Infrared (IR) Thermometers / Plant Stress Monitors

Infrared (IR) Thermometers / Plant Stress Monitors

Infrared (IR) Thermometers / Plant Stress Monitors

Infrared thermometers are useful tools for measuring temperature of plants, tools equipment and anywhere where you want to discover fast and accurate surface temperatures of objects.

Another great reason for using an infrared thermometer is to measure leaf temperatures. A study by the Department of Land Resource Science at the University of Guelph in canada concluded that an infrared thermometer more accurate than thermocouplesfor leaf temperature measurements. Errors are estimated to beless than 0.2 °C when measuring leaf temperatures in a typicalleaf chamber

It has been discovered that the differential between ambient air temperature and leaf temperature (ΔT) is a great indicator of a crops stress, especially from lack of irrigaion water and the subsequent yields.

A recent study by P. Mannini and S. Anconelli found that on irrigation controlled fruit, using the differential between ambient air temperature and leaf temperatuere (ΔT) was a better indicator of water stress and subsequent yields than other commonly used techniques as the tensiometer, evaporimeter and soil moisture.

  • They found that the best yields occured on irrigated strawberries when the ΔT was less than 1°C
  • They found the worst yields occured on a non-irrigated strawberries when the ΔT was more than 4°C.

The  USDA has also researched and discovered a correlation between plants stress levels and a rise in leaf temperature rise above ambient air temperature. The IR gun provides a simple, fast and accurate means of testing the differential temperature (ΔT).

Conditions for testing (ΔT):

  • Plant leaf surface must be dry
  • Use only when wind is blowing at less than 10 mph
  • Keep the sunlight at your back
  • Use a reliable thermometer for determining the ambient air temperature
  • Take I.R. readings from foliage only during the peak sunlit hours of the day, when stress is most likely to occur. Therefore, take readings from 11 AM to 4 PM during the summer and from 1 PM to 3 PM during the winter
  • If air temperature is much cooler than normal, then I.R. plant stress will not be measurable with this instrument
  • Remember that when you shine at an angle, the beam being cast will be oval, (similar to a flashlight beam) just something to bare in mind
  • You must have at least 2 minutes of sunlight immediately following an interruption by clouds
  • Remember that when you shine at an angle, the beam being cast will be oval, (similar to a flashlight beam) just something to bare in mind
  • Hold the sensor so that the beam will not see the sky or dirt, we are trying to eliminate these as confounding variables
  • As a good practice, take multiple leaf readings and average the results readings. The more samples you take the more reliable the results

Finally record the differential temperature between ambient air and average foliage temperature.The magnitude of this temperature difference is a measure of the stress level of the plants.

Distance : Spot Ratio

The infrared thermometers have a ratio such as 12:1  which is the distance to spot ratio. This means that the beam diameter cast on the leaf will be 1/12th of the distance from the sensor to the object. For example if you stood at 12 inches away from the leaf, the beam of light cast on the leaf would measure 1 inch in diameter, in other words the closer you are to the leaf the more concentrated the area will be that you measure. The higher the distance:spot ratio, the better quality the thermometer and you can also take accurate measurements from further away from the target.

In general, a differential temperature of +1or 2 degrees Celsius is very good. If the foliage is 10 °C warmer than ambient air, the plant is seriously stressed and cause of problem needs to be determined. Generally speaking  the cooler the crops stay over the course of the growing season, the higher the yield and the better the quality of the crop.
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