Optical IR sensors are able to measure the surface temperature of solids. But is there a range of the EM spectrum that can accomplish the same for measuring the temperature of nitrox (dry nitrogen, oxygen mixtures)?
If so what are the constraints?
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$\begingroup$ I suspect you could use could build one based on refractive index, if you can also control for pressure? $\endgroup$– ericksonlaCommented Aug 21, 2018 at 18:17
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$\begingroup$ Possibly interesting but not simple to implement: pdfs.semanticscholar.org/9ccd/… . Or.... mdpi.com/1424-8220/18/1/72/pdf $\endgroup$– Carl WitthoftCommented Aug 22, 2018 at 18:08
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$\begingroup$ @ericksonla good suggestion. Found this pdfs.semanticscholar.org/ee25/… $\endgroup$– docscienceCommented Aug 22, 2018 at 20:37
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$\begingroup$ There is a troll lurking about. Down voting answers. For no good reason. $\endgroup$– docscienceCommented Aug 22, 2018 at 20:41
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2 Answers
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The fundamental problem is: how do you reject the optical signal from solids or liquids "behind" the air mass you want to measure? The esoteric methods reported in : https://pdfs.semanticscholar.org/9ccd/6df082576a620a2a89f8e6b7203186e6b08c.pdf , http://www.mdpi.com/1424-8220/18/1/72/pdf may work, because they indirectly measure air temperature rather than trying to sense the spectral power distribution.
Otherwise, you essentially need a cold plate behind your sample volume to act as a near-zero-emission (not emissivity) baffle. So the answer is that it's really not a good way to go. Is there a reason you can't work with thermocouples?
answered Aug 22, 2018 at 18:15
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$\begingroup$ Good point; that makes sense. Yes - a thermocouple sticks out in a moving gas stream and can get 'fouled' or damaged; especially a fast one (low thermal mass). Looking for alternative methods of 'remote' sensing, so optical first thoughts. Optical, also if you can get it to work may have a very fast frequency response. Maybe take two temp measurements; background and gas + background and subtract the two? $\endgroup$ Commented Aug 22, 2018 at 20:17
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You can use a sensor such as an "Infrared Thermometer - MLX90614" and a simple circuit.
There are "SDA Temperature and Humidity monitors" available for making measurements in diving chambers and dive systems. They are certified to be accurate and used where safety is a concern, as such they won't be an inexpensive solution.
To measure using sound see: "An introduction to acoustic thermometry" (Apr 21 2011), by Jim Williams and Omar Sanchez-Felipe, of Linear Technology. Also a DIY solution.
"Acoustic thermometry is an arcane, elegant technique that measures temperature using the temperature-dependent transit time of sound in a medium. The medium can be a solid, a liquid, or a gas. Acoustic thermometers function in environments, including extreme temperatures, destructive physical abuse, and nuclear reactors, that conventional sensors cannot tolerate. Sonic speed in air varies predictably as the square root of temperature. The sonic transit time in a gas-path thermometer is almost entirely insensitive to pressure and humidity. Gas-path acoustic thermometers respond quickly to temperature changes. They have essentially no thermal mass or lag.".
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$\begingroup$ The device you cited from Sparkfun - I believe that measures temperature from a radiating (solid) surface, but not the temperature of the gas (air) between the sensor and surface. They mention measuring body (skin) temperature remotely as an example. $\endgroup$ Commented Aug 22, 2018 at 14:14
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$\begingroup$ But I suspect it's not measuring the air temperature directly but rather the temperature of, say, the duct walls. The temperature-sound speed relationship is fraught with uncertainties due to humidity level, altitude, etc. $\endgroup$ Commented Aug 22, 2018 at 18:04
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$\begingroup$ Your referenced sensor there does not appear to have any opto sensors. $\endgroup$ Commented Aug 22, 2018 at 18:06