What is the Humidity Deficit?

No, the concept of humidity deficit has not been invented just to complicate things. It is actually a key indicator to managing climate in a Greenhouse. In this article, we will try to make a clear distinction between Relative Humidity (%) and Humidity Deficit.

RELATIVE HUMIDITY (RH)

Relative humidity is commonly used in everyday life. It is the ratio between the quantity of water in the air and the quantity that air can hold. The hotter the air the more water it can hold before reaching saturation. The relative humidity is then displayed in %. Ex.: 40% RH means that you are at 40% max capacity of water in the air.

This value doesn't give a direct indication of plant transpiration. We do not recommend using this value for greenhouse management.

HUMIDITY DEFICIT (HD)

The HD is the water mass missing for the air to reach saturation. We can visualize it as the space available in the air to fill it with water. The more space available the stronger the evaporation force on the plants will be. The humidity deficit is directly linked to the health of the plant without having to consider temperature. We measure the HD in grams per meter cube (g/m3).

Ex.: A HD of 3 g/m3 is telling us that it is missing 3 grams of water to saturate the cubic meter of air.

How would the HD affect the plants?

The transpiration rate of a plant depends on the evaporation force applied to it and the opening of the stoma.

• The dryer the air, the higher the HD will be. That means the plant is going to have more transpiration. On the other hand, if the air is too dry, the plants will close their stoma to keep their water. This is going to negatively affect their growth.
  

• On the opposite, an HD too low (too humid) will not offer a strong enough evaporation force. The plant will stop its transpiration. This will limit water and mineral circulation in the plant and consequently limit gas exchange. Without this, the plant will have smaller growth.

Consequences of a bad humidity control

Humidity too low	Humidity too high
Wilt	Softness
Late growth	Increased leaf diseases
Smaller leaves	Nutrient deficiency
Burnt leaves tip	Root system diseases
Folding leaves	Edema

Advantages of Managing the Humidity Deficit

When we use Relative Humidity, the optimal interval for the plants changes with the temperature. By using the Humidity Deficit, the interval stays the same without any regard for temperature variation. This can simplify greenhouse climate management.

Temperature	15°C	20°C	30°C
RH management	50-75%	60-80%	80-90%
DH management	3-7 g/m3	3-7 g/m3	3-7 g/m3

The RH is not telling us the direct effect on the plant. 80% relative humidity is good for the plant at 20°C but too humid at 15°C. Because you need to factor in the temperature with the relative humidity reading makes evaluating optimal growing conditions harder.

Humidity Deficit in the Orisha System

Orisha's system will do a dehumidification pulse every time the HD value is below the asked value in the program. By doing so we make sure to expose the plants to humidity that will help their growth and prevent some diseases.

The pulse can be programmed to act with heating or not. You can also program a set opening of the side curtains and the roofs, the time of a pulse and the minimum time in between pulses.

In doubt, the default value in the program can be used. These are set at 25% opening, a 6-minute pulse, and a time of 40 minutes in between pulses (cycle) with heating.

The system will choose the right combination of the following actions:

• Heat;

• Start the positive pressure;

• Opening the side curtains.

Strategy	Heating	Fans	Roofs
Deactivated
Ongoing action	〰	一	一
Pulse	〰	〰	〰
Cold Pulse	〰	〰
Heating only	〰

〰: Action in a pulse

一: Ongoing action

The choice of strategy is made about the intervals of the temperature program and the outside temperature. The objective of the system is to:

• Maximize dehumidification;

• Minimize energy consumption;

• Minimize the impact on the plants.

Choose a Humidity Target

A good target will vary about the species of the plant and the development stage. For tomatoes, a good target would be 3 g/m3 in the day and 2,5g/m3 at night.

For specific advice on your production, ask a professional in greenhouse production.

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Sources

https://www.pthorticulture.com/fr/zone-du-savoir/comment-lhumidite-influence-t-elle-la-qualite-des-cultures/

https://www.mapaq.gouv.qc.ca/SiteCollectionDocuments/Regions/ChaudiereAppalaches/Espaceconferences/Jacques_Theriault_24_hrs_deshumidification_06022018.pdf

https://www.canr.msu.edu/news/why_should_greenhouse_growers_pay_attention_to_vapor_pressure_deficit_and_n

https://www.questclimate.com/fr/vapeur-pression-d%C3%A9ficit-int%C3%A9rieur-croissance-partie-3-different-stages-vpd/