Want to control VPD without breaking the bank?

We have mentioned the importance of VPD, or Vapour Pressure Deficit, in a few of our blog articles previously. To be absolutely blunt, maintaining an optimal grow environment in terms of temperature and humidity is key to getting the most out of your crop.

Although the concept of VPD might seem a little odd upon first encounter, what it really comes down to is this: VPD is the drying ability of the air.

In order to grow, plants need to "transpire". This is where the plant draws up water (containing nutrients) up from its roots, and then allows it to be lost through its leaves. If the plant did not lose water through the stomata (pores) in its leaves, the plant would not be able to draw up nutrients from its substrate.

For a plant to be able to transpire, the air around the leaves needs to have some "drying ability". If the air is too humid or too cold then the air will not have much drying ability. This means the plant cannot transpire properly and will be unable to get hold of adequate nutrients.

If the air is very warm and dry then it may have too much drying ability and there is a danger that the plant will lose water too quickly and therefore wilt. Fortunately, plants have a coping mechanism to help deal with this (in the short-term) but it is not exactly perfect. By closing the stomata on the undersides of the leaves, the amount of water loss is minimised which greatly extends the length of time it can survive in hot & dry conditions. However, the stomata are where the plant breathes in CO2 which it needs for photosynthesis.

There is an ideal range of VPD (drying ability) for optimum plant growth. If it is too high then the plant will not be able to get CO2. If it is too low then the plant will not be able to get the nutrients it needs.

So, getting the relative humidity right for the current temperature is very important. There are lots of useful charts out there which can help find the best humidity for any given temperature.

At this point it is well worth pointing out that when using these charts it is the leaf temperature that's important. This is usually between 1 and 3 ºC (with average of around 2 ºC) below the ambient air temperature in the grow-space. For an accurate leaf temperature we would highly recommend using an infrared thermometer like the Pocket IR thermometer.

The point-&-measure Pocket IR is a steal at just £19.95 (as on 27-3-19)

It's entirely possible for a grower to watch the temperature and humidity of their grow-space and adjust the humidity to achieve an optimal VPD by the switching on and off of a humidifier or dehumidifier as necessary. However, for virtually every grower, it is simply not practical to be constantly watching and attending their grow during the lights on period.

In recent times, solutions have appeared on the market for automating the control of VPD adjustment. However, they have mostly been aimed at large, industrial scale grows. For example, a combination of Dimlux Expert Lighting along with a controller, a humidity sensor and infrared plant leaf camera linked up to an Opticlimate Climate Control System can certainly help to achieve that goal. However, for the small-scale grower, there has been little, if not nothing, to assist with VPD control automation - until now.

Global Air Supplies (G.A.S.) are a highly innovative company who specialise in garden air environment products ranging from advanced fans & fan speed controllers to ozone generators. Now they have released an affordable way for the small-space grower to control VPD:

Their new Intelligent Humidity Controller is an absolute doddle to use. It consists of a control unit, a 3-in-1 sensor and a connection for a humidifier. On the standard model, the humidifier connection is a mains outlet socket via which the controller will switch your humidifier on and off as required. On the Humidity Controller model designed specifically for the G.A.S. SonicAir Pro Humidifier there is a cable and connection which plugs straight into the humidifier. There is no need for a mains connection for the controller because it draws its power straight from the humidifier through the control cable.

The Intelligent Humidity Controller has a ready-wired 3-in-1 sensor which is ideally placed in the centre of the grow-space. The sensor measures the air temperature, the relative humidity, and the light level (so that it knows whether the grow-space is in the dark cycle or the light cycle).

Your desired settings are programmed into the controller via the single control knob. Pressing the knob in has a similar effect to a select button, while turning the knob scrolls through menu options.

The menu is simple as there are only a few settings required.

First of all, the mode of operation needs to be selected. This is done by pressing the control knob in. There are 4 modes of operation to choose from:

  1. Relative Humidity 24Hours
  2. Relative Humidity Day / Relative Humidity Night
  3. VPD 24 Hours
  4. VPD Day / Relative Humidity Night

You will notice that the first 2 modes only control the relative humidity, not VPD. The third setting controls VPD but for 24 hours a day. Plants only need the VPD to be correct while they are photosynthesising which is during the light cycle. During the dark cycle, it is unnecessary. This is why the fourth mode - VPD Day / Relative Humidity Night - is going to be the most useful for growers. For the purposes of this article, we will assume that mode 4 will be chosen.

This is achieved by selecting the 4th menu option:

Next, because a VPD mode has been chosen, pressing in the control knob will make the G.A.S. Intelligent Humidity Controller ask what stage the plants are in:

Early Veg Setting maintains 0.4 - 0.8 kPa

Early Flowering Setting maintains 0.8 - 1.2 kPa

Late Flowering Setting maintains 1.2 - 1.6 kPa

The above are preset settings. However, the user can override these presets and set their own custom VPD target at the twist of a knob.

During Early Veg, plants prefer a lowish VPD (low air-drying ability) until they have established a root system that can support greater water loss.

The Early Flowering setting is also good for Late Veg too. Plants will hopefully have a good root system by this point and will be able to support faster transpiration and water loss. Hence plants at this stage can tolerate a higher drying ability of the air (VPD).

In late flowering, getting plants to transpire a bit more will make them even more productive. Also, a higher drying ability of the air creates a very difficult environment for mould & bacterial diseases to take hold. Of course, during late flowering is the real danger time for rot to set in. Keeping the RH low during the night and VPD high during the light-time really helps to keep fungi at bay.

The last main setting, "Plant Offset", is where the Pocket IR Thermometer mentioned above really comes into its own. The difference between ambient temperature and the actual leaf temperature is usually between 1-3 ºC.

Leaf temperature is always a little lower than ambient because the water evaporating from the leaves through transpiration has a cooling effect on them (a little like when a mammal "sweats"). The Pocket IR can tell you the actual leaf temperature by simply offering it up to a leaf (during the "lights-on" period) and then pressing the "measure" button. The difference between that measured leaf temperature and the ambient temperature can be entered into the "Plant Offset" setting. This allows the controller to work out a really accurate VPD.

There are a few other parts in the menu system such as choosing temperature measurement in either ºC or ºF. There is also a "Factory Reset" facility in case you lose track of what you have set and want all the settings to go back to how they were when you took it out of the box.

All-in all, the G.A.S. Intelligent Humidity Controller is a long-awaited piece of equipment that can help you take your growing to the next level without breaking the bank. Fantastic!

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