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
Sensors are key components of greenhouse monitoring systems. Each sensor continually measures a specific condition, such as 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. A sensor is a device placed in the system that produces an electrical signal directly related to the parameter that is to be measured. In general, there are two types of sensors: continuous and discrete.
Temperature Sensors
The single largest advantage of using greenhouses to grow crops is the ability to provide desirable temperatures for plant growth and development. Monitoring and controlling air temperature is common in many greenhouse 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. 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, especially in greenhouses that provide heat directly to the growing media rather than heating the air of the greenhouse. The temperature of the root zone can be measured by inserting a thermocouple or temperature probe into the media. Thermocouples (i.e., sensors) are typically used connected to data loggers or data loggers with internal sensors.
Plant Temperature. Besides the temperature of the greenhouse air, the temperature of the crop or the plant is also very important. 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. The temperature of the plant is measured with an IR (infrared) camera. The thermal radiation (IR radiation) emitted by the plant stands for a certain temperature of the plant.
Humidity Sensors
Humidity sensing is difficult even with the most expensive sensors, and these are typically not suitable or practical for the greenhouse industry (Figure 7.1). There are three common types of humidity sensors: capacitive, resistive, and wet/dry bulb. Both capacitive and resistive solid-state sensors are 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 the positioning of lights in indoor growth facilities. There are two common ways to measure light that are relevant to plants: (1) photosynthetically active radiation (PAR) and (2) global radiation, often referred to as the energy unit.
PAR Sensors. Photosynthetically active radiation (PAR) or quantum sensors measure photosynthetically active radiation in the 400 to 700 nm wave band. There are two different ways to measure quantum light: instantaneously or cumulatively. Light is quantified as micromoles per square meter per second (µmol∙m-2∙s-1) when measuring light instantaneously. Alternatively, when quantifying the amount of light over the course of a day, light is quantified as mole per square meter per day (mol∙m-2∙d-1).
Pyrometers. Global radiation is the most common light measurement for greenhouse control because it measures the entire spectrum of energy-producing light. A pyranometer provides radiometric units, which are typically expressed in watts per square meter (W∙m-2) or joules per square meter and second (J∙m-2∙s-1). (Note that 1 W = 1 J∙s-1.) This sensor measures total short-wave energy from 300 to 2,800 nm from the sun and sky.
Carbon Dioxide Sensors
Carbon dioxide (CO2) concentration measurement is often ignored, even though carbon dioxide is a critical factor for plant photosynthesis. On a cold winter morning, when greenhouse vents are tightly closed, the carbon dioxide concentration is often very low due to the photosynthesis of the plants in the greenhouse. Therefore, having the capability to at least monitor carbon dioxide is always important for plant production.
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. Tensiometers are simple instruments consisting of a plastic (typically) tube, a porous ceramic cup at one end, and a vacuum gauge at the other. The tube is filled with water to exclude air, and the tensiometer is inserted into 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.
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. 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.
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. 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.
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
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.
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