Environment control sensors provide critical data

Better environment control can result in improved plant quality and reduced energy use. Although the most common factors controlled are temperature, humidity and light levels, there are over 30 separate functions that can be controlled. Each of these requires at least one sensor to determine the existing conditions.

Sensor selection, location
Sensor selection and location are keys to good environment control. Selection should be based on accuracy, repeatability, output signal compatibility and interchangeability.
Placing the sensor in the right location to obtain measurements is also important. For example, the temperature sensor should be located in the plant zone to measure the air temperature around the plants, not located on a wall, near heating pipes or on an overhead truss.

Temperature measurement
To measure dry and wet bulb temperature measurements, electric resistance type sensors such as thermisters are used with electronic controls. Accuracy should be better than ±0.5ºF. Sensors should be shielded and aspirated. Place heating and cooling system sensors together so that they can sense the same environment.

Humidity measurement
Humidity can be measured using dry and wet bulb sensors. The wet bulb is kept wet by a wick. Location is inside an aspirated enclosure. The humidity level is calibrated to the psychrometric chart. Maintenance is important for good results. Some computer system manufacturers use a thin-film capacitive sensor that changes readings as humidity changes.

Carbon dioxide level
Air sampling is the standard method of measuring carbon dioxide. The air is passed through an infrared analyzer and compared to a reference sample. Moisture level of the air affects the measurement and the air may have to be passed through a dryer. Carbon dioxide levels may be maintained above ambient to as much as 1,500 parts per million to get additional plant growth. Calibration needs to be done every few months.

Weather station sensors
Outdoor weather data is important for making decisions on how the inside environment should be controlled and which pieces of equipment need to be operated. A weather station mounted on the peak of the greenhouse is usually an integral part of a computer control system. Sensors usually include measurements for temperature, wind, precipitation and light.

Wind speed and direction
A cup anemometer is the standard instrument for measuring outdoor wind speed. Revolutions per minute are counted and calibrated to wind speed. Wind direction is determined with a weather vane. The direction is detected by a calibrated potentiometer.

Rain detector
This sensor is common with open roof greenhouses. When it starts to rain, the roof usually needs to be closed. A common rain sensor consists of two honeycomb, gold plated electrodes on a circuit board. Raindrops will complete a circuit and signal the computer.

Radiation
Sunlight measurements are important to maximize plant growth. A pyranometer measures total shortwave irradiation. Measurement errors may occur from calibration, non-linearity, angular response and positioning. The pyranomete’s glass cover needs to be cleaned monthly.
Other light measuring instruments include silicon photocells that can be calibrated to measure photosynthetic active radiation (PAR) and a net radiometer that measures a combination of the short wave and long wave radiation. The photometer, similar to what is used in a camera, can measure artificial light in lux, footcandles, energy flux density and photosynthetic active radiation if a correction factor is applied.

Photosynthesis
Sensors for measuring plant photosynthesis continue to be developed. One system provides plant weight measurements excluding removal of fruit (vegetable number and weight) and leaves and the addition of irrigation water. Another system makes measurements on individual leaves by enclosing a leaf in a chamber and measuring the air exchange rate and carbon dioxide concentrations. The above measurements in addition to values for leaf and soil temperature, leaf dew point, transpiration index, stem diameter, leaf area, sap flow, hydraulic conductance, soil moisture, light level and daily fruit increment are fed into a software program to give a value for plant growth.

Irrigation
There are many sensors that have been developed for measuring the moisture content of the growing substrate. These can help in determining when to irrigate. Soil moisture tension is used in conjunction with evapotranspiration estimates to determine how strongly water is held in the substrate.
   
A common system available to manage irrigation is vapor pressure deficit (VPD). It is a measure of the difference in the humidity inside the leaf and the humidity of the greenhouse air. The computer calculates the amount of moisture lost by the plants and activates the irrigation system to make up this difference.
   
Two measurement systems are used for vapor pressure deficit. An infrared plant temperature sensor is used for crops with a large leaf surface. For propagation and potted plants, a “green leaf” sensor that measures the influence of light, radiation and wind on the plant temperature is used.

Fertigation
A concentrate solution of dissolved fertilizer and water is added to the irrigation water and mixed before it is applied to the plants. The system consists of tanks, pumps, sensors and controls that adjust the nutrient levels according to crop conditions and environmental factors.
   
For closed irrigation systems, the recycled water containing nutrients is analyzed and additional nutrients are added.  The pH and electrical conductivity can be adjusted before the water is reused. The water may also be filtered and treated by pasteurization, ozonation or ultraviolet radiation to disinfect it to remove nematodes, bacteria, fungi and viruses.
   
Solution pH and electrical conductivity are the standard electrodes used to measure fertigation. In systems that inject individual elements to maintain nutrient levels, individual element sensors are used. Selective semiconductor sensors that measure nitrate (NO3), potassium (K), calcium (Ca), sulfate (SO4), sodium (Na), chlorine (Cl) and ammonium (NH4) have been developed. Each element has to have its own nutrient supply tank, pump, solenoid valve and sensor.

Counters
These can be added to the system to provide useful information. They can record data on kilowatt hours of electricity used, fuel oil or gas consumption and water usage. Data can be stored and graphs can be printed out for analysis. This information can be important when comparing crops from one year to the next.
   
John Bartok Jr. is faculty emeritus, University of Connecticut, Department of Natural Resources Management and Engineering, jbartok@rcn.com.