Greenhouse Environmental Monitoring and Control
Greenhouse Environmental Monitoring Sensors
Greenhouses are closed environments where conditions are optimized for plant growth. Optimal controls require information both from the indoor and outdoor environments. Typically, carbon dioxide (CO2), relative humidity, solar radiation, vapor pressure deficit, and temperature are measured in the greenhouse among other parameters. Outside measurement parameters include wind speed and direction, humidity, and rain. These factors directly affect the growth of plants in the greenhouse.
Types of Greenhouse Sensors
Temperature Sensors
The single largest advantage of using greenhouses to grow crops is the ability to provide desirable temperatures for plant growth and development. Measuring and controlling air temperature is common in many production systems because it has the largest effect on plant temperature. Additionally, substrate temperature is important for propagators of cuttings and seeds because there are specific substrate temperature requirements for seed germination and callus and root development. Finally, by measuring plant temperature, you can determine whether plants are warmer or cooler than the air temperature.
Air Temperature. Temperature thermostats/sensors are typically housed in aspirated boxes that are suspended close to the crop they are monitoring (See Figure 6.1). Direct sunlight striking a thermostat/temperature sensor will result in elevated temperature measurements. 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.
Substrate Temperature. Root zone temperature is also an important factor in managing plant health. Theremocouples (i.e., sensors) are typically used connected to data loggers or data loggers with internal sensors. Theremocouples consists of two wires of different metals twisted and brazed or held together.
Plant Temperature. 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.
Humidity Sensors
Humidity sensing is difficult even with the most expensive sensors, and these are typically not suitable or practical for the greenhouse industry (See Figure 6.3). There are three common types of humidity sensors: capacitive, resistive, and wet/dry bulb. Both capacitive and resistive solid state sensors are fairly common in greenhouses because they offer reasonable accuracy and, in the humidity range typical of most horticulture applications, maintenance is generally limited to cleaning once or twice per year.
Light Sensors
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 often referred to as the energy unit and (2) Photosynthetically Active Radiation (PAR).
Pyrometers. Global radiation is the most common light measurement for greenhouse control because it measures the entire spectrum of energy producing light (See Figure 6.4). It is measured with a pyrometer, and is generally expressed in units of watts per square meter. The common unit of measurement is watts per square meter per second (W/m2).
PAR Sensors. PAR or quantum sensors measure Photosynthetically Active Radiation (PAR) in the 400 to 700 nm wave band (See Figure 6.5). The unit of measurement is micromoles per square meter per second (μmol m-2 s-1 or mmol/m2/day). They are primarily used in horticulture research applications to measure PAR within plant canopies, greenhouses, growth and germination chambers, and in laboratory applications and light studies. In commercial greenhouses, they can be used to compare the PAR values at various points in the plant canopy, and under screens, and to check the PAR uniformity and intensity when deploying new lighting systems.
Carbon Dioxide Sensors
Carbon dioxide (CO2) concentration measurement is often ignored despite the fact that carbon dioxide is a critical factor for plant photosynthesis (See Figure 6.6). In a cold winter morning, when greenhouse vents are tightly closed, we often see very low carbon dioxide concentration due to the photosynthesis of the plants in greenhouse. Therefore, having a capability to at least monitor carbon dioxide is always important for plant production. Most commonly used sensor for carbon dioxide is an infrared gas analyzer (IRGA) (See Figure 6.6).
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. Determining the water (or matric) potential of the substrate indicates how easily substrate water is available to plants, but does not provide information on how much water is present or available.
Tensiometers. 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 6.7). 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 6.8). 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. 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.
Wind Speed and Direction Sensors
Many greenhouse environment control computers have a “storm surge” protection feature that is dependent on the weather station. When the wind speed exceeds a preset threshold, the ridge vents are closed so that they are not damaged by high winds. The most common method to measure wind speed is with cup anemometers as shown on this weather station (See Figure 6.12). 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.
Wired vs. Wireless Installations
When it comes to installing sensors for monitoring systems, there are two main options: wired and wireless. Hard wired sensors transmit data through a wire-based communication platform and connect to the terminal block of a monitoring system base using two wires. Wired sensors can be placed up to 2,000 feet (610m) away from the main device and offer highly reliable communications. Generally, installation costs for a hardwired monitoring system can be costly but results are very reliable.
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