Greenhouse Environmental Control Systems
To improve crop management, a number of sensors and instruments can (and should) be used to gather information in the greenhouse. Medium and high technology greenhouses make use of a range of sensors which link into automated control systems. These systems can monitor temperature, relative humidity, vapor pressure deficit, light intensity, electrical conductivity (feed and drain), pH (feed and drain), carbon dioxide concentrations, wind speed and direction and even whether or not it is raining. The information is used to control heating, venting, fans, screens, nutrient dosing, irrigation, carbon dioxide supplementation and fogging or misting systems. Closer monitoring of the greenhouse environment with sensors and advanced software can greatly improve yields and economic performance by optimizing plant growth.
A sensor can be defined as any instrument that measures some type of physical or chemical characteristic and converts that measurement into a signal that can be read by an observer or automated data collection system. All sensors have measurement errors and it is important to understand the different limitations that affect measurements.
Accurate greenhouse air measurement necessitates that the sensor be shielded from the sun and lights, and that a constant stream of air moves by the sensor. Therefore, the sensor should be in an aspirated box or tube that is reflective (for example, white in color). The aspirated unit uses a fan to draw the air through, providing an actual ambient temperature reading, rather than radiant temperature. With the use of an aspirated unit, the temperature range may be only 2 or 3 degrees plus or minus the desired setting compared to a non-aspirated unit with a range of 4 or 5 degrees. Closer control of the greenhouse air temperature gives better control and timing of the crop being grown.
The temperature of the root zone can be measured by inserting a thermocouple or temperature probe into the substrate.
Monitoring plant temperature can be used to achieve better environmental control for growth and more efficient disease management. Plant temperature controls the rate of plant development. For instance, the temperature of plant tissue affects the rate of leaf unfolding, flower bud development and stem elongation. Although air temperature has the largest effect on plant temperature, light, humidity, media temperature and wind also have impacts.
Humidity measurement is expressed as a percentage of relative humidity. It is the amount of actual moisture in the air, relative to the capacity of the air to hold it. Humidity sensing is difficult even with the most expensive sensors, and these are typically not suitable or practical for the greenhouse industry. There are three common types of humidity sensors: capacitive, resistive, and wet/dry bulb.
Greenhouses require optimum lighting to maximize plant growth and productivity, while minimizing energy consumption. Light measurements help optimize growth, and can be used to automate supplemental light levels in greenhouses and guide positioning of lights in indoor growth facilities. There are two common ways to measure light that are relevant to plants: 1) global radiation and 2) photosynthetically active radiation (PAR). The difference between these two measures is that global radiation includes near-ultraviolet [ultraviolet (≈280 to 400 nm)], visible (400 to 700 nm), and near infra-red radiation (700 to 3000 nm).
Carbon dioxide (CO2) concentration measurement is often ignored despite the fact that CO2 is a critical factor for plant photosynthesis. In a cold winter morning, when greenhouse vents are tightly closed, we often see very low CO2 concentration due to the photosynthesis of the plants in greenhouse. Therefore having a capability to at least monitor CO2 is always important for plant production.
Wind Speed and Direction
The most common method to measure wind speed is with cup anemometers. Wind causes the cups to rotate around a vertical shaft and the number of rotations within a particular time interval is measured to determine the wind speed. Another type of anemometer is the windmill type, where a propeller is spun by the wind and the rotations of the propeller are measured.
These are sensors are mounted on outdoor weather stations to measure precipitation. Simple rain “grids” indicate either the presence or absence of precipitation but not the volume.
Irrigation Scheduling with Substrate Sensors
There are several soil moisture sensing technologies that may benefit greenhouse plant production including tensiometers, electrical resistance blocks, and dielectric sensors. The sensors are used to determine either water availability (i.e. soil water tension) or actual water content in the substrate.
Principles of Substrate Sensors
Substrate water content can be expressed in terms of the energy status of the water in the substrate (water or matric potential) or as the amount of water in the substrate (most commonly expressed on a volumetric basis). Both methods have advantages and disadvantages.
Tensiometers are simple instruments consisting of a plastic (typically) tube, a porous ceramic cup at one end, and a vacuum gauge at the other (See Figure 4.1). The tube is filled with water to exclude air and the tensiometer is inserted in to the soil. As the substrate dries, water is pulled from the tensiometer through the ceramic into the soil creating a vacuum within the tube that is measured by the gauge. The drier the substrate, the greater the pulling force and vacuum. When irrigation occurs, the vacuum in the tube pulls water back into the tube from the substrate, which reduces the vacuum. The “pulling” force of the soil on water is matric potential.
Electrical Resistance Blocks
Electrical resistance blocks are also known as gypsum block sensors, which are simply a plug or block of gypsum into which two electrodes are inserted (See Figure 4.2). The electrical resistance between the two electrodes is a function of the soil matric potential. The principle of operation is that the resistance of an electrodes-embedded porous block is proportional to its water content. Thus, the wetter a block is, the lower the resistance measured across two embedded electrodes.
Dielectric sensors measure the soil dielectric constant, an important electrical property that is highly dependent on substrate moisture content. The substrate dielectric constant can be considered as the substrate’s ability to transmit electricity and it increases with the increase of substrate water content. One advantage of this type of sensor is it gives an almost instantaneous reading. Growers can do a quick check of root zone moisture content without having to wait, as is the case with tensiometers and Watermark sensors. Another major advantage of this type of sensor is its maintenance requirement; very little or no maintenance is required. Disadvantages, on the other hand, include its susceptibility to influences by temperature, salinity, and substrate property, especially for systems operating lower than 20 MHz frequencies.
Frequency Domain Refractometer or Capacitance Sensors (FDR). Capacitance sensors are dielectric sensors that determine the substrate moisture by measuring dielectric permitivity of the soil (See Figure 4.3). They usually consist of two cylindrical shaped electrodes.
Time Domain Refractometer (TDR) Sensors. TDR sensors require two to four waveguides installed in the soil parallel to each other (See Figure 4.4).
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