"Absolute humidity, an indispensable tool in climate control"

For Ton van Kessel of Horti-Consult, absolute humidity (AH) is a natural in climate control. As a senior extension worker, he has seen many changes in greenhouse climate control over the years. The techniques for optimally controlling greenhouse climate have increased enormously. More screen cloths, lighting, dehumidification systems, and numerous sensors to monitor plant growth. He argues that to properly apply all these new greenhouse energy techniques in the growing climate, AH control is indispensable. AH is not affected by temperature but only by evaporation and moisture removal.

"No matter where in the world one grows fruiting vegetables, light, moisture, and plant temperature are the determining factors in the success of cultivation, and this is reflected in the AH value. If the key figures of the ideal AH values of a crop are known, registration and control of these is possible and gives a lot of peace of mind in controlling the greenhouse climate," says Ton.

Different control
"Most horticulturists still control their climate on relative humidity (RH) and/or moisture deficit (VD). This is quite understandable because, in the past, all climate control studies for optimal production were developed based on a ratio of temperature and light. The VD and RH can change values rapidly when temperatures change. So steering on these values is no longer a matter of course.

As cultivation is increasingly taking place according to the 'closed greenhouse principle,' this requires a different control, a more high-tech approach. Optimal growth and production are ultimately achieved on the basis of plant temperature. In the past, we always looked at the ideal 24-hour temperature to achieve production increases with this, but now we know that it is about optimizing the 'plant temperature.'

"A bit more self-managing"
Peet Withagen of tomato nursery Zonnekreek from Moerstraten started growing according to AH values five years ago. Peet: "Five years ago, we had a difficult tomato variety with a difficult setting. That was the moment for us to start growing differently. Ton van Kessel pioneered this by growing according to the absolute moisture content. By controlling the right AH values in the greenhouse, the greenhouse climate is in order. That's when the plant feels best; I don't look at RH or VD at all now, only at AH. It has also brought me a lot of peace of mind. Whereas before, I had to go back to work adjusting the settings on my climate computer when the weather changed, I now fully trust the AH values I have set. It has become a bit more self-managing."

Ideal AH values
Don Heijligers of BioVerbeek in Velden started growing on AH values in 2018. BioVerbeek grows tomatoes, peppers, and cucumbers, and they apply this principle for all crops. Don: "Growing in this way gives us a lot of peace of mind. There are several people responsible for cultivation at the company. Now that we have grown in this way, it is much easier to take over each other's work. Everyone knows what the ideal AH values are in the relevant crop. If you stick to this, you'll be fine. As a result, at the weekend, there is only one person busy keeping an eye on the climate in the crops while we were all working before this. Of course, it did take some time for me to get a feel for it and for us to figure out the right proportions. But I definitely don't want to grow in any other way anymore."

Climate computers
In an enthalpy graph, the grower can find the AH value depending on the temperature and relative humidity (RH) ratio. The value of AH can be expressed in gr/m3 or in gr/kg. Not all climate computers yet have settings to control greenhouse climate, according to AH. Growers who want to control their climate according to AH values advocate increasing the options in the software of climate computers.

Peet: "The current software is still not always sufficient to grow, according to AH. In addition, it does not always work in a user-friendly way. I hope there will soon be a new generation of climate computers that embrace growing according to this principle and thus also make it easier for growers to grow in this way." Meanwhile, a number of climate computer manufacturers have started working on this.

Dense greenhouse
Ton van Kessel: "From the point of view of energy, growers are increasingly growing in a 'closed greenhouse.' From one screen, we've gone to two or even three screens and lowered shower temperatures. To reduce disease pressure, vents are equipped with insect netting. When lighting with LED, we talk about 'cold light.' With fans, we aim for more ventilation. We install dehumidification systems for a drier climate and misting systems to add moisture again. All these measures affect the AH values in the greenhouse. So it is necessary to measure them properly and then be able to adjust the greenhouse climate accordingly."

For more information:
Ton van Kessel
Horti-Consult International

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