Remember when we talked about how important calibration is for your thermal sensors when calculating CWSI? ("Decoding VPD and CWSI for Optimized Crop Water Management").
Well, today we're going to explore how to get accurate readings without shelling out for a pricey blackbody calibrator.
Blackbody calibrators are the usual go-to for checking and adjusting your IRT or thermal camera. They provide a stable, known temperature source. But they can be expensive. And speaking of expensive, high-quality infrared thermometers (IRTs) in the $800-$1000+ range often include a significant cost for factory calibration—sometimes as much as half the total price. This is a labor-intensive process.
However, the raw materials used to make IRTs are relatively inexpensive, and the components are often very similar regardless of where the IRT is sold (U.S. vs. other countries). So, if you're comfortable with the calibration techniques described in this post, or plan to purchase a commercial blackbody calibrator, you can potentially save a lot by opting for a less expensive, but still reliable, IRT and calibrating it yourself.
So, what are our options?
I've got two ideas for you:
1. Water Works Wonders (for Quick Field Checks):
Did you know that water is practically opaque to thermal radiation? This means the surface of water can actually mimic a blackbody calibrator, albeit with some important caveats.
Water has a high emissivity (the ability to emit thermal radiation), around 0.96-0.98. While this is close to ideal (an emissivity of 1), it's not perfect, meaning it reflects a small amount of incoming radiation.
Here's what you do:
Warm/cool some water to a specific temperature.
Pour it into a bucket.
Use a regular temperature sensor (like a thermometer) to measure the water temperature.
Position your IRT right above the water surface, minimizing any reflections from the surrounding environment.
Take simultaneous readings from both your IRT and the water thermometer at set intervals (e.g., every 15 minutes).
Use these paired readings and a bit of linear regression to calibrate your IRT. Simple!
Important Considerations When Using Water:
Emissivity: While high, water's emissivity isn't perfect, meaning it reflects a small amount of thermal radiation.
Reflections: Minimize reflections from the surrounding environment (sky, buildings, etc.) as they can affect the readings.
Temperature Stability: Monitor the water temperature closely, as it can change relatively quickly, especially outdoors.
Surface Condition: Ensure the water surface is clean and free of contaminants.
Because of these limitations, the water method is best suited for quick checks in the field, not for precise laboratory calibration.
2. DIY Blackbody Calibrator (for Tinkerers):
My experience with thermal sensor and imaging system development provided me with access to a variety of blackbody calibrators. However, I was also interested in exploring the feasibility of constructing one myself. As it turns out, it's quite possible!
I'll outline the basic components here, and you can investigate the finer details independently.
Here's a simplified overview:
The Blackbody Surface: A metal plate (round or square) with a corrugated (wavy) surface, painted with high-emissivity infrared black paint. The corrugations increase the surface area, enhancing thermal radiation absorption and emission.
Components: You'll also need a Peltier thermo-electric cooler to control the surface temperature, a thermocouple to measure it, and a temperature controller (like one built with an Arduino) to maintain the desired temperature.
Important Considerations for DIY Blackbody:
Black Surface Quality: High-emissivity infrared paint is crucial. A well-constructed DIY version might achieve an emissivity of 0.90-0.95, but it likely won't match commercial calibrators (0.98+).
Temperature Uniformity and Control: Ensure even temperature distribution and a stable temperature using a quality controller.
Depending on your desired temperature range, you may need to incorporate several Peltier coolers. The construction of the blackbody surface can present some challenges (I sought assistance from a friend for this stage).
It's important to be aware that even with meticulous construction, a DIY blackbody calibrator may not achieve the same level of accuracy as a commercially manufactured device. However, for many intended uses, the performance should be adequate.
In Conclusion:
Both methods offer alternatives to expensive blackbody calibrators. The water method is great for quick field checks, while a DIY version can offer better accuracy. However, it's crucial to understand the limitations of each method and choose the one that best suits your needs and accuracy requirements. By calibrating your IRT yourself, you can potentially save significant money.
Consider comparing your results with other methods whenever possible.
Happy calibrating!
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