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Why Vapor Pressure Deficit (VPD) Is More Important than Relative Humidity (RH)

Relative humidity (RH) and vapor pressure deficit (VPD) are two different ways to measure humidity. While most growers traditionally use RH to track their greenhouse or indoor humidity, more and more growers are switching to monitoring VPD.

VPD provides a different approach to humidity, one which is more focused on plants. It allows growers to better optimize humidity for plant development, growth and health, resulting in larger yields and improved quality.

What Is the Difference between Vapor Pressure Deficit and Relative Humidity?

Relative humidity measures how saturated the air is with moisture, at a certain temperature.

Vapor pressure deficit, on the other hand, measures the difference between saturation and the current amount of moisture in the air, in terms of pressure.

For a more in-depth explanation about vapor pressure deficit, read our blog post – What is VPD?

The pressure described in VPD is what drives plant transpiration, making it a much better proxy for nutrient transport, and therefore, plant activity.

Vapor Pressure Deficit vs. Relative Humidity

We’ve recently received data from a tomato grower, comparing growing conditions in different greenhouses, some with DryGair (numbers 5-6), and some without (1-4).

vapor pressure deficit vs relative humidity

It’s clear to see that in the DryGair greenhouses, relative humidity (shown as RV in the image) was slightly lower. This is a smaller difference than normally expected. But it’s not surprising, as there are more plants in the DG greenhouses, placed more densely (higher leaf area index).

Naturally, more plants evaporate more water. So, without DryGair in the greenhouse, introducing more plants would result in a much higher RH level.

However, regardless of relative humidity, we can see that VPD (described as VD) was much higher. This means that despite having more plants in the greenhouse, which evaporate more water, the DryGair unit was able to create more “space” in the air for moisture.

Higher vapor pressure deficit means plants are able to transpire more water, improving metabolism. So, despite the greenhouse housing more plants, and relative humidity being only slightly lower, the plants received a significant boost to their growing conditions.

Using Vapor Pressure Deficit as a Growing Tool

This example demonstrates how tricky humidity can be to understand, monitor and optimize. It’s important not to rely on relative humidity as the only parameter for greenhouse humidity.

Vapor pressure deficit is a much more comprehensive growing tool than relative humidity, in understanding and stimulating plant activity. Which is why growers who use plant empowerment techniques often prefer VPD over RH.

Controlling vapor pressure deficit requires a dehumidifier to extract water vapor from the air. Actively dehumidifying increases VPD and allows plants to evaporate more water, even when RH isn’t very low.

DryGair users can track VPD using the SmartDG system, which allows real-time VPD monitoring. They can also use the platform to initiate or stop dehumidification remotely or based on predetermined setpoints.

Learn more about vapor pressure deficit and dehumidification in BellPark Horticulture’s video:

Additional Benefits of Dehumidification

Another great example of how VPD is a better tool than RH, is in this flower greenhouse in the Netherlands.

vapor pressure deficit relative humidity drygair

We can clearly see that when operating DryGair, relative humidity was ~10% lower. This isn’t a minor difference, but it’s not a huge gap either. However, it’s clear to see that the vapor pressure deficit was around twice as high. Which is a very substantial difference.

There’s no doubt that this difference in humidity creates better growing conditions, which ultimately affect quality and yields. But there are several more parameters in this photo that show just how much of a difference dehumidification can have in a greenhouse.

Energy Savings Less Heating

First of all, we can see a clear difference in the temperatures of the heat pipes (marked as Bu in the image). Without DryGair, they were heating at a temperature of 30°C. But with DryGair operating, heating was turned off.

This is because, by controlling humidity from inside, the grower was able to keep the greenhouse closed. Without ventilating, they were able to maintain their optimal temperaturelevels, and not lose heat to the environment. This is a big deal when it comes to energy savings, and in fact, saves growers an average of 50% on energy!

Avoiding CO2 Losses

Now, let’s take a look at CO2 – the concentration of which is almost double with DryGair. Without ventilating, they were able to maintain high CO2 levels inside the greenhouse, avoiding any losses. Of course, CO2 is a critical resource for plant activity, and higher levels contribute to improved growth.

There’s no doubt that using dehumidifiers to control the climate in greenhouses and grow rooms is crucial and has many benefits. But there are different ways to approach it and use it to optimize growth. Not only to prevent diseases, but to actively improve cultivation conditions.

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