For the rest of us, it’s good enough to use a VPD chart, which is a visual “ready reckoner” from which the VPD value can be picked out if we know the air temperature and Relative Humidity, which can be read from an Essentials Digital Min-Max Thermo Hygrometer

VPD Chart

From the above, we now know what VPD we have in our grow room, and also what VPD we wish to achieve. So, how do we adjust the VPD to ensure optimum transpiration levels for our crop?
To increase VPD:

• Increase the ambient temperature. This can be achieved by adding heat (using a heater or moving the grow lights closer to the plant canopy), or by reducing any cooling effect. Reduce the rate of air-change in your grow room, taking care to ensure that this doesn’t increase humidity. Adjust nutrient reservoir chillers.
• Decrease the humidity using a dehumidifier.
• Increase the lighting intensity (switch ballasts / controllers to boost setting or add supplementary lighting), to increase leaf temperature.

To decrease VPD:

• Decrease the ambient temperature. This can be achieved by removing heat (turn down any heaters, move the grow lights further away from the plant canopy) or by increasing the rate of air-change in your grow room.
• Increase the humidity using a humidifier.
• Decrease the lighting intensity (turn down ballasts / controllers or remove supplementary lighting) to lower the leaf temperature.

Having optimised your plant’s transpiration, there are other factors that you should consider to maintain plant health.
Air movement through and around your crop is highly beneficial:

• It ensures that humidity and temperature are uniform.
• It avoids temperature stratification, where hotter air gathers in the upper levels of the grow room, causing stagnation.
• It assists transpiration
• It reduces condensation, assists evaporation, and thus decreases the risk of disease.
• The gentle flexing of the plants enhances plant strength.
• It ensures an even availability of CO₂ throughout the crop.

Additional CO₂ can be beneficial, provided that your plants have everything else that they require. There is enough CO₂ in the atmosphere naturally (around 400 ppm) for plants to thrive, if the air in the grow room is renewed constantly. However, if your grow room is equipped with good lighting, and the plants have access to all the water and nutrients which they require, they can use up to about three times as much CO₂ as is available naturally.

The benefits of adding CO₂ include:

• Stronger, healthier plants. Your plants will have thicker stems and branches, and will be more resistant insect attack, and will be better equipped to cope with environmental stress and high temperatures.
• Faster growth (shorter vegetative cycle) and earlier flowering.
• Up to 30% increase in yield.

When and how much CO₂ should be added depends upon which stage of their lifecycle the plants are at:

• During the vegetative (growth) stage, aim for a target CO₂ level of 600 ppm – 700 ppm.
• During flowering, the target level of CO₂ should be 1000 ppm – 1200 ppm.

Never raise the CO₂ level above 1200 ppm. Plants have no use for such elevated levels which can create a toxic environment.
Never add CO₂ when the lights are off. In the dark period plants don’t photosynthesise, so any CO2 added then is wasted.

Remember that CO₂ is heavier than air, so it is best to arrange for it to delivered above the plant canopy so that it will reach every leaf of the plants. Also, you should consider using air circulators to move air upward from ground level.
Some RAM products that can help you to keep your grow room atmosphere ideal for your plants:
 
Coming soon – the RAM 12L Dehumidifier – 12L /day
Products already in stock:
RAM Inline Duct Fans

RAM Mixed-Flow Inline Fans

RAM EC Silenced Fans 

RAM EC Mixed-Flow Fans

RAM Inline Exhaust Fans

RAM Ducting

RAM Filters

RAM Air Circulators

RAM Wall Fans

RAM Pedestal Fans

RAM Small Fans

RAM Humidifiers

RAM CO₂ Controller & Release Kit