I’m looking for a small temperature sensor that can detect the temperature of a remote patch of air (a thermal in the sky). The thermal would be about 2-500 metres away.
The sensor would be mounted on an aircraft wing and as is would be flying, there would hopefully be no reflective surfaces around the thermal.
Any advice on the sources of such a sensor would be appreciated.
What temperature range do you need?
How many measurements per second?
precision ?
Accuracy?
Short of launching a temperature sensor into the thermal, I don't see any way of doing it. If you try to use IR, I can't see any way of picking up IR from the air you are interested in without also picking up IR from the air behind it. It might be possible in principle to use 2 cameras spaced well apart (e.g. one on each wingtip), focused on a remote object, and detect a change in the refraction of the air caused by the thermal by correlating the images; but I think it would need high-resolution cameras and lots of processing power.
dc42
I'm only looking at a small difference in the ambient temperature, probably only 3 or 4 degrees above the surrounding areas. That would be enough to cause a thermal. Probably only 2 measurements per second.
Accuracy is not important. All I need to know is that the patch of air (the thermal} is warmer than the surrounding air.
Rob Tillaart
I need to know that the patch of air is warmer or cooler than that immediately around the aeroplane. If the sensor is looking in a particular direction, and it senses the air temperature is more than that immediately surrounding the aeroplane, that is all I need to know - we have found a thermal! I would need to look at the highest of the two readings if as you say the sensor picks up from the air behind it. - I'm not sure how that can be accomplished.
How far can these sensors read?
Thanks both of you.
My understanding of IR is that the amount of IR emitted is related to the amount of heat and the amount of mass. Air has very, very little mass and so you would need a very, very sensitive IR sensor. I'm guessing the kind used for TV remote control receivers isn't going to cut it. Something like a darlington or avalanche maybe.
Assuming you can find a sensor sensitive enough, you'd be measuring the average IR of all of the air molecules in it's line of sight. This will further reduce accuracy, but perhaps it will still be accurate enough? Failing that, you could use two sensors aimed cross-eyed so they converge about 200-500m away.
In any case, you'd probably need to detect thermals based on the rate of change in IR, rather than an absolute reading.
Could you have a visual sensor that looks for the cloud at the top of the thermal?
Does such a thing exist commercially?
If not I suspect that it can't be done.
Does such a thing exist commercially?
I think they're called "pilots"
How is wind shear measured? I know that airports monitor for it and some aircraft have wind shear warning systems. It seems a thermal is like a slow wind shear.
ir-sensor MLX90614
useable?
Paco
See FLIR corporation
.045 degree resolution,
range from 4 F to 1200F
T series IR camera
I like Grumpy Mike's answer best.
Air is either transparent or opaque to far IR (the sort carrying heat at ambient temperatures).
If air is transparent then it won't be emitting any IR.
If air is opaque you'll only see the temperature of the nearby air, not the stuff further away, because its opaque
In practice you'd need to select a narrow set of wavelengths at which the average IR penetration is on the order of your 200 to 500m (in other words it looks misty in IR vision)
To measure a few degrees difference probably requires astonishingly expensive equipment of the sort put on IR astronomical satellite telescopes - highly accurate and sensitive bolometer in liquid helium?
Thanks for all you helpful advice. I'm moving forward!