Environmental Control Systems 



CHOOSING WHAT'S BEST 



Paul Fisher 



Being able to monitor the greenhouse environment 

 (temperature, light, humidity, etc.) and to control 

 these factors to provide optimum growing condi- 

 tions is a i<ey to growing high-quality plants in a timely 

 manner. Approaches for monitoring and control range 

 from completely manual operation through to automated 

 computer systems. These options vary widely in cost 

 and complexity and should be matched to the needs of 

 your business. In this article, I will describe some of the 

 equipment that is available, and discuss factors you 

 should consider in choosing what's best for you. 



Greenhouse Monitoring and Control Options 



1. IVIANUAL CONTROL The simplest approach to control- 

 ling the greenhouse is through a manual system: for ex- 

 ample, when vents are opened and closed by hand. A 

 manual system is fine for small operations with crops 

 that do not require precise environmental control and 

 where there is limited equipment for heating, cooling, 

 and lighting. Clearly, the cost of a manual system is in 

 labor — the decision on whether to upgrade to thermo- 

 stats, motor-controlled vents, etc., should be made 

 based on whether reliable and trained labor is available 

 to operate your equipment and whether the cost of that 

 labor will, in the long-run, exceed the cost of the upgrade. 



It is important in whatever control system you use that 

 there is a manual backup — that you can control all of your 

 equipment in a manual mode if a thermostat breaks down 

 or your computer 'dies' during a power failure. 



2. TIME CLOCKS A low-cost improvement over a manual 

 control system is to add time-clocks (around $50 and 

 upward each) that could turn lights or heating on and off 

 at set times of day or night. Time clocks will rapidly pay 

 for themselves in labor-saving, but do not dynamically 

 respond to changing greenhouse conditions: for example, 

 an unusually cool day in which heat is needed. 



3. SENSORS Even in a manual system, it is important to 

 monitor the greenhouse environment, especially tem- 

 perature. The cheapest option is to install a maximum- 

 minimum thermometer in each greenhouse area for a 

 cost of around $20 per zone. You can check and reset 

 the max-min thermometer either once a day or in the 



morning and evening to ensure that your greenhouse 

 temperatures are in an acceptable range and that your 

 heating and cooling equipment is working as intended. 



Digital temperature sensors or weather stations are 

 also available for around $300 upward. These are either 

 linked with a thermostat or environmental computer for 

 control or are stand-alone and purely for monitoring. 

 Some sensors have a unit which will measure light and 

 humidity in addition to temperature. Digital sensors can 

 typically provide information on current, average, and 

 (sometimes) day-and-night temperatures that allow you 

 to calculate DIP (day minus night) temperature. This ad- 

 ditional information can help you predict elongation of 

 plants and when crops will bloom. 



Where you position the temperature sensor in the 

 greenhouse is important, regardless of the type. Place 

 the sensor near the center of the greenhouse away from 

 the heating and cooling units to get a representative 

 reading. Temperatures generally increase by IF for every 

 one foot above the floor level. Therefore, place the sen- 

 sor near plant canopy height so that you are measuring 

 the air temperature experienced by the plant — the tem- 

 perature six feet above a bench is not the temperature 

 affecting plant growth. Shield the sensor from direct or 

 indirect rays of sunlight — sunlight on the sensor will give 

 a falsely high reading. The shield should be painted 

 white or with some other reflective surface to reduce 

 heat buildup. A fan can be mounted to blow air over the 

 sensor; this makes the temperature reading more repre- 

 sentative of air throughout the greenhouse. Sensors have 

 a limited life and need to be checked and either cali- 

 brated or replaced each year — a faulty instrument is 

 worth less than no instrument at all. 



4. INDIVIDUAL THERMOSTATS The next level in control 

 is to have each piece of equipment (e.g., the hot water 

 supply valve or the ridge vent) controlled by its own 

 thermostat. Individual thermostats are cheap ($200 and 

 upward) and work reasonably well if there is simple 

 heating and cooling equipment. Once you have more 

 than one thermostat in the greenhouse, you have the 

 potential problem of thermostats working against each 

 other (e.g., vents open while the heating is on). It is 

 very important to have thermostats calibrated at least 



• ECEMBER I996 4^|ANUARV 1997 



