Now that we understand what Vapor Pressure Deficit (VPD) is, we can delve into how to calculate it. If you're new to VPD, you may want to check out our previous blog post for an introduction to the concept. Knowing how to calculate VPD is essential for indoor growers who want to optimize their plants' growth and yield.
(Not a fan of crunching numbers? There's still an important takeaway from this blog regarding VPD, temperature, and humidity. At the end of the blog, I'll summarize the relationships and implications between these factors that you should keep in mind.)
To calculate VPD, you will need to know the air temperature and relative humidity in your grow room. Make sure you have a thermometer and hygrometer to measure these values and follow the steps below to calculate VPD.
Step 1: Measure the air temperature (T) in degrees Celsius (°C) using a thermometer.
For example, let's say the air temperature in your grow room is 26°C.
Step 2: Measure the relative humidity (RH) using a hygrometer.
In this example, let's say the relative humidity in your grow room is 60%.
Step 3: Calculate the saturation vapor pressure (SVP) at the measured air temperature (T) using the following formula
SVP = 0.611 x exp[(17.27 x T) / (T + 237.3)]
In this equation, exp is the mathematical constant e raised to the power of the value inside the parentheses.
Using our example, we can calculate the SVP as follows:
SVP = 0.611 x exp[(17.27 x 26) / (26 + 237.3)]
SVP = 3.675 kPa
Step 4: Calculate the actual vapor pressure (AVP) using the following formula
AVP = (RH / 100) x SVP
In this equation, RH is the relative humidity measured as a percentage.
Using our example, we can calculate the AVP as follows:
AVP = (60 / 100) x 3.675
AVP = 2.205 kPa
Step 5: Calculate the vapor pressure deficit (VPD) using the following formula
VPD = SVP - AVP
Using our example, we can calculate the VPD as follows:
VPD = 3.675 - 2.205
VPD = 1.470 kPa
Once you have calculated the VPD, you can use it to optimize your indoor grow environment. For example, if the VPD is too high, you may need to increase the humidity in your grow room. On the other hand, if the VPD is too low, you may need to reduce the humidity or increase the temperature. By monitoring and adjusting the VPD, you can create the ideal environment for your plants to thrive.
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VPD or humidity % values can be derived from each other given the same Saturated Vapor Pressure, which is affected by temperature and air pressure.
VPD and Humidity % moves the opposite direction (inversely related).