Greenhouse Climate Measurements Ensure Optimal Plant Growth
Greenhouses, structures with transparent roofing and walls, are designed for plant cultivation in controlled environmental conditions. Greenhouse cultivation has several advantages: it helps to maintain an optimal plant growth environment and protects the crops from pests and varying outdoor conditions like excess heat or cold, storms and blizzards, and droughts. Greenhouses are optimized for collecting and storing of solar energy. Thus greenhouses enable plant growth in areas otherwise unsuitable for cultivation, such as climates with a limited growing season. As certain crops can be grown in greenhouses throughout the year, greenhouses are becoming increasingly important for the food supply.
Points to Note when Selecting an Instrument
- Required accuracy and longterm stability
- Instrument degree of protection IP65/NEMA4 minimum
- Operating range in high relative humidity
- Capability to recover from condensation
- Sensor response time
- Solar shield for the temperature and humidity sensors
- Compatibility of the sensor signal output with the control system
- Required sensor calibration interval and ease of calibration
- Potential wear and tear of moving parts
- Spare parts availability
The Importance of Temperature and Humidity Control
The most important environmental parameters that need to be controlled for optimal greenhouse climate are temperature, relative humidity and carbon dioxide (CO2). Temperature is the most important single parameter in greenhouse controls as temperature has a significant role in plant growth and development.
The optimal temperature depends on the plant species grown and desired level of photosynthetic activity. Typical greenhouse temperatures vary between 10-20°C (50-68°F). Too high temperature reduces plant growth, eventually resulting in plant wilting and death whereas too low temperature limits plant growth.
In addition to optimizing the greenhouse temperature, humidity control is of vital importance as optimal plant growth can only be achieved within a certain humidity range. Too high relative humidity encourages mould growth, which causes plant diseases and can also damage the greenhouse structures. Too dry environment slows down the plant growth. The optimal relative humidity depends on the plant species grown, with a typical range varying between 50 and 70 percent.
Carbon Dioxide- the Growth Engine
Plants consume CO2 in the photosynthesis reaction combining it with water to form sugars and oxygen. The CO2 concentration in the greenhouse greatly influences the plant growth rate and thus CO2 level needs to be monitored and controlled for optimal growth.
The optimal CO2 concentration depends on the species grown, with the optimal growth levels reached around 1000 ppm (parts per million) for most crops. Active photosynthesis can bring down the greenhouse CO2 concentration to a level of 200 ppm, which is low enough to negatively impact the plant growth. Too low CO2 level limits growth but too high level of CO2 is not beneficial either. Plants are more sensitive towards high CO2 concentrations than humans and show damages like burnt leaves at elevated CO2 levels. Thus excess CO2 fertilization is not good for the crops; it increases costs and can be dangerous to humans, too (average exposure limit over 8 hours is 5000 ppm CO2).
During the summer, a suitable CO2 level can be maintained by ventilating and opening the roof windows, which brings the greenhouse to the background CO2 level around 380 ppm. This is not possible during colder periods, thus CO2 addition from CO2 burner or a gas bottle is needed.
Selecting Measurement Instruments for a Greenhouse
Tips for Transmitter Placement in the Greenhouse
- Select a location for the sensor that well represents the greenhouse climate.
- Temperature sensor should be placed within the plant zone. Temperature sensor on the wall, close to the roof or close to heating pipelines does not represent the climate around the plants.
- Humidity sensor should not be located close to heaters, heating pipelines, fans, walls or irrigation water sprays.
- CO2 sensor should not be placed near a vent or exhaust duct.
Greenhouses are challenging measurement environments. Constant high humidity, risk of condensation, potential spray irrigation, dust and dirt, and constant exposure to solar radiation are all factors of a challenging environment.
Only instruments designed to work in harsh environments will survive in a greenhouse. Accuracy, long-term stability, and integrability are important things to consider, among others. Check the box on the right for a list of things to take into account before selecting an instrument. The cheapest unit will not necessarily be the most economical in the long run.
Vaisala’s Sensor Technologies for Your Benefit
Vaisala introduced the Vaisala HUMICAP® thin-film capacitive humidity sensor in 1973.
Since then, the technology has developed from one company’s innovation into an industry standard. The HUMICAP® sensor consists of a substrate on which a thin film polymer is deposited between two conductive electrodes. The polymer film absorbs and releases water vapor as the relative humidity of the surrounding air changes. The capacitance of the polymer film depends on the amount of absorbed water. The capacitance is measured and converted into a humidity reading. Vaisala HUMICAP® technology is a promise of accuracy and stability even under demanding conditions.
The Vaisala CARBOCAP® sensor is a unique silicon-based non-dispersive infrared (NDIR) sensor measuring carbon dioxide (CO2). The light source of the sensor emits infrared light and the CO2 molecules present in the measurement chamber absorb a part of the light at a characteristic absorption wavelength. The IR detector measures the light intensity passing through a Fabry-Perot Interferometer (FPI) interference filter. The FPI filter is electrically tuned so that its pass band coincides with the absorption wavelength of CO2. The pass band of the FPI is then shifted to a wavelength having no absorptions. This wavelength serves as the reference signal. The ratio of these two signals, at the CO2 absorption wavelength and the reference wavelength, indicates the degree of absorption in the gas and thus the CO2 concentration. This unique reference signal compensates for the effects of sensor aging and contamination, making the sensor very stable over time. Vaisala CARBOCAP® sensors will prove themselves economical over time: their stability significantly decreases maintenance costs over the years.