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Member #161873

Member Since: November 11, 2010

Country: United States

  • The explanation of radiation implies that only infrared light warms things. That is false.

    Original: "Per the Stefan-Boltzman law, any object that isn't below absolute zero (0°K) emits (non-human-eye-visible) light in the infrared spectrum that is directly proportional to its temperature. The special infrared thermopile inside the MLX90614 senses how much infrared energy is being emitted by materials in its field of view, and produces an electrical signal proportional to that."

    What I think would be less roughshod: "Molecules emit light in proportion to their temperature—This is called blackbody radiation. For earthly temperatures, most of that is infrared. The thermopile is covered by a filter that lets through a wide band of the infrared part of the spectrum, but not visible light. In our common environments, there are many visible lights that aren't from "blackbody radiation" but rather from narrow-band fluorescent or LED lamps; so, we don't want to let those freaky lights affect our readings. The thermopile itself warms, compared to the material in which it's embedded, by receiving more radiation from warmer objects than it emits, or it cools by receiving less radiation from cooler objects than it emits. That changes its electrical properties, and those properties are compared to an unexposed internal thermometer, which is really a diode whose bandwidth changes with temperature. So, the device outputs a signal that is a comparison between its known temperature and its exposed temperature. For example, the device knows its internal temperature is 30°C, but the thermopile on the device is conducting electricity as if it were exposed to something 80°C hotter than itself, so the device reports that it is exposed to some source that is 110°C. Well, it's not quite that simple; the thermopile does not reach the same temperature as the object in its field of view, so we need to calibrate."

    "The thermopile itself is a pile of two different metals, where the interface between each layer of metals on one side is exposed and the interface on the other side is unexposed—Imagine cooking a triple-decker sandwich on its side. The electrical potential difference between the metals depends on temperature. It's a very small difference normally, so many layers are piled on top of each other to multiply the effect; and even then the small current still needs careful amplification for our purposes."

  • Students all over the world are going to get worse grades, then teachers like me will have to help them correct their models of "flowing voltage." If voltage "flowed" then you'd measure it with a connection like an ammeter has. If voltage was "at the base" then a voltmeter would need only one lead, not two. "But you told me SparkFun had useful guides!" "I did, and now I'm a bit sorry. Not every guide there is good."

  • Upvote on questioning the usefulness or consistency of the wording. In the commonly useful model, "voltage" is a difference between two points. The poster has so much room, so why does it use such short shorthand, as in "voltage at the base of the transistor," which should be "voltage between the base and the emitter"? I can always open in Adobe Illustrator and fix things. Yay for fair use!

  • "Description:...and can be used with the breakout board below." There is no breakout board shown below. Please update the description then delete this comment.

  • You can update the description but today's email from SF made the claim, so let the heads roll.

  • "Microbolometers are made up of materials which change resistance as they’re heated up by infrared radiation.">>"Microbolometers are made up of materials which change resistance as their temperature changes." It doesn't matter what kind of radiation changes their temp, and they change resistance when they're cooled too. The second form avoids misleading people into thinking that only infrared changes their resistance, and that they must be heated somehow to be read. Of course there's an IR-passing filter on the camera, but that is separate from the microbolometer. I only make this comment because I often see people, even my fellow physics teachers, make mistaken claims about infrared and energy transfer. "By measuring this resistance, you can determine the temperature of the object that emitted the radiation..." This claim is more complex to correct, because it misleads the reader under multiple scenarios: As an extreme example, point the camera at the sky...The microbolometer will be heated or cooled according to the sum of radiation from the volume, the cone, of air and space it "sees," not a single object. This effect can be generalized to many more terrestrial situations involving aerosols. As another extreme example, point the camera at shiny metal...The microbolometer will be more affected by radiation reflected by the object than the object itself. This effect can be generalized, to some degree, to all non-matte-black-in-IR objects, in fact all solid objects. This guide overall is very interesting and helpful. You might note which versions of software you are using, especially for Raspberry Pi, as that seems to be important. -Your friend in education at EnergyTeachers.org

  • My students were able to crimp about 60% of these successfully with regular needle-nose pliers. If you want a higher success rate, use crimping pliers, which in my lab are named "the precioussss."

  • My electronics students noticed a sentence in the description, as of 2015-10-25: "The two large exposed pads function as probes for the sensor, together acting as a variable resistor." Sparkfun might instead write "The two large exposed pads function as probes for the sensor, together acting as legs on a variable resistor." The variable resistor itself, in this case, is the variably moist soil between the probes. Perhaps a completely different sentence would be appropriate.

  • If the sun doesn't make a difference because it's such a small solid angle of the sky, how come it warms me when step into it from the shade? It may be a small part, but the radiative transfer depends on the cube of the temperature difference, so a small portion of the sky at 5000 Kelvin can overwhelm the effect of a colder sky much closer to the ambient temperature. Please see page 40 of the datasheet about sunlight. Visible light from the sun, as well as near infrared and ultraviolet, can heat the whole package, changing the temperature-difference between the thermopile and the ambient sensor, thus changing the reported temperature. The wavelength argument is much more reasonable, even though it also has limitations, than the solid-angle argument. I'd stick to that. It's interesting to think about which applications will depend on shielding from sunlight and which won't—Thanks for reporting on use avionics. I agree that pointing an infrared bolometer at the sky is interesting—It's so cold, even on a hot summer day, if clear. Of course cold isn't a thing, it's the absence of warming radiation, an absence natural to expect from space.