Infrared Heat Sensor (plant stress)

Professional Infrared Plant Stress Monitor

Click link at bottom to view instructions and specifications

Noncontact infra-red heat sensors measure plant stress by comparing temperature variances between ambient air & plant foliage.

Professional laser thermometer with advanced temperature processing; built-in socket accepts commercially available K-type thermocouple (not included)

• Measurement temperature range: -76F to 1157F
• Accuracy: +/- 1.8F
• Response time: 1 Second
• Distance:Spots:  16:1  (scroll down for explanation)

Plant Stress Monitoring Directions

Note: The USDA has researched the stress level in plants as a function of temperature rise above ambient air temperature. The IR gun provides a simple, fast and accurate means of testing the differential temperature (Δ T).

• Conditions required for testing:
• 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 sheet of plain white paper for determining 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.

You must have at least 2 minutes of sunlight immediately following an interruption by clouds.

Directions for taking differential temperature readings between ambient air and plant leaf surfaces:

Note: The sensor has 16:1 distance to spot ratio. This means that the beam diameter cast on the leaf will be 1/16th of the distance from the sensor to the object. For example if you stood at 16 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. 

Also 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.

The thing we are trying to eliminate is the beam measuring surrounding objects such as soil or the sky. 

1. Determine ambient air temperature. Point gun at center of 8.5" wide sheet of white paper. If you hold the sheet 24" from the IR gun, then the 3" diameter region will be providing the ambient temperature reading.
2. Hold the sensor so that the beam will not see the sky or dirt.
3. Take readings while in the field, not while in a pickup truck.
4. View only the vegetation. Notes on Corn: After the 5th leaf stage, aim at individual plants or down the rows. Once there is a full canopy, readings may be made from an elevated platform.
5. For readout of foliage temperature, take multiple samples, average the readings. Generally, the larger the area, the more samples you should take.
6. 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.

Factors affecting readings:

1. Tree crops will transpire water slowly.
2. Row crops transpire a greater amount.
3. Overcast skies, clouds, and cold fronts cause I.R. plant stress to decrease.
4. Early in the season, with small leaf area and shallow roots, stress will rise rapidly. Later in the season, large leaves and deeper roots will stabilize the stress factor.
5. Stress may increase immediately after irrigation or soaking rain, especially in heavy soils. The lack of oxygen to the roots inhibits the ability of the plant to absorb water from the soil.

I.R. Stress Reading related to crops:

1. 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.
2. Overall, the cooler a particular crop stays, the higher the yield and the better the quality of the crop.
3. In order to produce maximum yield on seeds, the plant stress should follow a sawtooth pattern over the course of the growing season. Start the season with low stress to promote plant growth. Then create stress in order to stimulate seed production. Thereafter, maintain the vigor of the crop in order to complete the production of seeds.

General notes:

• Leaf Temperature varies directly with:
• Ambient air temperature
• Sunlight level
• Relative humidity
• Leaf Temperature varies indirectly with:
• Water
• Oxygen
• Plant stomata will open if potassium is high.
• Plants evaporate water if stomata are open.
• As drying power increases, plant temperature goes down (evaporative cooling).
• If air is saturated with moisture, i.e., relative humidity is high, leaves will evaporate less.
• Late in season, as leaves dry, stress index will gradually increase to moderate levels.
• A leaf with closed stomata will be hotter.

Stomata will close if:

• Water levels are low
• Disease or insects are present
• Response may be slow due to:
• Slow water penetration into soil
• Delay of re-growth of new roots

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