Sunday, August 20, 2017

The Magic Of VPD

Support HighTek:

[ Intro ]

    What do you think of when you hear the word humidity?  For many many years, horticulturists of all types kept close track of the relative humidity in there grow environment.  The dictionary defines relative humidity as the amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature.  This means of expressing the amount of moisture in the air was great until our current day champion VPD came around.

[ What Is VPD ]

    VPD stands for vapor pressure deficit.  Vapor pressure deficit is the difference the moisture currently in the air and the total amount of moisture the air can possibly hold.  In other words, we take the difference between the current moisture level and the amount of moisture the air can potentially hold before it becomes fully saturated.  It is very important that we keep an eye on VPD.  If our air becomes over saturated it will release water molecules creating a film on our plants.  This will encourage rot in our greenhouse and that is a problem we don't want to deal with.  Below is an image of the VPD chart. The chart has green, yellow, and red sections.  The green section is the ideal range for your plant at the given temperature.  In the yellow, it is not ideal but should not cause any harm to your plants.  In the red range, you may see problems.

[ Calculating VPD ]

    Calculating VPD can seem like quite the daunting task but in all reality, it is not very difficult.  Big props to alchimiaweb for their article on calculating VPD.  I will be referencing a lot of information from them as it is the best article I found when learning about VPD.  In order to calculate our VPD we need three things.  We need our room temperature and relative humidity.  With these two things, we can calculate our SVP or saturated vapor pressure.  The saturated vapor pressure is the max amount of moisture the air can hold.  We can find our SVP using our temperature(in Celsius) and the chart below to the find SVP for our room at the current temp.

    Once we have our SVP it is very easy to calculate our VPD.  We can do this using the following formula.

VPD = ((100 - CRH) / 100) * SVP

   As you can see in the formula above all we need to do to find our VPD is first minus the current relative humidity from 100.  Next, we divide the result of that by 100 and multiply the result by the SVP we found using the chart above.  Now we know our VPD and we can start adjusting out room accordingly.

[ Controlling Your VPD ]

    If you would like to take a look at an open source system that can manage VPD and temperature in your greenhouse check out my open source grow room automation system here.  If you need any help or have any questions please comment or reach out using the information a the top of the page.  As always thanks for reading an happy growing! 

No comments:

Post a Comment