VPD Calculator & Chart (Vapor Pressure Deficit)
Online VPD Calculator & Chart (Vapor Pressure Deficit) tool is useful to generate the VPD chart for the given air temperature and relative humidity. It also helps to optimize the growing environment for plants. You can calculate the VPD chart by giving relative humidity or dew point or wet bulb temperature details as inputs.
Vapor Pressure Deficit Definition
VPD or vapor pressure deficit is defined as the difference between the quantity of moisture in the air and the quantity of moisture in the crop leaf. It is useful in metric horticulture as it roughly correlates to how plants transpire. Generally, in dry climate conditions, the dry air leads to the loss of body moisture quickly while in humid climates leads to feeling sticky or sweat because of slower evaporation rates.
Plants are able to regulate water loss to some extent through the opening and closing of the stomata in their leaves, the VPD affects the rate of plant transpiration. This is the reason for growers using vapor pressure deficit to create a healthy environment for plant growth.
What is Vapor Pressure Deficit for Plants?
The lower VPD indicates a minor difference between the air vapor pressure and the leaf vapor pressure. Or the air is humid, so plants transpire more slowly. Lesser VPD can be preferable at the starting stages of propagation. But at very lower vapor pressure deficits, plants are at great risk of mildew and diseases.
The higher VPD indicates that there is a greater difference between the air vapor pressure and the leaf vapor pressure. Or the air is more dry, so plants tend to transpire more fastly. At very high VPD, plants will choose a defensive mechanism in the case of dehydration. As a result, both transpirration and gas exhange are slowed, leading to slower plant growth.
How to Calculate Vapor Pressure Deficit with Relative Humidity?
Follow the below given instructions to calculate the VPD chart for the given inputs.
- Provide the relative humidity and air temperature as inputs to the caculator.
- Hit the calculate button.
- You will get the VPD of air along with the VPD chart.
Otherwise, you cna follow the manual process given here to calculate the vapor pressure deficit value.
For the standard VPD calculation, you need to measure relative humidity, air temperature and leaf tempertaure or canopy temperature.
The vapor pressure deficit formula is VPD = vapor pressure of leaf - vapor pressure of air
Normally, leaf temperature is few degrees lesser than air temperature because of the evaporative cooling.
Other different vapor pressure deficit equations are given here.
- Saturation vapor pressure = 0.61078 * exp[17.27 * T / (T + 237.3)] and T is the temperature
- Vapor pressure of air = saturation vapor pressure of air * relative humidity
- Vapor pressure deficit = saturation vapor pressure of leaf - saturation vapor pressure of air * relative humidity
Steps to Find VPD using Dew Point
If you are looking to set up an environmental control system to automatically regulate VPD. To simplify the control system, the dew point can be helpful. With the help of dew point, you only have to control one variable instead of two. The equation of vapor pressure deficit using dew point is here:
vapor pressure deficit = saturation vapor pressure of leaf - saturation vapor pressure at dew point
Calculate VPD with Wet Bulb Temperature
You can calculate VPD uisng dry and wet bulb temperature parameters. You can find the formula to find dew point based on dry and wet bulb temperature.
Solved Example of VPD
If we are in a greenhouse, the air temperature is 22oC, the relative humidity is 55%, and the temperature at the crop canopy is 20oc, find the vapor pressure deficit.
Solution:
Firstly, calculate the vapor pressure in the leaf.
vapor pressure in the leaf = 0.61078 * exp[17.27 * T / (T + 237.3)]
= 0.61078 * exp[17.27 * 20 / (20 + 237.3)]
= 2.338 kPa
Find the vapor pressure of air
vapor pressure of air = 0.61078 * exp[17.27 * T / (T + 237.3)] * relative humidity
= 0.61078 * exp[17.27 * 22 / (22 + 237.3)] * 0.55
= 1.454 kPa
Now calculate the vapor pressure deficit
vapor pressure deficit = 2.338 kPa - 1.454 kPa
= 0.88 kPa
FAQs on VPD (Vapor Pressure Deficit) Calculator
1. What is a VPD chart?
A vapor pressure deficit chart is a proven tool that can maximize your desired results when growing in a controlled environment. The chart suggests optimal ranges to run your plants during the different stages of growth.
2. How to calculate vapor pressure deficit?
To calculate the VPD, substitute the values in the formula vapor pressure deficit = saturation vapor pressure of leaf - saturation vapor pressure of air * relative humidity.
3. Why is VPD important?
VPD is very important for plants as it controls stomatal opening, transpiration rate, nutrient uptake, CO2 uptake and plant stress. Vapor pressure deficit directly impacts yields.
4. Does CO2 affect VPD?
The elevated CO2 can decrease the interval at high vapor pressure deficit conditions but increases the interval under low vapor pressure deficit conditions.