Equipment Every Greenhouse Should Have
by John Erwin
Professor, Department of Horticultural Science
University of Minnesota
How many times would you like to know your pH and EC immediately to decide what to fertilize with at that moment? What is your plant temperature? Is your media temperature what you think it is? Do you have a virus in your crop? Are your lights positioned to give you the amount of light you need for night break lighting or supplemental lighting? What temperatures did your plants get exposed to during shipping? New instruments available on today can answer these questions.
Quantum (PAR) Light Meters
We are beginning to appreciate how important it is to light different crops at different times in their production cycles. Sometimes we light to promote or delay flowering as is the case with photoperiodic plants. Sometimes we light to increase photosynthesis to increase plant mass or size, flower number, earliness of flowering, stockiness etc – which generally increases plant quality. Light levels needed for each of these purposes are outlined in many places in the literature, articles in magazines, and text books. However, if you want to know if you are at the levels that are recommended, you will need a light meter. More and more articles are referring to light in units of micromoles (umol m-2 s-1) and moles per day (mol d-1). These units are based on sensors called quantum or PAR sensors that only measure light in the 400-600 nm wavelengths – which is what is photosynthetically active in plants.
What is it?
The light meter of choice is a meter that senses light using a quantum or PAR sensor. A quantum sensor measures what light the plant can use to do photosynthesis (approx. 400-700 nm). Many light meters including those that are supplied by many environmental computer control companies use sensors that measure light in watts, lumens, lux, or footcandles. These other units are light units based on either light that the human eye can see or the total light available which includes a much broader range in light than what the plant uses for photosynthesis. Any light sensors other than quantum sensors, will not give you an accurate idea of how much light is available for plant growth. For this reason, a quantum sensor is the only thing that should be used for greenhouse plant production. There is no reason whatsoever for greenhouse environmental control companies to sell any other kind of meter. It is also important to place light meters inside a greenhouse at plant level rather than outside the greenhouse which is commonly done.
How Do They Work?
Light meters have a sensor below a protective window which converts light energy into electrical energy. Different sensors will give you different readings because they sense different amounts of light depending on the wavelengths or range they are sensitive to. It is very important to hold a light meter level as tilting it will give you an erroneous reading. For this reason, many light meters are sold with a leveling plate that holds them levels on not so level surfaces.
Where to Buy One?
There are fewer places to buy light meters with quantum sensors than the other meters mentioned above. There are three companies that I have had experience with that I would recommend: LiCor, Apogee, and Spectrum. The sensor plus meter costs generally range from $300-800. Some meters have the sensors attached to the meter with a wire; others have the sensor built in. In general, I would purchase the meters with the sensors attached with a wire rather than built in as it is easier to level the sensor itself with this sort of system. In general, Apogee produces a meter that is comparable to the more expensive LiCor meter (the ‘Cadillac’) for significantly less money.
Spectrum offers both an instantaneous light meter, plus a unit that measures instantaneous light, accumulated light per day (called moles of light per day), plus temperature (high, low , average) and a variety of degree day calculations called a Greenhouse WeatherTracker. These units can be moved from greenhouse to greenhouse to measure conditions over time. Most growers have found these to be very useful. A WeatherTracker costs approx. $500 as of the printing of this article. Other companies offer light integrators, but I would recommend the Spectrum system as it is the easiest to use and most greenhouse friendly at this point.
Where to buy one:
LiCor Portable Quantum Sensor and Meter:-
http://www.licor.com/env/Products/Sensors/190/li190_description.jsp
http://www.licor.com/env/Products/Sensors/250A/li250A_introduction.jsp
Apogee Portable Sensor and Meter:
http://webtrolley.org/mivastore/merchant.mv?Screen=PROD&Store_Code=apogee&Product_Code=BQM-SUN&Category_Code=BQM
http://webtrolley.org/mivastore/merchant.mv?Screen=PROD&Store_Code=apogee&Product_Code=QMSW-SS&Category_Code=BQM
Spectrum Greenhouse Weather Station:
http://www.specmeters.com/Weather_Tracker_Degree_Day_Recorders/Model_305_-_Greenhouse_Light.html |
