I've just got one of these from Maplins and it's a really good toy.
So after a certain amount of alcohol, we decide to measure temperature outside.
Ground is around 1 dec C as expected.
Most of the sky is about -9 to -11
But patches of clear sky between the clouds register as between -19 and -23
Can these things really measure the temperature that high up or is there something else I'm missing here (apart from another drink)
Mike
Oh, I've had -50 pointing it to the sky (it isn't supposed to go that low). With a completely clear sky, it can't get a reading (well, it would read -270C, i.e. 3 Kelvin I suppose, but you need a much better detector and liquid helium cooled for that;-).
You will often see temperatures of -40 reported at 38,000' in a plane, so I suspect it's not a complete fabrication.
On a cold day, you can go round the house looking for the colder areas on walls, ceilings, windows, etc to see where you are losing heat. One that I caught this way was the loft hatch which was at something like 12C when the rest of the ceiling was 22C. Quickly fixed with a bit of loft insulation positioned on top of it. You can also do the same on the outside of the house, looking for warm spots.
Another use is to measure temperature of things you're cooking. I've pointed it at a reheated casserole, to find it was only reheated to 55C and nowhere near as hot as I had thought.
They're great fun to play with. The only thing they are useless for is measuring temperature of copper -- copper is like silver foil in the infra-red and doesn't emit enough to get a reliable reading. If you want to measure temperature of copper pipes, you need to paint them or stick some tape on them.
BTW, my Maplin one died after about 9 months. Sent it back for warantee repair, and it came back unchanged (I don't believe anyone even looked at it). Sent it back again, and got a new one. Make sure you keep the receipt. I bought it on special offer -- I'm not sure I would have bought the Maplin one at full price. I bought one for my father too last Christmas, who has found it both quite useful and an interesting toy.
Yes, it is cold up there, great gadget, mine found an electrical fault in the theatre I work at, caused by a dodgy contactor, the electricians hadn't been able to pinpoint the cause of occasional flickery lights ! the giveaway was of course the odd few degrees up at this point. other interesting uses are checking thermostat opening temps on cars, and finding non running cylinders, stuck brake calipers, etc.
mrcheerful
Yeah. But I'm just amazed that a couple of mW laser reaches that far - and back again.
Also it was great to once and for all prove that my wide has cold feet in bed !! 10degC lower than mine - Ouch.
Can tell that from the subject spelling...;-)
It's one of life's little mysteries - why spool cheekers don't work on the subject line.
The laser has nothing to do with the temperature measurement, it is only used to highlight the measurement point.
Dave
In message , Mike wrote
If it wasn't for the atmosphere you would be looking at space which is close to absolute zero (-273C). When pointing at clouds you would be probably be seeing the temperature of the water in them, the reflected temperature from the ground and the effects of atmospheric attenuation.
I've never thought of having one of these, now I WANT ONE! Thanks guys. I'll bet they're sold out by now........
Biggles
Yep - definitely the best toy I've bought for ages. But don't get one of those £20 short range ones - aim for around £50 as they have laser finders and better optics.
Suddenly a light bulb shines brightly - of course. I could even outline the horizon with it today -
-6degC on the tops of the hills but another 20 deg lower just the slightest bit higher. Great toy.
I like to start my bath at 41C, as indicated, how sad is that?
AJH
Only on a planet with no atmosphere, surely? Earth's atmosphere with its greenhouse gases is anything but transparent in the infra-red. AIUI you'll only see anywhere near 3 K in the microwave 'window', roughly 1
- 10(+) GHz.
IANARadioAstronomer but Googling for produces several hundred hits.
I am sure I managed to get mine to show minus 270 when I aimed it at the girlfriend's shoulder after coming in very late and very drunk.
Adam
In message , Mike writes
Proper "High up" measurements would give a temp of slightly above absolute zero
Basically it's the temperature radiated by the immediate atmosphere
;-)
Basically-basically, it's measuring the total heat flux coming into the sensor within its cone of vision. From there to a meter reading called "temperature" can be a large leap of faith.
The heat flux is proportional to two separate things, absolute temperature^4 (to the fourth power ) and the emissivity of the radiating surface.
Most inexpensive IR thermometers *assume* a fixed emissivity of about
0.95, so if you point them at a surface that has a low emissivity (for instance, a polished metal surface) the heat flux is lower than it would be from a surface of emissivity 0.95, so the temperature will read low.More expensive instruments have an emissivity adjustment, but they still need to be calibrated. This is done by heating the test object up to a known temperature that has been measured some other way, and then twiddling the emissivity control until the IR thermometer reads correctly.
If you point one of these gadgets at the sky, it is most certainly not "measuring the temperature up there"! It is measuring the heat flux arriving down here, where the sensor is.
The reason these things work at all, to remotely measure the temperature of solid objects, is because the intervening air is almost transparent to radiated heat. It has a very low emissivity and heat absorption. So now you're trying to measure the actual temperature of that air...
Tricky(TM).
Air has a very low emissivity, so temperatures will tend to read very low. On the other hand, the sky contains a very large depth of air, which tends to compensate for that. On the other-other hand, the temperature decreases with height, so you're not measuring what you thought you were. But on the (other)^n hand... Even an idealised, perfect IR sensor is subject to all of those uncertainties; now add in the fact that you bought this sensor from Maplin.
And finally, the IR sensor's cone of vision is a helluva lot wider than that laser spot, which is only a visual aid to pointing.
...
You, sir are a party pooper
ha ha
It's still "toy of the year" as far as I'm concerned :-)
In message , Mike writes
Yeah, and I've got one
I use it for measuring the temperature of motor windings where +/- 5 degrees is good enough
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