APOGEE

Frequently Asked Questions
Topics



Q1.

   WHAT ARE SOME PROPERTIES OF SOLAR RADIATION?
 
A1.
 

Solar radiation is a term used to describe visible and near-visible (ultraviolet and near-infrared) radiation emitted from the sun. The different regions are described by their wavelength range within the broad band range of 200 to 100,000 nm (nanometers).

Terrestrial radiation is a term used to describe infrared radiation emitted from the earth. The components of solar and terrestrial radiation and their wavelength ranges are:
 
ULTRA VIOLET 250 to 400 nm UV SENSOR OR
SPECTRORADIOMETER
VISIBLE 400 to 700 nm QUANTUM SENSOR OR
SPECTRORADIOMETER
NEAR INFRARED 700 to 3000 nm  
INFRARED 3000 to 100,000 nm
(3 to 100 um)		 INFRARED RADIOMETERS


Approximately 99% of solar, or short-wave, radiation at the earth's surface is contained in the region from 300 to 3,000 nm while most of terrestrial, or long-wave, radiation is contained in the region from 3,500 to 50,000 nm.

Outside the earth's atmosphere, solar radiation has an intensity of approximately 1,370 watts per square meter. This is the value at mean earth-sun distance at the top of the atmosphere and is referred to as the Solar Constant. On the surface of the earth on a clear day, at noon, the direct beam radiation will be approximately 1,000 watts per square meter for many locations.


The availability of energy is affected by location (including latitude and elevation), season, and time of day. All of which can be readily determined. However, the biggest factors affecting the available energy are cloud cover and other meteorological conditions which vary with location and time.

Historically, solar measurements have been taken with horizontal instruments over the complete day. In the Northern US, this results in early summer values 4-6 times greater than early winter values. In the South, differences would be 2-3 times greater. This is due, in part, to the weather and, to a larger degree, the sun angle and the length of daylight.

Q2.
 
WHAT IS PHOTOSYNTHETIC RADIATION AND WHY SHOULD IT BE MEASURED TO PREDICT PLANT GROWTH?
A2.
 
Photosynthesis and plant growth depend on the energy in radiation but only specific wavelengths of radiation cause photosynthesis. We have known since Einstein that one photon excites one electron (the Stark-Einstein Law), which starts photosynthesis. In 1972 a scientist named Keith McCree showed that a meter that counted the number of photons in radiation would more accurately predict photosynthesis than the previously used foot-candle meters.  LI-COR (Lincoln, NE) started making meters to measure this radiation and scientists quickly switched to the new measurement system, which is called Photosynthetic Photon Flux (PPF). The energy in a photon is called a quantum so these meters are called quantum meters.  A quarter century later, LI-COR has sold thousands of high quality meters but their least expensive quantum meter with sensor is $780. The high cost means that they are used only by scientists and large commercial growers. Smaller growers have continued to use foot-candle meters, which measure light for humans. Footcandle meters have errors of up to 45% when used to measure light for photosynthesis.
Q3.
 
WHAT ARE THE CONVERSIONS FROM PPF (µmol m-2 s-1) TO OTHER UNITS OF MEASUREMENT?
A3.
 
Follow the links below to the desired conversion page.
      From µmol m-2 s-1 to footcandles

  From µmol m-2 s-1 to Lux

  From µmol m-2 s-1 to mol m-2 d-1

  From µmol m-2 s-1 to Einsteins

  From µmol m-2 s-1 to Watts m-2 when measuring UV light in sunlight

External conversion sites:
   The Eppley Laboratory has a conversion area that has
          "Instantaneous Conversion Calculator" and
          "Energy Over Time Conversion Calculator"

   Environmental Growth Chambers (EGC) has a comprehensive
          table and calculator to approximate conversion values for
          radiation from 400 - 700 nm from different lamp types.
Q4.
  WHAT ARE THE BEST PPF LEVELS FOR OPTIMUM PLANT GROWTH?
 A4.
 
The units of PPF are µmol m-2 s-1 (micromoles of photons per meter squared per second) or mol m-2 d-1.   The table below provides a guideline for light levels for certain plants.
 
   
TYPE OF PLANT / CONDITION
APPROXIMATE*
PPF FOR GOOD GROWTH
(µmol m-2 s-1) 
DAILY PPF (mol m-2 d-1) 
House plants
30 to 200
3 to 20
     
Leafy crop plants
(lettuce and basil)
200 to 600
10 to 30
     
Tomatoes and
other fruit crops
400 to 1000
20 to 50
     
Full sunlight at noon
in the summer
2000
50 in midsummer
25 inside a good glass greenhouse
     
Full sunlight at noon
in the winter
 
~1200
10 in midwinter
5 inside a good glass greenhouse
*Plant growth is determined by the daily PPF.  The daily PPF from instantaneous measurements under electric lights can be calculated by multiplying by the number of seconds in the photoperiod.
 
Q5.
  HOW DO I CLEAN MY SENSOR?
A5.
 
Isopropyl (rubbing) alcohol and a Q-tip work well for cleaning the sensor area.  Be careful NOT to use an abrasive cloth on the top as it will scratch the surface of the sensor.
Q6.
  CAN I USE MY SOLAR RADIATION SENSOR UNDERWATER?
A6.
 
Absolutely.  We have quite a few customers that purchase our sensors for continuous underwater applications in aquaculture and aquarium environments.  Model QSO and Model PYR are completely sealed and watertight.  The cable can be submerged underwater as well.

On Model QMSS, Model QMSW-SS or Model UVM-SS, you can still submerge the sensor and cable part of these units, but the meter part itself is only splash resistant. 
Q7.
  HOW DO I KNOW IF MY SOLAR RADIATION SENSOR NEEDS TO BE RECALIBRATED?
A7.
 
The frequency to recalibrate a sensor varies greatly with the application. Generally, we recommend a recalibration cycle of every two to three years, particularly when using the sensor in continuous outdoor applications. But rather than guessing, there is another method that allows the end user to make the determination. The Clear Sky Calculator is designed to calculate the intensity of radiation falling on a horizontal surface, at any time of day, in any location in the world. Essentially, the calculator outputs an estimated value that can be directly compared to the output of the sensor(s) in question. For best accuracy, comparison should be made on clear, non-polluted, summer days within one hour of solar noon. The test sensor(s) should also be leveled and cleaned to help produce consistent measurements.
Q8.
  WHAT IS THE PURPOSE OF THE OXYGEN SENSOR'S BUILT-IN HEATER?
A8.
 
In order to keep condensation from forming on the sensor’s Teflon membrane (where oxygen diffusion occurs), the built-in heater is designed to warm the sensor to a temperature slightly above the ambient temperature. This is particularly important in soil applications where the relative humidity is normally at 100%. For this reason, it is recommended that the heater be continuously powered. Once condensation forms on the membrane, the sensor must be removed from the humid environment and allowed to dry before the condensation evaporates and the signal returns. If the heaters are turned off and condensation forms, the heaters don't supply enough energy to evaporate the condensation once they are turned back on. The heater requires a 12 VDC input and consumes about 74 mW of power. This works out to about 6 mA of current draw.