How-To Read a Vapor Pressure Deficit (VPD) Chart

vpd
By Canncentral Editor

2019-07-24

When growing plants, what is vapor pressure deficit. How to calculate vapor pressure deficit. Calculation formula. Vapor pressure deficit chart.

Vapor Pressure Deficit

An important parameter to take into account when growing plants is VPD or vapor pressure deficit. Successful indoor gardeners only have a simple hydrometer which measures temperature relative humidity. A clear insight can be gained as to how much water is required by the plants to maintain its stomata moisture levels is through the use of a VPD chart which provides thorough information on the air’s drying ability around your plants.

But what is VPD exactly? It’s a measurement of the drying effects of the air around your plants. It’s the distinction between the pressure of the moisture content in the air (AVP) and the moisture content that the air can hold while it’s saturated (SVP). And how this relates to plant growth, it applies to the differences between the pressure inside the leaves and the surrounding atmosphere. It’s the clearer picture obtained when you combine the temperature and humidity all into one measurement. It indicates what’s actually going on around stomata as opposed to just knowing what the conditions are like in the actual room itself. Simply stated, the plants’ ability to transpire will be determined by the higher VPD which is the drying effects the air will have on the plant. As a gardener, one will want to ensure that the crucial measurements are in the right ranges. In most cases, if there is a moderate fluctuation between the amount of pressure deficit between the leaves and the outside air, that will yield a positive effect. This is due to some level of transpiration being required from the stomata to promote the water flow from the root to the rest of the entire plant. And it is from this flow that will supply the plant with all the necessary nutrients it needs to grow successfully.

A VPD Unit is a pressure reading written in millibars (mb). VPD may also be written in kilopascals (kPa). The conversion between the two is easy. Simply divide the millibar figures by 10 to obtain an approximate kPa number. That’s all there is to it.

The plants will lose too much water through transpiration and the stomata will close if the VPD level is too high. A plant’s transpiration rate will be slowed down if there is a low VPD level. A low transpiration rate means that the movement of water and plant nutrients will be greatly slowed down. This ultimately will cause a nutrient deficiency issue.

Typical ranges are:

Low transpiration (cutting / veg): 0.4 – 0.8 kPa

Healthy transpiration (flower): 0.8 – 1.2 kPa

High transpiration (useful during peak flower): 1.2 to 1.6 kPa

There is an easy way to calculate a VPD reading. One method is to use the Maxie controller. This accurate, high-end, complete growing system device will do all the work for you. In a typical grow room, when connected to the mat and controller, Light, Co2 generators, Co2 sensors, humidifiers, heaters can all be conveniently controlled and regulated digitally from their central points.

In order to monitor VPD you’ll need the following:

  • temperature probe
  • humidity probe
  • a plant camera

A plant camera takes readings from the leaf temperature starting at the top of the canopy. The VPD readings are then calculated by the Maxi controller when all these readings are combined. With these readings in hand, you can then make the necessary adjustments to your setup to keep the VPD in its optimal performance “sweet spot”. These humidity adjustments can be done with the use of a humidifier or dehumidifier. You’ll want to ensure that the room is being properly cooled by adjusting atmospheric temperatures. An increase in temperature may be required. Or an adjustment to the plant canopy temperature. This can be achieved by simply lifting or moving the lights closer or farther away from your plants. The Maxi controller is a recommended piece of equipment to invest in. It will dim the light if the plants go above your required settings. But it is cautioned to use it as a last resort as the plants will always require maximum light power.

Another piece of equipment you’ll want to pick up is an infrared pocket thermometer which measures leaf temperature. You’ll also need a hygrometer to take temperature and relative humidity readings from the room. Bear in mind you’ll want to keep the distance routed to the size of the area you want to measure. Simply point the infrared thermometer at the canopy and pressure measures to get the leaf temperature. If the leaf measures roughly 30 centimetres, keep the infrared pocket thermometer to a distance of 30 centimetres away.

And now, the terminology will get a bit technical. In order to calculate VPD you’ll need the ambient air temperature, the relative humidity (RH) and, the canopy air temperature. Then the saturation pressure (SVP) must be calculated. You can either use a handy psychrometric chart or from the following equation:

SVP = 610.78 x exp(T / (T +238.3) x 17.2694))

By computing the pressure for both the ambient and canopy temperatures, we can then compute the actual pressure of the water vapor in the air (AVP). The formula for calculating this is by multiplying to the relative humidity:

AVP = SVP x RH/100

Lastly, subtract SVP from AVP and you’ve got your vapor pressure deficit.

VDP=SVP-AVP

If math is not one of your strongest skill, use a VPD chart. It will show the degrees in Fahrenheit for flower stage, with a 0° offset for leaf temperature. Through the VPD chart you can find out what the pressure would be for that temperature add 100 percent relative humidity. Of if you like, the VPD chart can be be used to create your own customized chart. With this you can modify the stage, the units, and the leaf temperature adjustment. Now you’re all set to get growing.

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