Ton van Kessel

Absolute humidity: an essential tool in climate control

For Ton van Kessel of Horti-Consult, the absolute humidity (AH) is a matter of course in climate control. As a senior advisor, he has seen many changes in climate control in glasshouses over the years. There are now many more techniques for best managing the climate in greenhouses. There are more screen cloths, lighting options, dehumidifier systems, and countless sensors to monitor plant growth. To use all these new energy technologies for the growth climate in the greenhouse effectively, control over the AH is essential. AH is not affected by temperature, but by condensation and humidity control.    

Regardless of where fruit and vegetables are grown in the world, light, humidity and plant temperature are the determining factors in successful cultivation, and these are expressed in the AH value. If the key figures of the ideal AH values of a crop are known, they can be registered and managed. This makes controlling the glasshouse climate a lot easier.

Ton van Kessel explains. “Most horticulturalists regulate their climate on relative humidity (RH) and/or humidity deficit (HD). This is very understandable because the research on climate control for optimal production in the past was on the ratio of temperature and light. If temperatures change, the HD and the RH values can change quickly. Using these values to manage the climate is no longer the norm. As more and more cultivation is done using the ‘closed greenhouse’ principle, a different method of control and a more high tech approach are needed. Optimal growth and production are achieved using the temperature of the plant itself. In the past we always looked at the ideal daily temperature to increase production, but we now know that it is really about optimising the ‘plant temperature’.”

Peet Withagen of the Zonnekreek tomato nursery in Moerstraten started using AH values in growing his tomatoes. Withagen says that “Five years ago we had a difficult strain of tomatoes that had a difficult setting. This was when we decided to use a different method of growing. Ton van Kessel was very much the pioneer by growing according to absolute humidity. By controlling the right AH values in the glasshouse, you regulate the climate in the glasshouse. The plant then feels at its best. I no longer look at the RH or HD, but just at the AH. It is also makes everything a lot easier. While I previously had to adjust the settings of my climate control computer with every change in the weather, I now put my faith in the AH values that I have preset. The system manages itself much more now.”

Don Heijligers of BioVerbeek in Velden started growing according to AH values in 2018. BioVerbeek grows tomatoes, peppers, and cucumbers, and they have adopted this system for all their crops. “Using this method to grow our crops has brought much peace and quiet,” says Heijligers. “There are a number of us that do the cultivation for the company. It is now much easier to take over each other’s work when necessary. Everyone knows the ideal AH values of each crop, so if you stick to them it will be fine. It means that only one person is needed at the weekend to keep an eye on the crops’ climate, while previously we all had to work. Of course, it took a while before I developed a feel for it and before we found the right ratios. But I never want to go back to the old way of growing.”

Depending on the temperature and humidity (RH) ratio, the AH value is listed in the enthalpy graph. The AH value can be given in gr/m3 or in gr/kg. As yet, not all climate control computers have settings to control the greenhouse climate according to AH. Growers who wish to manage their climate according to AH values are requesting more functions in the software of climate control computers. Withagen continues. “The current software is not always able to grow according to AH. On top of that, it is not always user friendly. I hope that a new generation of climate control computers will be available soon that will embrace cultivation according to this method and make it easier for growers to work.” In the meantime, a few producers of climate control computers are working on this.

Ton van Kessel adds that “In terms of energy, more and more growers are growing in ‘closed’ greenhouses. We have moved from one to two or even three screens and are reducing the pipe temperatures. To reduce the occurrence of disease, window vents are fitted with insect mesh. LED lighting is sometimes described as ‘cold’ light. We use fans to increase the ventilation rate. We set up dehumidification systems to create a drier climate and misting systems to add more humidity. These all affect the AH values in the glasshouse. They thus need to be monitored well and to control the glasshouse climate accordingly.”

Do you want to know more about the impact of absolute humidity and how to manage it? If so, contact Ton van Kessel of Horti-Consult International at

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